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These materials are to be used only for the purpose of individual, private study and may not be reproduced in any form or medium, copied, stored in a retrieval system, lent, hired, rented, transmitted, or adapted in whole or in part without the prior written consent of Jeppesen. Copyright in all materials bound within these covers or attached hereto, excluding that material which is used with the permission of third parties and acknowledged as such, belongs exclusively to Jeppesen. Certain copyright material is reproduced with the permission of the International Civil Aviation Organisation, the United Kingdom Civil Aviation Authority, and the Joint Aviation Authorities (JAA). This book has been written and published to assist students enrolled in an approved JAA Air Transport Pilot Licence (ATPL) course in preparation for the JAA ATPL theoretical knowledge examinations. Nothing in the content of this book is to be interpreted as constituting instruction or advice relating to practical flying. Whilst every effort has been made to ensure the accuracy of the information contained within this book, neither Jeppesen nor Atlantic Flight Training gives any warranty as to its accuracy or otherwise. Students preparing for the JAA ATPL theoretical knowledge examinations should not regard this book as a substitute for the JAA ATPL theoretical knowledge training syllabus published in the current edition of “JAR-FCL 1 Flight Crew Licensing (Aeroplanes)” (the Syllabus). The Syllabus constitutes the sole authoritative definition of the subject matter to be studied in a JAA ATPL theoretical knowledge training programme. No student should prepare for, or is entitled to enter himself/herself for, the JAA ATPL theoretical knowledge examinations without first being enrolled in a training school which has been granted approval by a JAA-authorised national aviation authority to deliver JAA ATPL training.

Contact Details: Sales and Service Department Jeppesen GmbH Frankfurter Strasse 233 63263 Neu-Isenburg Germany Tel: ++49 (0)6102 5070 E-mail: [email protected] For further information on products and services from Jeppesen, visit our web site at: www.jeppesen.com

JA310101-000

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© Jeppesen Sanderson Inc., 2004 All Rights Reserved ISBN 0-88487-350-1

Printed in Germany

PREFACE_______________________

As the world moves toward a single standard for international pilot licensing, many nations have adopted the syllabi and regulations of the “Joint Aviation Requirements-Flight Crew Licensing" (JAR-FCL), the licensing agency of the Joint Aviation Authorities (JAA). Though training and licensing requirements of individual national aviation authorities are similar in content and scope to the JAA curriculum, individuals who wish to train for JAA licences need access to study materials which have been specifically designed to meet the requirements of the JAA licensing system. The volumes in this series aim to cover the subject matter tested in the JAA ATPL ground examinations as set forth in the ATPL training syllabus, contained in the JAA publication, “JAR-FCL 1 (Aeroplanes)”. The JAA regulations specify that all those who wish to obtain a JAA ATPL must study with a flying training organisation (FTO) which has been granted approval by a JAA-authorised national aviation authority to deliver JAA ATPL training. While the formal responsibility to prepare you for both the skill tests and the ground examinations lies with the FTO, these Jeppesen manuals will provide a comprehensive and necessary background for your formal training. Jeppesen is acknowledged as the world's leading supplier of flight information services, and provides a full range of print and electronic flight information services, including navigation data, computerised flight planning, aviation software products, aviation weather services, maintenance information, and pilot training systems and supplies. Jeppesen counts among its customer base all US airlines and the majority of international airlines worldwide. It also serves the large general and business aviation markets. These manuals enable you to draw on Jeppesen’s vast experience as an acknowledged expert in the development and publication of pilot training materials. We at Jeppesen wish you success in your flying and training, and we are confident that your study of these manuals will be of great value in preparing for the JAA ATPL ground examinations. The next three pages contain a list and content description of all the volumes in the ATPL series.

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ATPL Series Meteorology (JAR Ref 050) • The Atmosphere • Wind • Thermodynamics • Clouds and Fog • Precipitation

• Air Masses and Fronts • Pressure System • Climatology • Flight Hazards • Meteorological Information

General Navigation (JAR Ref 061) • Basics of Navigation • Magnetism • Compasses • Charts

• Dead Reckoning Navigation • In-Flight Navigation • Inertial Navigation Systems

Radio Navigation (JAR Ref 062) • Radio Aids • Self-contained and External-Referenced Navigation Systems

• Basic Radar Principles • Area Navigation Systems • Basic Radio Propagation Theory

Airframes and Systems (JAR Ref 021 01) • Fuselage • Windows • Wings • Stabilising Surfaces • Landing Gear • Flight Controls

• Hydraulics • Pneumatic Systems • Air Conditioning System • Pressurisation • De-Ice / Anti-Ice Systems • Fuel Systems

Powerplant (JAR Ref 021 03) • Piston Engine • Turbine Engine • Engine Construction

• Engine Systems • Auxiliary Power Unit (APU)

Electrics (JAR Ref 021 02) • Direct Current • Alternating Current • Batteries • Magnetism

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• Generator / Alternator • Semiconductors • Circuits

Instrumentation (JAR Ref 022) • Flight Instruments • Automatic Flight Control Systems • Warning and Recording Equipment • Powerplant and System Monitoring Instruments

Principles of Flight (JAR Ref 080) • Laws and Definitions • Aerofoil Airflow • Aeroplane Airflow • Lift Coefficient • Total Drag • Ground Effect • Stall • CLMAX Augmentation • Lift Coefficient and Speed

• Boundary Layer • High Speed Flight • Stability • Flying Controls • Adverse Weather Conditions • Propellers • Operating Limitations • Flight Mechanics

Performance (JAR Ref 032) • Single-Engine Aeroplanes – Not certified under JAR/FAR 25 (Performance Class B) • Multi-Engine Aeroplanes – Not certified under JAR/FAR 25 (Performance Class B) • Aeroplanes certified under JAR/FAR 25 (Performance Class A)

Mass and Balance (JAR Ref 031) • Definition and Terminology • Limits • Loading • Centre of Gravity

Flight Planning (JAR Ref 033) • Flight Plan for Cross-Country Flights • ICAO ATC Flight Planning • IFR (Airways) Flight Planning • Jeppesen Airway Manual

• Meteorological Messages • Point of Equal Time • Point of Safe Return • Medium Range Jet Transport Planning

Air Law (JAR Ref 010) • International Agreements and Organisations • Annex 8 – Airworthiness of Aircraft • Annex 7 – Aircraft Nationality and Registration Marks • Annex 1 – Licensing • Rules of the Air • Procedures for Air Navigation

• Air Traffic Services • Aerodromes • Facilitation • Search and Rescue • Security • Aircraft Accident Investigation • JAR-FCL • National Law

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Human Performance and Limitations (JAR Ref 040) • Human Factors • Aviation Physiology and Health Maintenance • Aviation Psychology

Operational Procedures (JAR Ref 070) • Operator • Air Operations Certificate • Flight Operations • Aerodrome Operating Minima

• Low Visibility Operations • Special Operational Procedures and Hazards • Transoceanic and Polar Flight

Communications (JAR Ref 090) • Definitions • General Operation Procedures • Relevant Weather Information • Communication Failure • VHF Propagation • Allocation of Frequencies

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• Distress and Urgency Procedures • Aerodrome Control • Approach Control • Area Control

Table of Contents

CHAPTER 1 The Atmosphere Introduction ...................................................................................................................................................1-1 Definition of the Atmosphere.........................................................................................................................1-1 Properties of the Atmosphere .......................................................................................................................1-1 Composition of the Atmosphere....................................................................................................................1-1 Water (H2O) ..................................................................................................................................................1-3 The Water Cycle ...........................................................................................................................................1-3 Particles and Dust.........................................................................................................................................1-3 Carbon Dioxide (CO2) ..................................................................................................................................1-4 Structure of the Atmosphere .........................................................................................................................1-4 Troposphere..................................................................................................................................................1-4 Tropopause...................................................................................................................................................1-5 Stratosphere .................................................................................................................................................1-7 Stratopause...................................................................................................................................................1-7 Mesosphere ..................................................................................................................................................1-7 Mesopause ...................................................................................................................................................1-7 Thermosphere...............................................................................................................................................1-7 International Standard Atmosphere (ISA) .....................................................................................................1-8 ISA deviation.................................................................................................................................................1-8 Jet Standard Atmosphere (JSA) ...................................................................................................................1-9 Answers to ISA deviation questions............................................................................................................1-10

CHAPTER 2 Pressure and Pressure Systems Introduction ...................................................................................................................................................2-1 Atmospheric Pressure...................................................................................................................................2-1 Measuring Atmospheric Pressure .................................................................................................................2-2 Mercury Barometer .......................................................................................................................................2-2 Aneroid Barometer........................................................................................................................................2-3 Units of Measurement ...................................................................................................................................2-3 Pressure Variation ........................................................................................................................................2-4 Horizontally ...................................................................................................................................................2-4 Diurnally ........................................................................................................................................................2-4 Vertically .......................................................................................................................................................2-4 The Relationship between Pressure and Temperature.................................................................................2-5 Pressure/Height Calculations........................................................................................................................2-6 Pressure Values............................................................................................................................................2-8 QFE ..............................................................................................................................................................2-8 QNH..............................................................................................................................................................2-8 QFF...............................................................................................................................................................2-8 The Standard Pressure Setting.....................................................................................................................2-8 Synoptic Charts.............................................................................................................................................2-8 Pressure Systems.........................................................................................................................................2-9 Depressions ..................................................................................................................................................2-9 Depression Weather ...................................................................................................................................2-10 Anticyclones................................................................................................................................................2-10 Troughs.......................................................................................................................................................2-12 Trough Weather ..........................................................................................................................................2-12 Ridges.........................................................................................................................................................2-13 Ridge Weather ............................................................................................................................................2-13 Cols.............................................................................................................................................................2-13 Col Weather ................................................................................................................................................2-14 Movement of Pressure Systems .................................................................................................................2-14

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CHAPTER 3 Altimetry Introduction .................................................................................................................................................. 3-1 Pressure Calculations .................................................................................................................................. 3-1 Converting between Height and Altitude ...................................................................................................... 3-2 Converting between Altitude and Pressure Altitude/Flight Level .................................................................. 3-4 Pressure Change ......................................................................................................................................... 3-5 Correcting for Temperature .......................................................................................................................... 3-6 Converting between QNH and QFF ............................................................................................................. 3-8 Mountain Flying.......................................................................................................................................... 3-10 Altimeter Settings ....................................................................................................................................... 3-10 Calculation of Minimum Usable Flight Level .............................................................................................. 3-11

CHAPTER 4 Temperature Introduction .................................................................................................................................................. 4-1 Temperature Scales..................................................................................................................................... 4-1 Fahrenheit .................................................................................................................................................... 4-1 Celsius ......................................................................................................................................................... 4-1 Kelvin ........................................................................................................................................................... 4-1 Conversion Factors ...................................................................................................................................... 4-1 Measurement of Temperature...................................................................................................................... 4-2 Heating of the Atmosphere .......................................................................................................................... 4-3 Solar Radiation............................................................................................................................................. 4-3 Terrestrial Radiation..................................................................................................................................... 4-4 Conduction ................................................................................................................................................... 4-4 Convection ................................................................................................................................................... 4-5 Latent Heat of Condensation ....................................................................................................................... 4-5 Advection ..................................................................................................................................................... 4-5 Diurnal Variation of Temperature ................................................................................................................. 4-5 The Greenhouse Effect ................................................................................................................................ 4-7

CHAPTER 5 Water in the Atmosphere Introduction .................................................................................................................................................. 5-1 Water States and Latent Heat ...................................................................................................................... 5-1 Evaporation .................................................................................................................................................. 5-1 Melting ......................................................................................................................................................... 5-1 Sublimation .................................................................................................................................................. 5-2 Condensation ............................................................................................................................................... 5-2 Freezing ....................................................................................................................................................... 5-2 Saturation..................................................................................................................................................... 5-2 Humidity ....................................................................................................................................................... 5-2 Absolute Humidity ........................................................................................................................................ 5-3 Saturation Content ....................................................................................................................................... 5-3 Relative Humidity ......................................................................................................................................... 5-3 Humidity Mixing Ratio .................................................................................................................................. 5-3 Super-saturation........................................................................................................................................... 5-4 Saturation and Dewpoint.............................................................................................................................. 5-4 Condensation Level ..................................................................................................................................... 5-5 Diurnal Variation of Humidity........................................................................................................................ 5-6 Water Vapour Pressure................................................................................................................................ 5-6 Saturation Vapour Pressure Curve .............................................................................................................. 5-7 Measurement of Humidity ............................................................................................................................ 5-8 Psychrometer ............................................................................................................................................... 5-8 Humidity Method .......................................................................................................................................... 5-9 Answers to Exercises................................................................................................................................. 5-10

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CHAPTER 6 Density Introduction ...................................................................................................................................................6-1 The Ideal Gas Laws ......................................................................................................................................6-1 Boyle’s Law...................................................................................................................................................6-2 Charles’s Law ...............................................................................................................................................6-2 The Gas Equation .........................................................................................................................................6-2 Effect of Water Vapour on Air Density ..........................................................................................................6-3 Variation of Surface Air Density with Latitude ...............................................................................................6-3 Variation of Air Density with Height...............................................................................................................6-3 Variation of Air Density with Latitude and Height ..........................................................................................6-4 Diurnal Variation of Density...........................................................................................................................6-5 Density Altitude .............................................................................................................................................6-5 Calculating Density Altitude ..........................................................................................................................6-6 Effect of Density on Aircraft Performance .....................................................................................................6-7 Answers to Exercises....................................................................................................................................6-8

CHAPTER 7 Stability Introduction ...................................................................................................................................................7-1 Adiabatic Processes .....................................................................................................................................7-1 The Dry Adiabatic Lapse Rate ......................................................................................................................7-1 The Saturated Lapse Rate ............................................................................................................................7-1 The Environmental Lapse Rate.....................................................................................................................7-2 Summary of Adiabatics .................................................................................................................................7-2 Stability of the Air ..........................................................................................................................................7-2 Absolute Stability ..........................................................................................................................................7-3 Absolute Instability ........................................................................................................................................7-3 Conditional Instability ....................................................................................................................................7-3 Summary of Stability .....................................................................................................................................7-5 Neutral Stability.............................................................................................................................................7-5 Convective or Potential Instability .................................................................................................................7-6 Inversions......................................................................................................................................................7-7 Cloud Formation ...........................................................................................................................................7-8 The Dry Thermal ...........................................................................................................................................7-8 Formation of a Cloud ....................................................................................................................................7-9 Calculating Cloud Base...............................................................................................................................7-10 Forecasting Cloud Formation......................................................................................................................7-11

CHAPTER 8 Clouds Acknowledgements.......................................................................................................................................8-1 Introduction ...................................................................................................................................................8-1 Cloud Terms .................................................................................................................................................8-1 Cloud Classification ......................................................................................................................................8-2 Layer Clouds.................................................................................................................................................8-2 Clouds of Great Vertical Extension ...............................................................................................................8-2 Low Clouds ...................................................................................................................................................8-3 Stratus, ST ....................................................................................................................................................8-3 Stratocumulus, SC ........................................................................................................................................8-4 Medium Clouds .............................................................................................................................................8-4 Altostratus, AS ..............................................................................................................................................8-4 Altocumulus Castellanus, ACC .....................................................................................................................8-5 Altocumulus Lenticularis, ACL ......................................................................................................................8-5

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CHAPTER 8 (Continued) High Clouds ................................................................................................................................................. 8-5 Cirrus, CI ...................................................................................................................................................... 8-5 Cirro-Stratus, CS.......................................................................................................................................... 8-6 Cirro-Cumulus, CC....................................................................................................................................... 8-6 Clouds with Great Vertical Development...................................................................................................... 8-7 Cumulus ....................................................................................................................................................... 8-7 Cumulonimbus ............................................................................................................................................. 8-8 Cloud Amounts............................................................................................................................................. 8-9 Cloud Base .................................................................................................................................................. 8-9 Cloud Ceiling.............................................................................................................................................. 8-10 Measuring Cloud Base............................................................................................................................... 8-10 AIREPS...................................................................................................................................................... 8-10 Human Observations ................................................................................................................................. 8-10 Balloons ..................................................................................................................................................... 8-10 Ceilometer.................................................................................................................................................. 8-10 Alidade ....................................................................................................................................................... 8-10 Vertical Visibility ......................................................................................................................................... 8-10 Summary of Cloud Type and Characteristics ............................................................................................. 8-11

CHAPTER 9 Cloud Formation Introduction .................................................................................................................................................. 9-1 Turbulence ................................................................................................................................................... 9-1 Conditions .................................................................................................................................................... 9-1 Mechanism................................................................................................................................................... 9-2 Cloud Types ................................................................................................................................................. 9-4 Convection ................................................................................................................................................... 9-4 Conditions .................................................................................................................................................... 9-4 Mechanism................................................................................................................................................... 9-5 Advection ..................................................................................................................................................... 9-6 Cloud Types ................................................................................................................................................. 9-6 Orographic Uplift .......................................................................................................................................... 9-6 Conditions .................................................................................................................................................... 9-6 Mechanism................................................................................................................................................... 9-7 Cloud Types ............................................................................................................................................... 9-10 Frontal Uplift............................................................................................................................................... 9-10 Conditions .................................................................................................................................................. 9-10 Mechanism................................................................................................................................................. 9-10 The Warm Front ......................................................................................................................................... 9-10 The Cold Front ........................................................................................................................................... 9-11 Cloud Types ............................................................................................................................................... 9-12 Convergence.............................................................................................................................................. 9-13 Conditions .................................................................................................................................................. 9-13 Mechanism................................................................................................................................................. 9-13 Cloud Types ............................................................................................................................................... 9-13 CHAPTER 10 Precipitation Introduction ................................................................................................................................................ 10-1 Precipitation Processes.............................................................................................................................. 10-1 Bergeron Theory (The Ice Crystal Effect)................................................................................................... 10-1 Coalescence Theory (Capture Effect))....................................................................................................... 10-2 Intensity of Precipitation ............................................................................................................................. 10-2 Continuity of Precipitation .......................................................................................................................... 10-2 Precipitation Types..................................................................................................................................... 10-3 Hail............................................................................................................................................................. 10-4

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CHAPTER 11 Thunderstorms Introduction .................................................................................................................................................11-1 Conditions ...................................................................................................................................................11-1 Trigger Actions............................................................................................................................................11-1 Thunderstorm Classification........................................................................................................................11-1 Heat/Airmass Thunderstorms .....................................................................................................................11-2 Convection ..................................................................................................................................................11-2 Orograohic Uplift .........................................................................................................................................11-2 Advection ....................................................................................................................................................11-2 Convergence...............................................................................................................................................11-2 Frontal Thunderstorms................................................................................................................................11-2 Identification of Thunderstorms...................................................................................................................11-3 Stages of Development...............................................................................................................................11-3 Growth Stages ............................................................................................................................................11-3 Mature Stage ..............................................................................................................................................11-3 Dissipating Stage ........................................................................................................................................11-4 Supercell Thunderstorms ............................................................................................................................11-5 Movement of Thunderstorms ......................................................................................................................11-5 Squall Lines ................................................................................................................................................11-5 Hazards ......................................................................................................................................................11-5 Turbulence and Windshear .........................................................................................................................11-5 Gust Front ...................................................................................................................................................11-6 Microbursts .................................................................................................................................................11-6 Hail..............................................................................................................................................................11-7 Icing ............................................................................................................................................................11-7 Lightning .....................................................................................................................................................11-8 Static ...........................................................................................................................................................11-8 Water Ingestion...........................................................................................................................................11-8 Tornadoes...................................................................................................................................................11-9 Pressure Variation ......................................................................................................................................11-9 Weather Radar............................................................................................................................................11-9 Avoidance Criteria.....................................................................................................................................11-10

CHAPTER 12 Visibility Introduction .................................................................................................................................................12-1 Types of Visibility Reduction .......................................................................................................................12-1 Types of Visibility ........................................................................................................................................12-1 Meteorological Visibility...............................................................................................................................12-1 Runway Visual Range.................................................................................................................................12-1 Oblique Visibility..........................................................................................................................................12-2 Measurement of Visibility ............................................................................................................................12-2 Measurement of Runway Visual Range......................................................................................................12-3 RVR Reporting............................................................................................................................................12-3 Visibility While Flying ..................................................................................................................................12-4 Types of Fog ...............................................................................................................................................12-6 Radiation Fog..............................................................................................................................................12-6 Advection Fog .............................................................................................................................................12-7 Steaming Fog (Artic Sea Smoke)................................................................................................................12-8 Frontal Fog..................................................................................................................................................12-8 Hill Fog........................................................................................................................................................12-9 Other Reducers of Visibility.......................................................................................................................12-10 Smoke Fog (Smog)...................................................................................................................................12-10 Dust and Sand ..........................................................................................................................................12-10

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CHAPTER 12 (Continued) Precipitation ............................................................................................................................................. 12-10 Visual Illusions ......................................................................................................................................... 12-11 Shallow Fog ............................................................................................................................................. 12-11 Rain Showers........................................................................................................................................... 12-11 Layer Cloud.............................................................................................................................................. 12-11 Rain Effects.............................................................................................................................................. 12-11

CHAPTER 13 Icing Introduction ................................................................................................................................................ 13-1 Conditions .................................................................................................................................................. 13-1 Effects of Icing............................................................................................................................................ 13-1 Icing Definitions.......................................................................................................................................... 13-2 Supercooled Water Droplets ...................................................................................................................... 13-3 Size of Supercooled Water Droplets .......................................................................................................... 13-3 Freezing Process ....................................................................................................................................... 13-3 Types of Icing............................................................................................................................................. 13-4 Clear Ice..................................................................................................................................................... 13-4 Rime Ice..................................................................................................................................................... 13-4 Mixed Ice.................................................................................................................................................... 13-5 Rain Ice...................................................................................................................................................... 13-5 Hoar Frost .................................................................................................................................................. 13-5 Factors Affecting the Severity of Icing........................................................................................................ 13-6 Engine Icing ............................................................................................................................................... 13-7 Piston Engine Icing .................................................................................................................................... 13-7 Jet Engine Icing.......................................................................................................................................... 13-8 Ice Protection ............................................................................................................................................. 13-9

CHAPTER 14 Wind Introduction ................................................................................................................................................ 14-1 Terms Associated with Wind ...................................................................................................................... 14-2 Forces Acting upon the Air ......................................................................................................................... 14-2 The Pressure Gradient Force..................................................................................................................... 14-3 The Geostrophic Force .............................................................................................................................. 14-3 The Geostrophic Wind ............................................................................................................................... 14-5 The Geostrophic Wind Scale ..................................................................................................................... 14-7 The Gradient Wind ..................................................................................................................................... 14-7 Winds Near the Equator ............................................................................................................................. 14-9 The Surface Wind ...................................................................................................................................... 14-9 Diurnal Variation of the Surface Wind ...................................................................................................... 14-10 Measurement of Surface Wind................................................................................................................. 14-11 Isallobaric Effect....................................................................................................................................... 14-12

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CHAPTER 15 Local Winds Introduction .................................................................................................................................................15-1 Land and Sea Breezes ...............................................................................................................................15-1 Sea Breeze .................................................................................................................................................15-1 Land Breeze................................................................................................................................................15-2 Operational Implications of the Land and Sea Breezes ..............................................................................15-2 Katabatic and Anabatic Winds ....................................................................................................................15-3 Katabatic Wind............................................................................................................................................15-3 Anabatic Wind.............................................................................................................................................15-4 Foehn Wind/Effect ......................................................................................................................................15-5 Valley/Ravine Wind.....................................................................................................................................15-6 Headland Effect ..........................................................................................................................................15-7 Low-Level Jet..............................................................................................................................................15-7 Nocturnal Jet...............................................................................................................................................15-7 Valley Inversion...........................................................................................................................................15-7 Coastal Jet ..................................................................................................................................................15-8 Low Level Jet in Front of an Extra-Tropical Cold Front ...............................................................................15-8

CHAPTER 16 Air Masses Introduction .................................................................................................................................................16-1 Origin and Classification .............................................................................................................................16-1 Modification of Air Masses ..........................................................................................................................16-2 Air Masses Affecting Europe.......................................................................................................................16-3 Arctic ...........................................................................................................................................................16-3 Polar ...........................................................................................................................................................16-3 Tropical .......................................................................................................................................................16-5 Air Mass Summary......................................................................................................................................16-5

CHAPTER 17 Fronts and Occlusions Introduction .................................................................................................................................................17-1 Types of Front.............................................................................................................................................17-1 Warm Front .................................................................................................................................................17-1 Cold Front ...................................................................................................................................................17-2 Quasi-Stationary Front ................................................................................................................................17-2 Pressure Situation at a Front ......................................................................................................................17-2 Semi-Permanent Fronts of the World..........................................................................................................17-3 Arctic Front..................................................................................................................................................17-3 Polar Front ..................................................................................................................................................17-3 Mediterranean Front ...................................................................................................................................17-3 Inter-Tropical Convergence Zone (ITCZ) ....................................................................................................17-4 Characteristics of Fronts .............................................................................................................................17-4 Warm Front .................................................................................................................................................17-4 Cold Front ...................................................................................................................................................17-5 Polar Front Depressions .............................................................................................................................17-6 Weather Associated with the Polar Front Depression.................................................................................17-8 Occlusions ................................................................................................................................................17-11

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CHAPTER 18 Upper Winds Introduction ................................................................................................................................................ 18-1 Thermal Wind Component ......................................................................................................................... 18-1 Calculating the Thermal Wind Component................................................................................................. 18-2 Upper Wind ................................................................................................................................................ 18-3 Global Upper Winds ................................................................................................................................... 18-5 Jet Streams ................................................................................................................................................ 18-5 Common Jet Streams ................................................................................................................................ 18-6 Sub Tropical Jet Stream............................................................................................................................. 18-6 Polar Front Jet Stream ............................................................................................................................... 18-8 Winds Around a Polar Front Depression.................................................................................................... 18-9 Clear Air Turbulence .................................................................................................................................. 18-9 Identification of Jet Streams ..................................................................................................................... 18-10 Contour Charts......................................................................................................................................... 18-10 Thickness Charts ..................................................................................................................................... 18-11

CHAPTER 19 Windshear and Turbulence Windshear .................................................................................................................................................. 19-1 Definitions and the Meteorological Background ......................................................................................... 19-1 Definition .................................................................................................................................................... 19-1 Low Altitude Windshear ............................................................................................................................. 19-1 Meteorological Features............................................................................................................................. 19-2 Thunderstorms ........................................................................................................................................... 19-2 Frontal Passage ......................................................................................................................................... 19-2 Inversions................................................................................................................................................... 19-3 Turbulent Boundary Layer.......................................................................................................................... 19-3 Topographical Windshears ........................................................................................................................ 19-3 The Effects of Windshear on an Aircraft in Flight ....................................................................................... 19-4 Techniques to Counter the Effects of Windshear ....................................................................................... 19-8 ICAO Definitions....................................................................................................................................... 19-10 Nature of Turbulence ............................................................................................................................... 19-11 Turbulence, Meteorological Factors ......................................................................................................... 19-11 Thermal Turbulence ................................................................................................................................. 19-11 Mechanical/ Frictional Turbulence............................................................................................................ 19-11 Mountain Waves ...................................................................................................................................... 19-12 Flight Over and in the Vicinity of High Ground ......................................................................................... 19-12 Conditions ................................................................................................................................................ 19-12 Visual Detection of Mountain Waves........................................................................................................ 19-13 Turbulence ............................................................................................................................................... 19-14 Turbulence at Low and Medium Levels.................................................................................................... 19-14 Turbulence in the Rotor Zone .................................................................................................................. 19-14 Turbulence in Waves ............................................................................................................................... 19-14 Turbulence at High Levels (near and above the tropopause) .................................................................. 19-15 Turbulence Near the Jet Stream .............................................................................................................. 19-15 Turbulence in the Stratosphere ................................................................................................................ 19-15 Downdraughts .......................................................................................................................................... 19-15 Icing ......................................................................................................................................................... 19-15 Flying Aspects.......................................................................................................................................... 19-15 Low Altitude Flight.................................................................................................................................... 19-15 High Altitude Flight ................................................................................................................................... 19-16 Inversions................................................................................................................................................. 19-16 Marked Temperature Inversion ................................................................................................................ 19-16 Reporting Turbulence............................................................................................................................... 19-17

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CHAPTER 20 Non-Frontal Pressure Systems Introduction .................................................................................................................................................20-1 Low, Cyclone or Depression, and Trough ...................................................................................................20-1 Low Pressure Types ...................................................................................................................................20-2 Secondary Depression................................................................................................................................20-2 Icelandic Low ..............................................................................................................................................20-3 The Origin Of Low Pressures And Weather ................................................................................................20-5 Orographic or Lee Side Lows or Troughs ...................................................................................................20-5 Thermal Depressions ..................................................................................................................................20-6 Instability Lows............................................................................................................................................20-7 Mediterranean Low .....................................................................................................................................20-7 Polar Lows ..................................................................................................................................................20-8 Baltic Sea Cyclones ....................................................................................................................................20-8 Cells of Cold Air Aloft (Cold Pools) .............................................................................................................20-8 Anticyclone or High, and Ridge or Wedge ..................................................................................................20-9 Nature of a High..........................................................................................................................................20-9 High Pressure Systems ............................................................................................................................20-10 Subtropical Highs (Warm Anticyclones) ....................................................................................................20-10 Continental Highs (Cold Anticyclones) ......................................................................................................20-10 High Pressures And High Pressure Ridges (Or Wedges) In Series Of Travelling Depressions................20-11

CHAPTER 21 Meteorological Observations and Meteorological Services Types of Service .........................................................................................................................................21-1 Pre-Flight Briefing .......................................................................................................................................21-1 Meteorological Charts .................................................................................................................................21-1 Broadcast Text Meteorological Information.................................................................................................21-2 Special Aerodrome Meteorological Reports (SPECI)..................................................................................21-2 Terminal Aerodromes Forecast (TAF) ........................................................................................................21-2 Special Forecasts and Specialized Information ..........................................................................................21-3 SIGMET Service .........................................................................................................................................21-3 Aircraft Reports ...........................................................................................................................................21-4 Routine Aircraft Observations .....................................................................................................................21-4 Special Aircraft Observations......................................................................................................................21-4 Clear Air Turbulence (CAT).........................................................................................................................21-5 Airframe Icing..............................................................................................................................................21-6 Aerodrome Closure.....................................................................................................................................21-6 In-flight Procedures.....................................................................................................................................21-6 Accuracy of Meteorological Measurement or Observation..........................................................................21-7 Marked Temperature Inversion ...................................................................................................................21-7 Aerodrome Warnings ..................................................................................................................................21-7 Special Facilities .........................................................................................................................................21-8 Windshear Alerting......................................................................................................................................21-8 Windshear Reporting Criteria......................................................................................................................21-8 Observing Systems and Operating Procedures ..........................................................................................21-9 Cloud Height ...............................................................................................................................................21-9 Temperature ...............................................................................................................................................21-9 Horizontal Surface Visibility.........................................................................................................................21-9 Runway Visual Range (RVR)....................................................................................................................21-10

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CHAPTER 22 Meteorological Messages Introduction ................................................................................................................................................ 22-1 Aerodrome Meteorological Report ............................................................................................................. 22-1 Special Aerodrome Meteorological Reports ............................................................................................... 22-1 Terminal Aerodrome Forecasts.................................................................................................................. 22-1 Actual Weather Codes ............................................................................................................................... 22-1 Identifier ..................................................................................................................................................... 22-2 Surface Wind Velocity ................................................................................................................................ 22-2 Horizontal Visibility ..................................................................................................................................... 22-2 Runway Visual Range (RVR) ..................................................................................................................... 22-3 Weather ..................................................................................................................................................... 22-4 Significant Present and Forecast Weather Codes...................................................................................... 22-4 Cloud.......................................................................................................................................................... 22-5 CAVOK ...................................................................................................................................................... 22-5 Air Temperature and Dewpoint .................................................................................................................. 22-5 Sea Level Pressure (QNH) ........................................................................................................................ 22-6 Supplementary Information ........................................................................................................................ 22-6 Recent Weather (RE)................................................................................................................................. 22-6 Windshear (WS)......................................................................................................................................... 22-6 Trend.......................................................................................................................................................... 22-6 Runway State Group.................................................................................................................................. 22-6 Runway Designator (First Two Digits)........................................................................................................ 22-7 Runway Deposits (Third Digit).................................................................................................................... 22-7 Extent of Runway Contamination (Fourth Digit) ......................................................................................... 22-7 Depth of Deposit (Fifth and Sixth Digits) .................................................................................................... 22-7 Friction Coefficient or Braking Action (Seventh and Eighth Digits)............................................................. 22-8 'Auto' and 'Rmk'.......................................................................................................................................... 22-8 Missing Information .................................................................................................................................... 22-8 Examples of METARS ............................................................................................................................... 22-8 Aerodrome Forecasts (TAF) Codes ........................................................................................................... 22-9 TAF Contents and Format.......................................................................................................................... 22-9 Significant Changes ................................................................................................................................... 22-9 Other Groups ........................................................................................................................................... 22-10 VOLMET Broadcasts ............................................................................................................................... 22-11

CHAPTER 23 The Synoptic Chart Introduction ................................................................................................................................................ 23-1 The Station Circle Decode ......................................................................................................................... 23-3 Pressure (1 o'clock) ................................................................................................................................... 23-3 Pressure Tendency (3 o'clock)................................................................................................................... 23-3 Past Weather (5 o'clock) ............................................................................................................................ 23-4 Additional Past Weather Symbols .............................................................................................................. 23-4 Low Cloud or Vertical Visibility (6 o'clock) .................................................................................................. 23-5 Vertical Visibility ......................................................................................................................................... 23-5 Dewpoint (7 o'clock) ................................................................................................................................... 23-5 Visibility (9 o'clock Outer Position) ............................................................................................................. 23-5 Present Weather (9 o'clock Inner Position) ................................................................................................ 23-6 Weather in the Past Hour But Not at the Time of Observation ................................................................... 23-7 Surface Air Temperature or Dry Bulb Temperature (11 o'clock) ................................................................ 23-8 Medium Level Cloud (12 o'clock Lower Position)....................................................................................... 23-8 High Level Cloud (12 o'clock Upper Position) ............................................................................................ 23-9 Total Cloud Cover (Shown in the Centre of the Circle) ............................................................................ 23-10 Surface Wind............................................................................................................................................ 23-10

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CHAPTER 24 Upper Air Charts Introduction .................................................................................................................................................24-1 Symbols for Significant Weather .................................................................................................................24-1 Fronts and Convergence Zones and Other Symbols ..................................................................................24-2 Cloud Abbreviations ....................................................................................................................................24-2 Cloud Amount .............................................................................................................................................24-2 Clouds Excerpt............................................................................................................................................24-2 Cumulonimbus Only....................................................................................................................................24-3 Weather Abbreviations................................................................................................................................24-3 Lines and Symbols on the Chart .................................................................................................................24-3 Significant Weather Chart ...........................................................................................................................24-4 Upper Wind and Temperature Charts .........................................................................................................24-6 Averaging Wind Velocities ..........................................................................................................................24-8

CHAPTER 25 Climatology – The World Climate Introduction .................................................................................................................................................25-1 Ideal Global Circulation ...............................................................................................................................25-1 Rotation of the Earth ...................................................................................................................................25-2 Idealised Pressure Zones ...........................................................................................................................25-3 The Earth’s Tilt............................................................................................................................................25-3 Pressure Zones...........................................................................................................................................25-4 Equatorial Low (Trough) .............................................................................................................................25-4 Sub-Tropical Highs .....................................................................................................................................25-4 Temperate Low ...........................................................................................................................................25-4 Polar High ...................................................................................................................................................25-4 Prevailing Surface Winds ............................................................................................................................25-4 Westerly Winds ...........................................................................................................................................25-4 Easterly Winds ............................................................................................................................................25-5 Climatic Zones ............................................................................................................................................25-5 Equatorial Climate (0° to 10° Latitude)........................................................................................................25-5 Tropical Transition Climate (10° to 20° Latitude) ........................................................................................25-5 Arid Sub-Tropical (20° to 35° Latitude) .......................................................................................................25-5 Mediterranean Climate (35° to 40° Latitude)...............................................................................................25-6 Disturbed Temperate (40° to 65° Latitude) .................................................................................................25-6 Polar Climate (65° to 90° Latitude)..............................................................................................................25-7 Modifications to the Idealised Circulation ....................................................................................................25-7 Global Temperature Distribution .................................................................................................................25-7 Mean Sea Level Temperatures – January ..................................................................................................25-7 Mean Sea Level Temperature – July ..........................................................................................................25-8 Seasonal Variations in Temperature...........................................................................................................25-8 Upper Air Temperature Distribution ............................................................................................................25-9 World Pressure Distribution ........................................................................................................................25-9 Mean Sea Level Pressure – January ..........................................................................................................25-9 Mean Sea Level Pressure – July ..............................................................................................................25-10 Upper Winds .............................................................................................................................................25-11 Mean Upper Wind – January ....................................................................................................................25-11 Mean Upper Wind – July...........................................................................................................................25-12 Inter Tropical Convergence Zone (ITCZ) ..................................................................................................25-12 ITCZ – January .........................................................................................................................................25-13 ITCZ – July................................................................................................................................................25-13 Stability and Moisture Content of the ITCZ ...............................................................................................25-14 ITCZ Weather ...........................................................................................................................................25-14 Inter Tropical Front (ITF/FIT).....................................................................................................................25-14 Low Level Winds.......................................................................................................................................25-15 Low Level Winds – January ......................................................................................................................25-15 Low Level Winds – July ............................................................................................................................25-16 Climatic Summary.....................................................................................................................................25-17

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CHAPTER 26 Climatology – Prevailing Winds and Ocean Currents Introduction ................................................................................................................................................ 26-1 Europe and the Mediterranean................................................................................................................... 26-1 Africa.......................................................................................................................................................... 26-7 Asia ............................................................................................................................................................ 26-9 The Indian Monsoon ................................................................................................................................ 26-11 The Far East Monsoon............................................................................................................................. 26-13 North America .......................................................................................................................................... 26-15 South America.......................................................................................................................................... 26-15 Australia ................................................................................................................................................... 26-16 Ocean Currents........................................................................................................................................ 26-17 Cold Water Coast..................................................................................................................................... 26-18 Warm Water Coast................................................................................................................................... 26-18 Summary of the Local Winds of the World ............................................................................................... 26-18

CHAPTER 27 Climatology – Tropical Revolving Storms and Tornadoes Tropical Revolving Storm (TRS)................................................................................................................. 27-1 Characteristics ........................................................................................................................................... 27-2 Visual Indications of the Advance of the TRS ............................................................................................ 27-6 Tornado...................................................................................................................................................... 27-6 Tropical Revolving Storm Areas................................................................................................................. 27-8

CHAPTER 28 Climatology – Regional Climatology Europe ....................................................................................................................................................... 28-1 Mediterranean ............................................................................................................................................ 28-5 North Atlantic And North America .............................................................................................................. 28-8 Africa........................................................................................................................................................ 28-13 Asia .......................................................................................................................................................... 28-17 Australia and the Pacific........................................................................................................................... 28-19 South America and the Caribbean ........................................................................................................... 28-22

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INTRODUCTION Meteorology is the study of the Earth’s atmosphere and the physical processes that occur within it. The study of Meteorology is important for the pilot because the atmosphere is the medium through which the aircraft moves. It is essential to know what conditions are present along a route, and knowledge of the processes in which weather forms is useful for predicting what conditions may occur during flight.

DEFINITION OF THE ATMOSPHERE The term atmosphere refers to the gaseous envelope that surrounds the Earth. It is held to the Earth by the force of gravity. This gaseous envelope moves with the rotation of the Earth and extends from the surface of the planet up to the boundary of space.

PROPERTIES OF THE ATMOSPHERE The atmosphere acts as a fluid, is a poor conductor of heat, and only supports life in the lower levels. Due to the extent of the volume of air, variations are found both horizontally and vertically in the following properties: ¾ ¾ ¾ ¾

Pressure Temperature Density Humidity

Later chapters cover each of these properties in detail.

COMPOSITION OF THE ATMOSPHERE The density of the atmosphere decreases with altitude. This does not affect the composition up to an altitude of at least 60 km. Ozone and some trace elements are affected by the chemical reactions in the upper reaches towards 60 km.

Meteorology

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Chapter 1

The Atmosphere

Above 70 km the lower force of gravity causes the atmospheric composition to vary with height. The following percentages show the composition of dry air in the lower levels: Nitrogen: Oxygen: Argon: Carbon Dioxide:

78.09% 20.95% 0.93% 0.03%

The graph below represents this composition:

Other trace elements include: ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾

Neon Helium Krypton Xenon Hydrogen Methane Iodine Nitrous Oxide Ozone Sulphur Dioxide Nitrogen Dioxide Ammonia Carbon Monoxide

The above list is background information and needn’t be memorised. The composition of dry clean air shown above does not allow for the effects of water in the atmosphere (up to 4% in volume), dust and smoke, or carbon dioxide.

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Meteorology

The Atmosphere

Chapter 1

WATER (H2O) Water can assume all three physical states in the atmosphere, the solid state (ice), the liquid state (water), and the gaseous state (water vapour). Water is unique in that it can readily change from one state to another and can co-exist in all three states.

THE WATER CYCLE

The water cycle starts when solar radiation strikes moist ground or a water surface. The water then becomes vapour in the air. The concentration of water vapour is greatest in the lower parts of the atmosphere. When conditions are correct, water vapour forms clouds and then condenses, becomes droplets, and falls as precipitation. These clouds and the precipitation they produce make up part of what is known as weather.

PARTICLES AND DUST The solid particles in the atmosphere consist mainly of dust and sand from the ground and salt particles from the oceans. In addition, man has added all sorts of soot and dust. These solid particles can restrict visibility, for example, with haze or during sand storms. The amount of solid particles in the air varies, but the existence of these particles is of fundamental importance to processes such as condensation and the formation of ice. The condensation process occurs in the lower parts of the atmosphere. Without condensation nuclei, it would be difficult for water vapour to convert into precipitation and for the formation of ice. Meteorology

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Chapter 1

The Atmosphere

CARBON DIOXIDE (CO2) Carbon dioxide is to be found both naturally in the atmosphere and as a waste product from burning fossil fuels (carbon compounds). A large part of the carbon dioxide, which is released into the air, is returned to nature’s own circulation via the oceans. Carbon dioxide plays a large role in the heating of the atmosphere.

STRUCTURE OF THE ATMOSPHERE There are five layers in the atmosphere. From the surface upwards these are the troposphere, stratosphere, mesosphere, ionosphere, and exosphere.

Note: In the diagram above, the ionosphere and the exosphere combine to form the thermosphere.

TROPOSPHERE The troposphere extends from the surface up to an average height of 11 km. Within the layer, temperatures generally decrease as altitude increases. It is an area of relatively low stability where the over-turning of air is frequent. It holds virtually all the water vapour in the atmosphere and is the layer where most flying occurs. The troposphere contains over 75% of the mass of the total atmosphere.

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Meteorology

The Atmosphere

Chapter 1

TROPOPAUSE The upper boundary of the troposphere is known as the tropopause. It separates the troposphere from the stratosphere. The temperature ceases to decrease with height at the boundary of the tropopause. The height of the tropopause varies with latitude, season of the year, and the weather conditions. The tropopause is lowest over the poles (approximately 26 000 ft or 8 km) and highest over the equator (approximately 52 000 ft or 16 km). Its average height is 36 090 ft (11 km) at about 45° latitude. Since the thickness of the troposphere is determined by the amount of solar energy and the vertical mixing, the tropopause is lower over areas where the air is cold than where it is warm. The left-hand picture below shows that the tropical tropopause height is greater than the polar tropopause height. The right hand picture shows that, for a given region such as the poles, the summer tropopause is higher than the winter tropopause.

As the temperature decreases with height, so the temperature at the tropopause over the poles will be higher than over the equator because the tropopause is closer to the ground here. This is the opposite situation to the surface temperature. Typically, the tropopause temperature is -50°C over the poles and -80°C over the equator. Another feature of the tropopause is that, rather than show a gradual change in height between the equator and the poles, there are breaks in the tropopause where large temperature differentials occur.

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Chapter 1

The Atmosphere

The first of these breaks occurs at about 40° latitude, where warm air circulating from the equator meets colder air from higher latitudes. The second break is at 55° latitude, where tropical air meets polar air. The third break is between 60° and 70° latitude, where polar air meets arctic air. This break is more common in the Northern Hemisphere. The diagram below shows the breaks: 55° lat

60°-70° lat

40° lat

The presence of these breaks can cause strong winds called jet streams. These will be discussed in detail in a later chapter. The table below shows the approximate height of the tropopause at various latitudes in winter and summer: Latitude

1-6

Winter

Summer

0°

56 000 ft

55 000 ft

10°

55 000 ft

52 000 ft

20°

52 000 ft

51 000 ft

30°

45 000 ft

47 000 ft

40°

38 000 ft

43 000 ft

50°

35 000 ft

38 000 ft

60°

33 000 ft

35 000 ft

70°

29 000 ft

31 000 ft

80°

25 000 ft

29 000 ft

Meteorology

The Atmosphere

Chapter 1

STRATOSPHERE The stratosphere extends from the tropopause to approximately 50 km above the surface of the Earth. Some flying occurs in the lower parts of the stratosphere, so the combination of the troposphere and lower parts of the stratosphere is therefore often referred to as the aviation atmosphere. The stratosphere is relatively stable. Initially, the temperature remains constant and then starts to increase so that it is around 0°C at the top of the layer. This is due to the absorption of ultra-violet radiation by ozone in the lower layers of the stratosphere and the retransmission of this radiation as infra-red heat. The concentration of ozone varies with the latitude, being greater over the poles than the equator. Therefore, the stratosphere is warmer at higher latitudes. The region is not an area of still conditions but one of slow vertical movement and strong horizontal winds.

STRATOPAUSE This is the boundary that separates the stratosphere from the mesosphere.

MESOSPHERE In the mesosphere, temperature again decreases with height. The lowest temperature of approximately -90°C occurs between 80 and 90 km.

MESOPAUSE This is the upper boundary of the mesosphere.

THERMOSPHERE This is the outermost layer of the atmosphere that holds the exosphere in its upper regions (at heights greater than 700 km) and the ionosphere in its lower regions. The ionosphere is a region where the air becomes ionised by solar radiation. It consists of several sub-layers. These layers, named the D, E, F1, and F2 layers are important in the transmission of certain radio waves and will be covered in more depth in Radio Navigation. The thermosphere is characterised by an increase in temperature with height. At 200 km, the temperature is generally 600°C. At times of sunspot activity, it can be up to 2000°C.

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The Atmosphere

INTERNATIONAL STANDARD ATMOSPHERE (ISA) The conditions of the atmosphere are constantly changing. This causes problems for aviation, especially with the calibration of pressure instruments. For this reason, the International Standard Atmosphere (ISA) was devised. It is a purely hypothetical atmosphere that represents an average picture of the actual atmosphere. ISA has been in use since 1964 and is the most widely used hypothetical atmosphere. It possesses the characteristics laid out below: Mean Sea Level (MSL)

Temperature Pressure Density

15°C 1013.25 hPa 1225 g/m3

From MSL to 11 km (36 090 ft)

Temperature decreases at 1.98°C per 1000 ft (6.5°C per km)

From 11 km to 20 km (65 617 ft)

Temperature constant at –56.5°C

From 20 km to 32 km (104 987 ft)

Temperature rises with height at 0.3°C per 1000 ft (1°C per km)

The chart shows that the ISA temperature is constant above 36 090 ft in the aviation atmosphere.

ISA DEVIATION In aviation, it is important to know how the atmosphere differs from ISA at any particular time. Such information is used in performance calculations and in correcting for instrument errors. ISA deviation is the difference between the ISA temperature and the actual temperature. It can be a positive or a negative deviation. Example 1:

You are flying at 30 000 ft. The outside air temperature is -50°C. What is the ISA deviation?

Answer 1:

The ISA temperature would be 15 – (1.98 × 30) = -44.4°C. The difference between this and the actual temperature is 5.6°C. The actual temperature is the lower figure, so the deviation is negative (-5.6°C).

Example 2:

You are flying at 22 000 ft. The ISA deviation is +10°C. What is the outside air temperature?

Answer 2:

The ISA temperature would be 15 – (1.98 × 22) = -28.56°C. ISA deviation is +10°C, so the ambient temperature must be higher than this: -28.56 + 10 = -18.56°C

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Meteorology

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Chapter 1

The following table is given for you to practice doing ISA calculations. Answers can be found at the end of this chapter:

Height (ft)

Ambient temperature (°C)

10 000

-10

ISA Temperature (°C)

17 000

-12 -34.5 -59.32

+8 -7

38 000 8000

ISA Deviation

+10 -15.84 -48.36 -32.7

+22 -18

Note: For the JAR exams, it is sufficient to round the lapse rate up and use 2°C/1000 ft for ISA calculations.

JET STANDARD ATMOSPHERE (JSA) The Jet Standard Atmosphere (JSA) is often used by engine manufacturers. It assumes a mean sea level temperature of +15°C. The temperature then lapses at 2°C per 1000 ft to infinity. There is no tropopause in the JSA. So an aircraft at 40 000 ft with an outside air temperature of –65°C would have: ¾ ¾

An ISA temperature deviation of –8.5°C A JSA temperature deviation of 0°C

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Chapter 1

The Atmosphere

ANSWERS TO ISA DEVIATION QUESTIONS

1-10

Height (ft)

Ambient temperature (°C)

ISA Temperature (°C)

ISA Deviation

10 000

-10

-4.8

-5.2

17 000

-30.66

-18.66

-12

25 000

-26.5

-34.5

+8

34 000

-59.32

-52.32

-7

38 000

-46.5

-56.5

+10

8000

-15.84

-0.84

-15

32 000

-26.36

-48.36

+22

15 000

-32.7

-14.7

-18

Meteorology

INTRODUCTION Chapter 1 introduced the concept of the atmosphere as a fluid. The chapter also discussed the fact that certain properties of the atmosphere vary both horizontally and vertically. The fluidity of the air means that it tends to flow from a region of high pressure to a region of low pressure. It is these pressure differences and the consequent movement of air that are the main cause of weather. An understanding of pressure and pressure systems is vital for pilots.

ATMOSPHERIC PRESSURE Air is made up of particles that, small as they are, are nevertheless under the force of gravity. A surface must support the weight of the air directly above it. Atmospheric pressure is the force per unit area exerted by the molecules of air over a specific surface. Consider the column of air below:

h2

h1

s2

s1

The height of the column above s2 (h2) is less than that above s1 (h1). There is a larger weight of air above s1, hence a larger pressure. The cross-sectional area of both surfaces is the same. Meteorology

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Chapter 2

Pressure and Pressure Systems

MEASURING ATMOSPHERIC PRESSURE MERCURY BAROMETER

Vacuum

Mercury

Scale

The simplest means of measuring atmospheric pressure is the Mercury Barometer. A 1 metre tube of mercury is upturned in a reservoir of mercury. Atmospheric pressure is exerted on the surface of the mercury in the reservoir. The mercury in the tube then sinks to about 760 mm above the reservoir at mean sea level. The atmospheric pressure is therefore said to be 760 millimetres of mercury (760 mmHg). As the atmospheric pressure varies, so does the height of the mercury.

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Chapter 2

ANEROID BAROMETER

Another way of measuring pressure is by using the aneroid barometer. This consists of a partially evacuated capsule that expands and contracts as the air pressure outside the capsule changes. A scale indicates these changes by using a system of linkages. The diagram shows the basic ideas behind the system.

UNITS OF MEASUREMENT One method of expressing atmospheric pressure was introduced above, that is, mmHg. The SI unit for force is the Newton. The SI unit of pressure then becomes the N/m2, as pressure is force per unit area. The N/m2 is also known as the Pascal (Pa). 100 000 N/m2 is known as the Bar. Within one bar is 1000 millibars. This is the unit most widely used in aviation. The millibar may also be known as the hectoPascal. To further complicate the issue, some countries use inches of mercury—the United States for example. Use the following conversion when moving between units: 1000 mb = 1000 hPa = 29.53 inHg = 100 000 N/m2 = 750.1 mmHg The ISA values at mean sea level are: 1013.25 mb = 1013.25hPa = 29.92 inHg = 101 325 N/m2 = 760 mmHg

Meteorology

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Chapter 2

Pressure and Pressure Systems

PRESSURE VARIATION Pressure varies horizontally, diurnally, and vertically.

HORIZONTALLY Pressure varies from place to place and also changes over time. Horizontal pressure differences lead to movement of air and hence, weather.

DIURNALLY Pressure also has a twelve-hour oscillation period. In one day there are two peak pressure values, which occur at around 1000 and 2200 hours. There are two lows, one at around 1600 and another at 0400 hours. The difference between the high and low values is very small in temperate latitudes (only about 1 hPa), but is much more significant in tropical and sub-tropical latitudes (about 3 hPa). Although the diurnal pressure change in temperate latitudes is often masked by other events, absence of the expected change in lower latitudes is often a warning of impending severe weather, such as a tropical revolving storm.

Tropical/subtropical latitudes – typically 3 hPa

Temperate latitudes – typically 1 hPa

VERTICALLY Pressure always decreases with increase of height. In the ISA we assume that the surface pressure is 1013.25 hPa. From this we can calculate the pressure for any height.

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Pressure and Pressure Systems

Chapter 2

Pressure (hPa)

Approximate Height (ft)

850 700 500 400 300 200 100 50

5 000 amsl 10 000 amsl 18 000 amsl 24 000 amsl 30 000 amsl 40 000 amsl 53 000 amsl 68 000 amsl

Be sure to learn the figures in the above table.

THE RELATIONSHIP BETWEEN PRESSURE AND TEMPERATURE

120 ft

120 ft

ISA + 1°C

ISA

ISA 1°C

The diagram above shows three columns of air: one at ISA, one slightly warmer than ISA, and one slightly colder than ISA. The pressure at the base of all columns is the same. Cold air is denser than warm air and tends to sink. Therefore, the same pressure is found at a lower height in the cold column. The pressure decreases more quickly with height than in the ISA column. Conversely, warm air is less dense and rises. The same pressure is found at a higher height than the colder columns. The pressure decreases less quickly with height than in the ISA column. For a given height interval the decrease in pressure depends on the mean temperature of the column of air. For the same height interval the pressure change will be greater in a cold column of air than in a warm column of air.

Meteorology

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Chapter 2

Note:

Pressure and Pressure Systems

This results in a difference in height of 120 ft per degree Celsius. This is addressed in more detail in later chapters

This phenomenon is important to understand because the altimeter is calibrated to ISA. While flying in an environment that is colder than ISA, the altimeter detects the same pressure at a lower height, so you are actually flying at a lower height than you think you are, which is obviously a potentially dangerous situation. Thus the phrase: ‘Warm to cold – don’t be bold!’

PRESSURE/HEIGHT CALCULATIONS It is unlikely that you will have to make pressure/height calculations in the JAR exams, but the formulae are included here nonetheless. For calculations involving small intervals of less than 50 hPa, the following formula can be used to calculate the height change per hectoPascal change in pressure: H = 96T/P Where: H height in feet T mean temperature in K P pressure in hectoPascals Example:

Using the values for ISA MSL. T = 15 + 273 = 288; P = 1013.25 H = (96 × 288) / 1013.25 = 27.3 ft

Therefore, at mean sea level, the height change is 27.3 ft per hPa. However, as you go higher the rate of pressure fall lessens because the temperature is also falling. The changes at various heights are laid out below: Height

Height change per hPa MSL 2000 ft amsl 20 000 amsl 40 000 amsl

27 ft 30 ft 50 ft 100 ft

For JAR-FCL examinations, use 1 hPa change as equivalent to 27 ft near the surface.

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Meteorology

Pressure and Pressure Systems

Chapter 2

p2

p1

h2

h1

Use the following formula to calculate an unknown height from knowledge of its pressure: H2 = H1 + 221.1T(LOG P1 – LOG P2)

Where: H2 H1 T P1 P2

height required known height the mean temperature of the column of air in K pressure at h1 pressure at h2

Example:

At MSL the pressure is 1016 hPa, 12°C At 700 hPa the temperature is 2°C What height is the 700 hPa level: The mean temperature of the column is 7°C h2 = h1 + 221.1T(log P1 – log P2) h2 = 0 + (221.1 x 280) x (log 1016 – log700) h2 = 61 908 x 0.1618 h2 = 10 017 feet This is the height of the 700 hPa level

Meteorology

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Chapter 2

Pressure and Pressure Systems

PRESSURE VALUES The following are the most likely pressure values that pilots will encounter:

QFE QFE is the pressure at the datum level of an aerodrome (usually the highest useable point on the aerodrome). Since it is generally not possible to place a measuring device at this point it is usually measured elsewhere with corrections applied for the height difference between the measuring point and the aerodrome datum. These corrections take into account prevailing temperature. When you have QFE set, the altimeter reads zero when you are sitting at the datum level of the aerodrome. When flying on QFE, the reading on your altimeter is the height above aerodrome level and is often just referred to as height.

QNH QNH is the QFE reduced to mean sea level using ISA conditions. With QNH set, the altimeter reads aerodrome elevation when you are sitting at the datum level of the aerodrome. When flying on QNH, the altimeter reading is your height above mean sea level and is generally referred to as your altitude.

QFF QFF is the QFE reduced to mean sea level using actual outside air temperature. It is an important term for meteorology but must never be used in altimetry. Never fly on QFF.

THE STANDARD PRESSURE SETTING The standard pressure setting of 1013 hPa is often used. The resulting figure is usually divided by 100 and referred to as a Flight Level.

SYNOPTIC CHARTS A synoptic chart depicts the pressure situation at a particular time. The chart features lines called isobars. These lines connect places of equal pressure. They are normally drawn for every even whole millibar. Note that the pressure represented is the QFF. Another type of line found on some pressure charts is the isallobar, which connects places of the same pressure tendency and is annotated in millibars per hour. This may be a decrease or an increase. Isallobars are useful in predicting the movement of pressure systems.

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Meteorology

Pressure and Pressure Systems

Chapter 2

PRESSURE SYSTEMS When looking at a synoptic chart, you can see certain patterns. These are called ”pressure systems” and understanding the properties of these systems can help us forecast the weather.

DEPRESSIONS A depression is a region of low pressure. It can also be referred to as a low or a cyclone. The size of depressions can vary quite considerably, for example: Temperate low Tropical Revolving Storms Tornado

up to 1500 km in diameter approximately 300 km in diameter tens of metres in diameter

It appears on a synoptic chart as a series of concentric, roughly circular isobars with the lowest pressure in the centre. The low pressure in the centre causes air to flow into the low. This is called convergence. This then causes air in the centre to rise, producing a relatively high pressure at height. The result is a circulation of air as shown in the diagram below:

H

30 – 35 000 ft ASCENDING

L

The surface wind blows counter clockwise around a low in the Northern Hemisphere and clockwise in the Southern Hemisphere. In both cases, wind also blows in toward the centre. The mechanisms of this are discussed in a later chapter.

Meteorology

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Chapter 2

Pressure and Pressure Systems

The diagram below represents this: 1004 1002 1000 998 996

L

There are many different kinds of depressions. These will be described in later chapters.

DEPRESSION WEATHER Due to the lifting at the centre of the low, cloud will form and there will be associated precipitation. The mechanisms of this are described in later chapters. Typical weather is described in the table below: Cloud

Full cover from near the surface to the tropopause.

Precipitation

Generally continuous light or moderate. Heavy showers and thunderstorms possible because of the unstable nature of the air.

Visibility

Good out of precipitation but poor in precipitation.

Temperature

Mild.

Winds

Depends on the pressure gradient of the isobars but normally strong.

ANTICYCLONES This is a region of relatively high pressure, appearing as roughly circular, concentric isobars on the synoptic chart, with the highest pressure in the centre. It is also referred to as a high. Isobars are generally more widely spaced than in a depression. Air will flow out of the centre of the high pressure toward areas of lower pressure. This is called divergence. To replace the diverging air, air descends. This is called subsidence. This results in a relatively low pressure at height. Air circulates clockwise around a high in the Northern Hemisphere and counter clockwise around a high in the Southern Hemisphere, as well as flowing out of the high. 2-10

Meteorology

Pressure and Pressure Systems

1004

Chapter 2

L

1006 1008 1010 30 – 35 000 ft

H DESCENDING

H

There are two main types of anticyclone: the warm anticyclone and the cold anticyclone.

WARM ANTICYCLONES Warm anticyclones are a result of an excess of air at height. Air descends and is warmed. The main example is the sub-tropical highs caused by the circulation of air known as the Hadley cells.

COLD ANTICYCLONES Cold anticyclones are caused by low surface temperatures and are found in high latitudes. The low temperatures cause the density of the air to increase and air to subside.

ANTICYCLONIC WEATHER When anticyclonic weather is present, air is descending, which prevents cloud from forming and gives generally good weather. There may be some cloud and precipitation at the edge of the system. Temperature inversions are possible due to the subsidence.

Meteorology

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Chapter 2

Pressure and Pressure Systems

The table below shows typical weather associated with an anticyclone: Cloud

None because of the warming effect of subsidence.

Precipitation

None.

Visibility

In summer, hazy conditions can occur; in winter, foggy conditions.

Temperature

Depends on the type. Hot in summer, cold in winter.

Winds

Light.

TROUGHS A trough is the extension of isobars out from a depression in the shape of a V, with the pressure getting lower moving out from the centre. Troughs may be frontal or non-frontal. In frontal troughs, the front forms the centre line of the trough. The weather depends on the type of front. Frontal weather is discussed in a later chapter. In non-frontal troughs, the convergence of air at the centre line causes lifting and unstable weather.

996

1000

998

1002

1004

Centre line

TROUGH WEATHER

2-12

Cloud

For frontal troughs, the cloud types depend on the type of front. With cold fronts clouds with a large vertical development are expected. With warm fronts, layer clouds are more likely. For non-frontal troughs, CB and CU can be expected.

Precipitation

Showers, thunderstorms, hail with either cold frontal or non-frontal systems. Light to moderate rain and drizzle with warm fronts.

Visibility

Good except in precipitation.

Winds

Moderate with possibility of gusts and squalls.

Meteorology

Pressure and Pressure Systems

Chapter 2

RIDGES Ridges are an extension from a high pressure system. They are more rounded than troughs; more like a U shape.

1010

1008

1006

1004

Ridges are often found between two polar front depressions (see later chapters). They provide periods of good weather.

RIDGE WEATHER Ridge weather is very similar to anticyclone weather.

COLS A col is a region of very little pressure variation between two highs and two lows. Winds are therefore very light and the air remains mostly stationary, so it remains in contact with the ground for an extended period of time. 1004 1000 1008

996

H

L

COL

1004

1000 996

L

Meteorology

1008

H

2-13

Chapter 2

Pressure and Pressure Systems

COL WEATHER In summer, extended contact with the hot ground can lead to instability cloud and thunderstorms. In winter, extended contact with the cold ground can result in the formation of fog or low stratus.

MOVEMENT OF PRESSURE SYSTEMS Anticyclones tend to be long-lasting (up to 6 months) and move quite slowly. Depressions move more quickly and generally only last about 2 weeks. Cols generally get quickly absorbed into other systems, lasting only a few days.

2-14

Meteorology

INTRODUCTION Altimeters measure altitude, or height, by using the fact that pressure reduces with height. The altimeter measures the local pressure but presents this as an altitude in feet rather than as a pressure in hPa. The altimeter is an aneroid barometer that detects pressure by way of a capsule. Knowledge of the detailed workings of the altimeter are not required for Meteorology. The instrument is calibrated to ISA, so altimeters only read accurately in standard conditions. The altimeter has a baroscale, a knob that allows the pilot to set the particular reference to which he wishes to relate the aircraft’s height. This is usually one of the following:

QNH The altimeter reads height above mean sea level, which is generally referred to as altitude. In non-ISA conditions, mean sea level is not the same place as actual sea level. Hence the altimeter only reads height above actual sea level in ISA conditions.

QFE The altimeter reads height above aerodrome level in ISA conditions — generally just referred to as height.

THE STANDARD PRESSURE SETTING, 1013 hPA The resulting figure is usually divided by 100 and referred to as a Flight Level.

PRESSURE CALCULATIONS When making calculations in altimetry, you can assume that 1 hPa corresponds to 27 ft for the JAR exams, even though in the real atmosphere the pressure lapse rate decreases as altitude increases, as discussed in the previous chapter.

Meteorology

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Chapter 3

Altimetry

CONVERTING BETWEEN HEIGHT AND ALTITUDE As discussed above, the vertical distance above aerodrome level is known as height. The vertical distance above mean sea level is altitude. QFE is the pressure at aerodrome level. QNH is QFE reduced to sea level using ISA conditions. Therefore, if the airfield is above sea level, the QFE is of a lower pressure than the QNH, and the height is lower than the altitude. If the airfield is below sea level (a rare occurrence, but not impossible) the QFE is higher than the QNH and the altitude is lower than the height. Example 1:

An aircraft is flying at an altitude of 3500 ft. The QNH is 1010 hPa. The QFE is 988 hPa. What is the aircraft’s height? 1010 – 988 = 22 hPa Using 27 ft per hPa, the elevation of the airfield must be 22 × 27 = 594 ft. Hence the aircraft height must be 3500 – 594 = 2906 ft.

This situation is depicted graphically below:

3-2

Meteorology

Altimetry

Example 2:

Chapter 3

An aerodrome has an elevation of 1500 ft. The QFE is 965 hPa. Calculate an approximate QNH. 1500 / 27 = 56 hPa The airfield is above sea level so the QNH will be higher, hence: QNH = 965 + 56 = 1021 hPa.

This situation is depicted graphically below:

Meteorology

3-3

Chapter 3

Altimetry

CONVERTING BETWEEN ALTITUDE AND PRESSURE ALTITUDE/FLIGHT LEVEL Flight level and pressure altitude is based on a pressure setting of 1013 hPa. For example, if you have 1013 set on your altimeter and your reading is 35 000 ft, you are at a pressure altitude of 35 000 ft. Flight level is simply pressure altitude divided by 100. In this example, you are at Flight Level 350. Altitude is based on the QNH at any particular time. This varies from place to place and with time. Look at the following example. As you can see, if QNH is lower than 1013 hPa, the altitude is lower than the pressure altitude. If QNH is higher than 1013 hPa, the altitude is higher than the pressure altitude. As before, use 27 ft per hPa. The diagram below shows the corresponding altitudes for FL 350 with a low QNH and with a high QNH.

35 000 – (27 × 10) = 34 730 ft

35 000 + (27 × 10) = 35 270 ft

FL 350

FL 350

1003 hPa

1013 hPa

1023 hPa

3-4

Meteorology

Altimetry

Chapter 3

PRESSURE CHANGE If the pressure falls at a place and the altimeter is not reset, the value it shows will be the height in feet above an incorrect datum. If the pressure falls, for example, the pressure datum to which the altimeter was originally set will have lowered. The aircraft height in relation to it will have increased.

1800 hrs – 1020

0600 hrs – 1010 10 hPa = 270 ft

1020 hPa If an aeroplane flies from one location to another one with a lower pressure, it will be flying with reference to a particular datum. If the datum lowers, the aeroplane descends. In the example below, the aircraft is flying at 500 ft on QNH 1020. It flies towards an area with a QNH of 1010.

500

500 – 270 = 230 ft 1020 hPa

1010 hPa 500 10 × 27 = 270 ft

1020 hPa When flying to an area of lower pressure the altimeter over-reads. Conversely, when flying to an area of higher pressure, the altimeter under-reads. This gives rise to the memory aid: ‘High to low – beware below!’

Meteorology

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Chapter 3

Altimetry

CORRECTING FOR TEMPERATURE The altimeter is calibrated so it reads correctly in ISA conditions. However, in the real world it is rarely ISA conditions. If it is warmer or colder than ISA, the altimeter reads incorrectly. Look at the following diagram showing three columns of air: one is at ISA, one is ISA - 10°C, and one is ISA + 10°C.

ISA +10 ISA ISA -10

If the air is warmer than ISA, it expands upward; the pressure at the top remains the same so the aeroplane actually is higher than indicated. If the air is colder than ISA, the air sinks. The pressure at the top remains the same so the aeroplane is lower than indicated. The altitude at which the aeroplane is actually flying is called the true altitude. In order to convert between indicated altitude and true altitude, use one of the following formulas. Both give the same result so it is a matter of personal preference which one you use. 1% per 2.5°C deviation from ISA 4 feet per 1000 feet per 1°C deviation from ISA Add if warmer than ISA; subtract if colder than ISA. To summarise, if flying into warmer air, you climb if maintaining the same reading on your altimeter. If flying into colder air, you descend if maintaining the same reading on your altimeter. This gives rise to the memory aid: ‘Warm to cold – don’t be bold!’

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Meteorology

Altimetry

Chapter 3

EXAMPLE 1 You are flying at 6000 feet on a QNH of 1008 hPa. The temperature is 8°C. What is your true altitude? At 6000 ft the ISA temperature is 15 – (2 × 6) = 3°C. Hence the temperature is ISA + 5. Using formula 2 you get: 4 × 6 × 5 = 120 ft It is warmer than ISA so the true altitude is 6000 + 120 = 6120 ft (using formula 1: 1% of 6000 ft × (5 / 2.5) = 120 ft)

EXAMPLE 2 You are flying at FL 300. The QNH is 976 hPa. The temperature is -58°C. What is your true altitude? First you have to convert from FL to altitude. The QNH is lower, therefore altitude will be lower than FL. (1013 – 976) × 27 = 999 ft Altitude is 30 000 – 999 = 29 001 ft ISA temperature is 15 – (29 × 2) = -43°C. So it is ISA – 15. Temperature correction is 4 × 29 × 15 = 1740 ft. Colder than ISA so true altitude is 29 001 – 1740 = 27 261 ft.

Meteorology

3-7

Chapter 3

Altimetry

CONVERTING BETWEEN QNH AND QFF You are not expected to calculate precise values of QFF from QNH and vice-versa in the JAR exams. However, you are required to say whether QFF is higher or lower than QNH in given conditions.

QFE 998

540 ft

>ISA

QFF QNH

QFF > QNH

QFF < QNH

3-9

Chapter 3

Altimetry

MOUNTAIN FLYING There is a tendency for air to collect on the windward side of a mountain range. This leads to an increased pressure on the windward side. Conversely, a lower pressure is experienced on the leeward side. This means that if you use the QNH from the windward side you may be exposing yourself to danger as you will be flying into a region of lower pressure. To ensure adequate terrain clearance you should always use the lowest QNH for the area. As air tries to flow through a mountain range, the air is obstructed by the range. In order to pass over the mountain the air must speed up. Bernoulli’s theorem states that if the velocity goes up, the pressure goes down. This leads to a lower pressure over the top of the mountain — a potentially dangerous situation. The greater the wind speed over the mountain, the greater the height loss. In order to ensure safe terrain clearance you should add an extra margin to your minimum safe altitude, depending on the wind speed, as shown below: < 30 kt 31 – 40 kt 41 – 50 kt 51 – 60 kt > 60 kt

no addition necessary add 500 ft add 1000 ft add 1500 ft add 2000 ft

In summary, when calculating the actual altitude above high terrain: Use the correct reference pressure (the lowest QNH). Correct the reading for temperature deviation as described above. Allow an extra margin of safety for strong winds.

ALTIMETER SETTINGS For take-off and landing, the QNH is normally used — never the standard pressure setting. When climbing, the standard pressure setting is set when passing the Transition Altitude. When descending, the QNH is set on passing the transition level. Transition Altitude

The altitude at or below which the vertical position of an aircraft is referenced to altitude

Transition Layer

The airspace between the transition altitude and the transition level

Transition Level

The lowest flight level available for use above the transition altitude

Note: Pilots may, at their discretion, use QFE for take-off and landing in which case the aircraft should have two altimeters and the second one should be set to QNH. If the barometric pressure is very low (e.g. below the altimeter sub-scale minimum setting) then the standard pressure setting is used and the aircraft is landed with a false altitude indicated on the altimeter (the QNE). 3-10

Meteorology

Altimetry

Chapter 3

CALCULATION OF MINIMUM USABLE FLIGHT LEVEL Minimum usable flight level is important to know in the event of a decompression over high terrain. Sometimes it is defined by the authorities; if not, you can calculate it as follows: 1.

2. 3. 4. 5.

Note the highest elevation within 5 nm of track, then: a) In the case of the terrain being higher than 6000 ft (1800 m), add 2000 ft to this value. b) In the case of the terrain being lower than or equal to 6000 ft (1800 m), add 1000 ft to this value. Correct for temperature deviation as described earlier in the chapter. Correct for wind as described earlier. Convert from an altitude to a pressure altitude. Round up to the nearest higher flight level.

Meteorology

3-11

Chapter 3

3-12

Altimetry

Meteorology

INTRODUCTION Temperature is one of the most important variables that affect the atmosphere. The temperature changes that occur on the Earth’s surface initiate both vertical air movement (leading to cloud development) and horizontal air movement (wind). Temperature normally decreases with height. If there is an increase with height, this is called an inversion. If temperature stays the same with change in height, this is called an isothermal layer.

TEMPERATURE SCALES There are three scales of measurement for temperature. These are:

FAHRENHEIT In the Fahrenheit scale, the freezing point of water is 32°F and the boiling point of water is 212°F. This scale is not used in meteorology.

CELSIUS The Celsius scale is widely used. The freezing point of water is 0°C and the boiling point is 100°C.

KELVIN The Kelvin scale does not have units, but intervals of the scale are equal to 1°C. The scale relates to absolute zero (−273°C) which is defined as 0K. The freezing point of water is 273K and the boiling point is 373K. 0K is called absolute zero and is the temperature at which all molecules stop moving completely.

CONVERSION FACTORS To convert from Celsius to Fahrenheit: °F = (°C x

9

) + 32

5

Meteorology

4-1

Chapter 4

Temperature

To convert from Fahrenheit to Celsius: °C = (°F – 32) X

5 9

To convert from Celsius to Kelvin: K = °C + 273 To convert from Kelvin to Celsius: °C = K – 273

MEASUREMENT OF TEMPERATURE Surface temperatures are measured using mercury thermometers housed in a Stevenson screen. This is a louvred wooden box that allows air to circulate around the thermometers but protects them from draughts and direct sunlight. It is held 4 ft above the ground so the temperature won’t be adversely affected by the ground temperature.

High level temperatures are measured using a Radio Sonde, a radio transmitter that is carried high into the atmosphere (up to 150 000 ft) by a hydrogen balloon and sends back continuous readings of pressure, temperature, and humidity to stations on the ground. Temperature is measured to the nearest 0.1°C and reported to the nearest whole number. If the temperature ends in 0.5, it is rounded to the nearest odd whole number.

4-2

Meteorology

Temperature

Chapter 4

HEATING OF THE ATMOSPHERE The atmosphere is heated by five different processes: 1. Solar radiation 2. Terrestrial radiation 3. Conduction 4. Convection 5. Latent heat of condensation A sixth process, advection, is responsible for the horizontal transfer of heat. We will look at each of these processes in turn.

SOLAR RADIATION Radiation from the sun is of the short-wave type. Most of the radiation that reaches the Earth’s surface is of wavelengths less than 2 microns. Nearly all the radiation passes through the Earth’s atmosphere without heating it. Ultra-violet radiation is absorbed by ozone in the stratosphere. Still more is reflected by cloud cover. But on a clear day, about 85% of the sun’s radiation will reach the Earth’s surface. The radiation does not heat the atmosphere directly but does heat the surface of the Earth. This process is called insolation. The atmosphere then becomes heated by the other processes described below. The amount of insolation (heating of the surface) depends on the angular elevation of the sun. This in turn depends on latitude, season, and time of day. Latitude As can be seen from the diagram below, as you move further from the equator, the curvature of the Earth means that the same amount of solar radiation is spread over a larger area of the Earth’s surface. So insolation is less at higher latitudes

Meteorology

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Chapter 4

Temperature

Season For the same reasons mentioned above, the sun heats the Earth more efficiently if it is directly overhead. Where this occurs depends on the time of year. At the equinoxes, the sun is overhead the equator; at Summer Solstice (21st June) it is overhead the Tropic of Cancer (23.5°N); at Winter Solstice (21st December) it is overhead the Tropic of Capricorn (23.5°S). Time of day The amount of insolation is greatest at noon when the sun is highest in the sky.

TERRESTRIAL RADIATION The Earth’s surface absorbs large amounts of solar radiation at short wavelengths and retransmits it as smaller amounts of long-wave radiation, between 4 and 80 microns. This is the main method by which the atmosphere is heated. Since the atmosphere is heated from below, it gets colder as you move away from the surface of the Earth. This is the reason for the temperature lapse rate.

CONDUCTION Conduction occurs when two bodies are touching one another. Heat passes from the warmer body to the colder body. For example, heat passes from a warm ground surface to the air. At night, the ground cools quickly due to lack of insolation from the sun. The air in contact with the ground loses heat by conduction. As air is not a very good conductor, air at a higher level remains warm, which results in a temperature inversion.

4-4

Meteorology

Temperature

Chapter 4

CONVECTION As air is heated by conduction or radiation, it becomes less dense and tends to rise. Likewise, cold air is more dense and subsides. This vertical movement of air is called convection. This process helps heat the upper levels of the atmosphere.

LATENT HEAT OF CONDENSATION When heat is used to alter temperature it is called sensible heat. Heat used to alter the state of a substance is referred to as latent heat (latent meaning hidden), as no temperature change occurs. For example, when water turns from vapour to droplets in the atmosphere, it is turning from the gaseous state to the liquid state. Heat is released when this occurs. Likewise, when it turns from liquid to gas, it absorbs heat to effect the change, but the actual temperature remains constant within the substance. As air is lifted it cools and is no longer able to hold as much water vapour. This condenses out as water droplets and latent heat is released, warming the atmosphere.

ADVECTION Advection is the process by which air moves horizontally. The movement is caused by variations in pressure, but the air takes with it its characteristics, including its temperature.

DIURNAL VARIATION OF TEMPERATURE The maximum amount of insolation occurs at noon when the sun is high in the sky. As the earth takes time to heat up, it does not immediately transfer the heat out to the atmosphere — there is a slight lag. This means that the highest air temperature occurs at about 1500 local time. The lowest temperature occurs about a half an hour after sunrise, again due to lag.

Meteorology

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Chapter 4

Temperature

THE EFFECT OF CLOUD COVER ON DIURNAL VARIATION During the day, clouds prevent some solar radiation from reaching the Earth, hence reducing the maximum temperature that the air near the surface reaches during the day. At night, clouds trap some of the heat between them and the ground, hence raising the minimum temperature that the air drops to at night. The overall effect is to reduce the diurnal variation.

THE EFFECT OF WIND ON DIURNAL VARIATION During the day, wind causes surface air to be mixed with cooler air above. The amount of time that any air is in contact with the warm ground is short, so the maximum temperature the air near the surface reaches is lower compared to calm conditions. During the night, terrestrial radiation leads to a reduction in air temperature close to the ground. Any wind causes mixing of the cold surface air with warmer air above. Therefore, the minimum temperature of the air above the surface at night is not as low as it would be in calm conditions. The overall effect is to reduce diurnal variation.

THE EFFECT OF SURFACE ON DIURNAL VARIATION How much a surface heats up when exposed to insolation depends on its specific heat. The specific heat is the amount of heat required to raise the temperature of the surface by 1°C. Some examples of surfaces listed in the order of increasing specific heat follows: 1. Bare rock/stone 2. Concrete 3. Dry soil 4. Wet soil 5. Oceans 6. Snow surfaces Those surfaces that take a long time to heat up also lose their heat very slowly, so the diurnal variation over the sea is minimal but is much greater over the land. Not only does water have a much higher specific heat than land, but due to the movement of the sea surface, the energy is spread to a depth of several metres, whereas solar radiation only heats the top few inches of the land surface. Topics found later in the course detail why the different properties of land and sea are important.

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Chapter 4

SUMMARY In summary, greatest diurnal variation can be found over the land, with clear skies and no wind. Least diurnal variation can be found over the sea and over the ice caps, when skies are cloudy and it is windy.

THE GREENHOUSE EFFECT Water vapour and carbon dioxide are transparent to short wavelength radiation, but they are less permeable to long wavelengths. This means they allow solar radiation to reach the surface, but do not allow all of the terrestrial radiation to leave the atmosphere and go back into space. This leads to an increase of temperature at ground level, a process called the greenhouse effect, since the glass in a greenhouse works in a similar way.

Meteorology

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Chapter 4

Temperature

4-8

Meteorology

INTRODUCTION Most water in the atmosphere is in the form of water vapour, which is water in its gaseous state. This water cannot be seen. In order for water to become visible in the form of clouds, mist, or fog it must turn into water droplets or ice crystals.

WATER STATES AND LATENT HEAT Water can exist in three basic states: solid (ice), liquid (water), and gas (water vapour). When changing from one state to another, latent heat is either released or absorbed.

EVAPORATION This is the change of state from a liquid to a gas. Gas is a higher energy state than liquid so latent heat is “absorbed” during this process. Evaporation can take place at any temperature above absolute zero, but the rate of evaporation is greater at higher temperatures.

MELTING This is the change of state from a solid to a liquid. Liquid is a higher energy state than solid so latent heat is “absorbed” during this process.

Meteorology

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Chapter 5

Water in the Atmosphere

SUBLIMATION Sometimes a substance can turn directly from a solid to a gas or from a gas to a solid without passing through the intermediate liquid state. The term sublimation can be used to describe this process in both directions. The change from gas to solid, however, can also be referred to as deposition. Latent heat is “absorbed” when a solid turns to a gas. Latent heat is “released” when a gas turns to a solid. This process is important in the formation of frost, hail, and some airframe icing.

CONDENSATION This is the change of state from a gas to a liquid. Liquid is a lower energy state so latent heat is “released”. Condensation nuclei must be present in order for condensation to occur in the atmosphere. Condensation nuclei are tiny particles of hygroscopic (water attracting) material, such as dust and pollution.

FREEZING This is the change of state from a liquid to a solid. Solid is a lower energy state so latent heat is “released”. For this to occur, freezing nuclei are required, similar to those for condensation. Without them, the water droplets in the atmosphere become supercooled, which means they remain as a liquid state despite being lower than freezing temperature. Supercooled droplets are a major cause of airframe icing. They are discussed again later in the course.

SATURATION As water evaporates into the air, there comes a point in which the air can no longer accept any more water vapour. The amount of vapour that air can hold is dependent on its temperature and pressure. The higher the temperature, the more water vapour the air can hold. When the air contains the maximum amount of water vapour it can hold, it is described as being saturated. The air can become saturated in two ways: extra water vapour can be added, or the air can be cooled, since cooler air holds less water vapour.

HUMIDITY Humidity refers to the amount of water vapour in the air. It is often expressed as a percentage and is known as relative humidity.

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Water in the Atmosphere

Chapter 5

ABSOLUTE HUMIDITY Absolute humidity is the actual mass of water in a given volume of air and is generally expressed in g/m3.

SATURATION CONTENT Saturation content is the mass of water a given volume of air can hold, not that which it is actually holding, again expressed as g/m3.

RELATIVE HUMIDITY Relative humidity is an expression of how much water vapour is in the air, expressed as a percentage of the maximum amount the air could hold at that temperature and pressure. Hence: RELATIVE HUMIDITY (RH)

=

AMOUNT OF WATER VAPOUR IN THE AIR % AMOUNT OF WATER VAPOUR THE AIR CAN HOLD

=

ABSOLUTE HUMIDITY % SATURATION CONTENT

Example:

If the absolute humidity is 12 g/m3 and the saturation content is 26 g/m3, what is the relative humidity? Relative Humidity = Absolute Humidity / Saturation Content = (12 ÷ 26) = 0.462 = 46.2%

Please attempt the following simple RH calculations. The answers can be found at the end of the chapter: Exercise 1: Absolute Humidity (g/m3)

Saturation Content (g/m3)

6

20

Relative Humidity (%)

34 14

45 30

HUMIDITY MIXING RATIO Humidity mixing ratio (HMR) is similar to absolute humidity but is the mass of water in a certain mass of air. The unit for this is therefore g/kg rather than g/m3. Typically, the HMR is between 5 and 50 g/kg in temperate latitudes.

Meteorology

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Chapter 5

Water in the Atmosphere

HMR FOR SATURATION CONTENT / SATURATION MIXING RATIO The saturation mixing ratio (SMR) is the HMR when the parcel of air is saturated. Hence relative humidity can also be expressed as: RELATIVE HUMIDITY (RH)

=

HMR % HMR FOR SATURATION CONTENT

SUPER-SATURATION As mentioned earlier, condensation only occurs if there are condensation nuclei present. If no nuclei are present, then the water remains as vapour and the air is described as super-saturated. This means there can conceivably be a relative humidity greater than 100%.

SATURATION AND DEWPOINT The graph below shows the HMR for saturation plotted against the temperature in °C. The higher the temperature, the larger the amount of water the air can hold. However, the relationship is not linear, it is logarithmic.

30

HMR for Saturation in g/kg

25

20

15

10

5

0 -30

-20

-10

0

10

20

30

Temperature in degrees C

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Water in the Atmosphere

Chapter 5

It follows that if a parcel of air contains a certain amount of water vapour and is cooled, it will be able to hold less water vapour. If it continues to cool, it eventually reaches a point where the amount of vapour it can hold is equal to the amount it is actually holding. The air is said to be saturated. The temperature at which this occurs is called the dewpoint. A parcel of air at 20°C with a HMR of 7 g/kg (as seen on the graph) is not saturated. Air at 20°C can hold up to 14 g/kg. What happens if air is cooled to 10°C? Based on the graph, the HMR for saturation is 7 g/kg. Therefore, the air is saturated — the relative humidity is 100%. So the dewpoint for air containing 7 g/kg is 10°C. Cooling the air beyond this point results in water vapour condensing to become droplets, which causes clouds, fog/mist, or dew. Relative humidity also has an effect on the rate of evaporation. Evaporation does not occur if the air is saturated. Warmer air can take more vapour so is less likely to be saturated. However, evaporation can still occur if the air above the liquid is cold, especially if there is a breeze to take away the saturated air and replace it with dry air. Note:

The term dry air is used to describe any air that is not saturated. So, even air with a RH of 99% is still dry. Completely dry air, that is air with an RH of 0%, does not occur in the atmosphere.

Using the graph above answer the following questions: Exercise 2: The HMR is 4 g/kg. The temperature is 20°C. What is the RH? Exercise 3: The HMR is 15 g/kg. What is the dewpoint? Exercise 4: The dewpoint is 18°C. The RH is 40%. What is the HMR?

CONDENSATION LEVEL When unsaturated air is cooled, it eventually reaches its dewpoint and water vapour condenses out as water droplets. One way in which a pocket of air may cool is if it is lifted. As the air rises it cools. Once it reaches a level where the RH becomes 100%, any further lifting leads to condensation. This level is referred to as the condensation level. As air rises it is said to cool adiabatically. Likewise, as air descends it is said to warm adiabatically. This process of adiabatics and how it relates to dewpoint and cloud formation is discussed more fully in the chapter on Stability. Meteorology

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Chapter 5

Water in the Atmosphere

DIURNAL VARIATION OF HUMIDITY Assuming the absolute humidity of the air remains constant, the relative humidity varies as the temperature varies. Cold air can hold less water, so just after dawn, when temperature is at its lowest, RH is at its highest. This is why mist and fog are most likely to form around dawn. Throughout the day as the temperature increases with increased insolation, the relative humidity decreases, dropping to its lowest value at about 1500 LMT when the air temperature is at its greatest. After this, the temperature starts to drop again, so the RH starts to rise.

WATER VAPOUR PRESSURE This is the part of the atmospheric pressure that is exerted by the water vapour present. When the air is saturated, the water vapour pressure is called Saturation Vapour Pressure. The dewpoint depends on the vapour pressure. The lower the vapour pressure, the lower the dewpoint. As air rises, it expands and cools. Its overall pressure goes down so the pressure exerted by the water vapour also goes down. This leads to the dewpoint decreasing as well. The dewpoint decreases by about .5°C per 1000 ft gain in height. Yet another formula for dewpoint arises from the relationship between water vapour pressure and saturation vapour pressure: RELATIVE HUMIDITY (RH)

=

VAPOUR PRESSURE (hPa)

%

CORRESPONDING VAPOUR PRESSURE FOR SATURATION

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Chapter 5

SATURATION VAPOUR PRESSURE CURVE 12

Vapour Pressure in hPa

10

8

Ice

6

Water

4

2

0 -25

-20

-15

-10

-5

0

5

10

Temperature in degrees C

Saturation vapour pressure depends on a number of factors. The graph above shows that the saturation vapour pressure is higher over ice than over water. Other factors affecting saturation vapour pressure are: 1. Higher above a curved surface than a flat surface 2. Higher over clean water than a salt solution 3. Higher around a supercooled droplet than an ice crystal

Meteorology

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Chapter 5

Water in the Atmosphere

MEASUREMENT OF HUMIDITY PSYCHROMETER DRY BULB

WET BULB

MUSLIN CLOTH

DISTILLED WATER

To calculate humidity and dewpoint, a Psychrometer or Wet and Dry Bulb Hygrometer is used. This apparatus consists of two mercury thermometers. One, the dry bulb thermometer, is an ordinary thermometer that measures the air temperature. The other, the wet bulb thermometer, has a piece of muslin cloth wrapped around the bulb. The other end of this cloth is dipped in a container of distilled water. As the water evaporates from the cloth, latent heat is drawn from the immediate surroundings. This causes the wet bulb temperature to be lower than the dry bulb temperature. The wet bulb temperature is the lowest temperature to which the air can cool by evaporation. Note that if the air is already saturated, no evaporation occurs and the two readings are the same. In this case the temperature displayed will also be the dewpoint. The two figures obtained can be used to look up the dewpoint, RH, and HMR from tables. An approximation of the dewpoint can be made using the following method: 1. Subtract the wet bulb temperature from the dry bulb temperature. 2. Subtract this figure (the wet bulb depression) from the wet bulb temperature.

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Meteorology

Water in the Atmosphere

Example:

Chapter 5

Dry Bulb Temperature Wet Bulb Temperature

20°C 15°C

Wet Bulb Depression = Dry Bulb Temperature – Wet Bulb Temperature = 5°C Dewpoint Temperature = Wet Bulb Temperature – Depression = 15° - 5° = 10°C Please complete the following dewpoint calculations: Exercise 5: Dry Bulb Temperature (°C)

Wet Bulb Temperature (°C)

22

15

Dewpoint (°C)

18

10 12

3

HUMIDITY METHOD Another method of approximating the dewpoint is from the RH and air temperature. The formula is: DIFFERENCE BETWEEN TEMPERATURE AND DEWPOINT =

(100 – RH) 5

Example:

The temperature is 23°C and the relative humidity is 80%. What is the dewpoint? Difference = (100 – 80) ÷ 5 = 4°C Dewpoint = 23°C - 4°C = 19°C

Test your understanding of the formula by completing the following table: Exercise 6: Air Temperature (°C)

Relative Humidity (%)

18

70

12

4 85

Meteorology

Dewpoint (°C)

2

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Chapter 5

Water in the Atmosphere

ANSWERS TO EXERCISES Exercise 1: Absolute Humidity (g/m3)

Saturation Content (g/m3)

Relative Humidity (%)

6

20

30

15.3

34

45

14

46.7

30

Exercise 2: 28.6% Exercise 3: 21°C Exercise 4: 4.8 g/kg Exercise 5: Dry Bulb Temperature (°C)

Wet Bulb Temperature (°C)

Dewpoint (°C)

22

15

8

18

14

10

21

12

3

Exercise 6: Air Temperature (°C)

Relative Humidity (%)

Dewpoint (°C)

18

70

12

12

60

4

5

85

2

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Meteorology

INTRODUCTION The density of a substance is its mass per unit volume. Density in the atmosphere is usually expressed as grams per cubic metre (g/m3). It may also be expressed as a percentage of the standard surface density. This is called relative density. Example 1:

As chapter one detailed, the standard surface density is 1225 g/m3. Hence if the actual density is 900 g/m3, the relative density would be: 900 × 100 = 73.47% 1225

Example 2:

If the actual density is 1500 g/m3 what is the relative density? 1500 × 100 = 122.45% 1225

A third way in which density may be expressed is as density altitude. This is described later in the chapter.

THE IDEAL GAS LAWS An ideal gas is one that is incompressible and without viscosity. The atmosphere is assumed to be an ideal gas. There are several gas laws that apply. In the next few formulae, the following key applies: P = Pressure V = Volume T = Temperature ρ = Density

Meteorology

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Chapter 6

Density

BOYLE’S LAW At constant temperature, as the pressure of gas increases, its volume must decrease. Therefore the pressure is inversely proportional to volume:

Pα

1 V

To remove the proportional sign, use:

P=

Constant V

so: PV = Constant or: P1V1 = P2V2

CHARLES’S LAW At constant pressure, if the temperature of a gas increases, the gas expands. In other words, its volume increases. The temperature is proportional to volume: TαV To remove the proportional sign, use: V = Constant x T so: V T

= Constant

or: V1 T1

=

V2 T2

THE GAS EQUATION Combining Boyle’s and Charles’s laws, the gas equation becomes (where R is the gas constant): PV = RT

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Meteorology

Density

Chapter 6

Density can also be a part of the equation. In an ideal gas, as volume increases, density decreases. This is due to the same mass of air being contained in a larger volume. So:

ρα

1 V

Substituting this into the ideal gas equation: P ρ

= RT

Re-arranging to make density the subject of the equation:

ρ=

P RT

So, maintaining a constant temperature: if pressure goes up, density goes up. Maintaining a constant pressure: if temperature goes up, density goes down.

EFFECT OF WATER VAPOUR ON AIR DENSITY Water vapour is less dense than air: approximately 5/8 of the density of dry air. Therefore, all other things being equal, the density is lower in more humid atmospheres. This difference is usually insignificant and can be ignored for aviation purposes. In the tropics, however, where it can be very humid, it can make a large difference.

VARIATION OF SURFACE AIR DENSITY WITH LATITUDE Air density is lowest with low pressure and high temperature. So in the equatorial regions, density at the surface is low. High pressure and low temperature equates to high density. Examples of this can be found at the poles or at the centre of a large land mass in winter, (e.g. Siberia). So, in general, density increases with increasing latitude. The lowest density can be found at an aerodrome that is not only hot and high, but humid. An example is Nairobi, which is very close to the equator, so experiences high temperatures and humid conditions. It is also at an elevation of about 5500 ft, so has all the attributes that contribute to low density.

VARIATION OF AIR DENSITY WITH HEIGHT As height increases, both the temperature and pressure decrease. Based on the gas laws, a decrease in temperature leads to an increase in density and a decrease in pressure leads to a decrease in density.

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Chapter 6

Density

So, with one law trying to increase the density and one trying to decrease it, will it therefore stay constant? The answer is no. Since pressure near the surface decreases by about 10 hPa per 300 ft, this would produce a reduction in density of about 1%. A similar height increase would cause a drop in temperature of less than 1°C. This would lead to an increase in density of about 0.3%. The change in pressure has more of an effect, therefore, density decreases with height. This leads to the following observations: 20 000 ft

Density is 50% of the surface value.

40 000 ft

Density is 25% of the surface value.

60 000 ft

Density is 10% of the surface value.

VARIATION OF AIR DENSITY WITH LATITUDE AND HEIGHT As already mentioned, the air density at the surface tends to increase with increased latitude and density decreases with increased height. Now it’s time to bring those two factors together. Consider two columns of air of equal heights. Both columns have the same pressure at the base, but one column of air is cold and the other warm.

LOW PRESSURE

HIGH PRESSURE

1013 hPa

The cold air has a higher density, so as height increases there is a greater reduction in mass and the change in pressure is greater. Conversely, the warm air is less dense, so there is a small reduction of mass above as height increases. The change in pressure is less, so pressure at the top of the cold column is lower than at the top of the warm column. 6-4

Meteorology

Density

Chapter 6

This is important when considering global patterns in density. At the equator, the air temperature is high, so density at the surface is relatively low, as is pressure. At the Poles, the air temperature is low, so density at the surface is relatively high, as is pressure. However, as height increases over the equator, pressure, and therefore density, decreases relatively slowly, like in the warm column of air described above. As height increases over the Poles, pressure, and therefore density, decreases relatively quickly, like in the cold column of air described above. At the equator there is a relatively low density at the surface, compared to the Poles, but a relatively high density at height, as the density decreases only slowly. At the Poles there is a relatively high density at the surface, but a relatively low density at height, as the density decreases quickly. At approximately 26 000 ft the density is constant at all latitudes.

DIURNAL VARIATION OF DENSITY Density is highest when temperatures are lowest, that is, just after dawn. It is at its lowest at about 1500 LMT when temperatures are highest.

DENSITY ALTITUDE The density altitude at which you are flying is the pressure altitude in the International Standard Atmosphere at which that density would occur.

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Chapter 6

Density

Logically, if it is warmer than ISA, your density altitude is higher than your pressure altitude and vice versa for colder than ISA conditions. The diagram below shows two columns of air: one is at ISA and the other is warmer than ISA. ISA

WARMER THAN ISA

1000 g/m

1000 g/m

3

3

1225 g/m

10 000 ft

3

0 ft

At surface level in ISA the density is 1225 g/m3. At 10 000 ft, the density is 1000 g/m3. The warmer column of air has been heated such that the air density at the surface has decreased to 1000 g/m3, the same as that at 10 000 ft in ISA conditions. Hence the density altitude at the surface is 10 000 ft.

CALCULATING DENSITY ALTITUDE 120 ft

120 ft

ISA + 1°C

ISA

ISA 1°C

Density altitude differs from pressure altitude by 118.8 ft per 1°C deviation from ISA. In the JAR exams it is sufficient to use 120 ft per 1°C deviation from ISA. Add the difference to the pressure altitude if warmer than ISA, subtract if colder.

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Meteorology

Density

Chapter 6

Example:

The pressure altitude is 20 000 ft. The ISA deviation is 4°C. What is the density altitude? It is warmer than ISA so: Density altitude = Pressure altitude + (120 × ISA deviation) = 20 000 + 480 = 20 480 ft

Exercise 1: The pressure altitude is 15 000 ft. The ISA deviation is -5°C. What is the density altitude? Exercise 2: The pressure altitude is 8000 ft. The ambient temperature is 9°C. What is the density altitude (use a lapse rate of 2°C/1000 ft)? Exercise 3: The density altitude is 26 000 ft. The ISA deviation is +8°C. What is the pressure altitude?

EFFECT OF DENSITY ON AIRCRAFT PERFORMANCE Low density reduces the performance of engines and aerofoils. Engines work by accelerating air backward in order to produce thrust. Less dense air has lower mass. The lower the mass, the less thrust the engine produces. The production of lift by aerofoils such as the wings also depends on the density. The formula for lift is shown below: 2

LIFT = CL ½ ρV S

Where: CL = COEFFICIENT OF LIFT ρ = DENSITY V = TRUE AIRSPEED S = SURFACE AREA OF AEROFOIL The amount of lift produced is directly proportional to the density. So if density is low the aircraft will not produce as much lift, all other factors being equal. This is very important on take-off and landing. In order to generate enough lift, the aircraft either has to fly at a lower weight or a higher TAS. If a higher speed is chosen, then the aircraft requires a longer take-off and landing run. At an airport such as Nairobi, aircraft often have to operate with reduced weight at the hottest time of the day.

Meteorology

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Chapter 6

Density

ANSWERS TO EXERCISES Exercise 1: Density altitude = 15 000 - (120 × 5) = 14 400 ft Exercise 2: ISA temp for 8000 ft = 15 – (2 × 8) = -1°C. ISA deviation is therefore +10°C. Density altitude = 8000 + (120 × 10) = 9200 ft Exercise 3: Density altitude = Pressure altitude + (ISA deviation × 120) Hence: Pressure altitude = Density altitude – (ISA deviation × 120) = 26 000 – (8 × 120) = 25 040 ft

6-8

Meteorology

INTRODUCTION The processes leading to cloud formation and precipitation depend greatly on the stability of the atmosphere. In order to understand the concepts of stability and instability, you must understand the concept of adiabatics.

ADIABATIC PROCESSES As a bubble of air rises, the pressure in the surrounding atmosphere goes down and the bubble expands. This leads to the temperature within the bubble decreasing. This is called adiabatic cooling. Conversely, if a bubble of air descends, it compresses and the temperature increases. This is called adiabatic warming. Air is not a very good conductor, so there is very little exchange of heat with the surrounding environment. Hence, an adiabatic process is one in which the temperature changes within the system but there is no exchange of energy with the surroundings.

THE DRY ADIABATIC LAPSE RATE When dry air (unsaturated air) is forced to rise, it cools at what is called the Dry Adiabatic Lapse Rate (DALR). This has been found to be 3°C/1000 ft. This is the same regardless of how close to saturation the air is. It is also independent of pressure and temperature.

THE SATURATED ADIABATIC LAPSE RATE Once the air reaches saturation, water vapour starts to condense if the air is cooled any further. This process of condensation releases latent heat, as discussed in earlier chapters. This means that the temperature does not decrease as much as if it were dry, due to this extra heat being added into the system. The rate is referred to as the Saturated Adiabatic Lapse Rate (SALR). The actual amount of heat released as latent heat depends on the amount of condensation that occurs. In cold temperatures, even when the air is saturated, the actual amount of water vapour present is low, so very little latent heat is released. In this situation, the SALR is nearly as high as the DALR.

Meteorology

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Chapter 7

Stability

In hot temperatures, saturated air contains a large amount of water vapour and condensation releases large amounts of latent heat. The SALR, therefore, is considerably lower than the DALR. The average SALR is taken to be 1.5°C/1000 ft.

THE ENVIRONMENTAL LAPSE RATE This is the lapse rate of the air in the environment, that is, the air surrounding the adiabatic system, not within the system itself. This air is not in vertical motion. The ELR is variable. As discussed in Chapter 1, the average ELR is 1.98°C/1000 ft.

SUMMARY OF ADIABATICS The following diagram shows the DALR and the SALR. The DALR is constant at 3°C/1000 ft, but the SALR is not constant. As the height increases, the SALR approaches the DALR.

SALR

Height DALR

Temperature

Where the ELR falls in this picture is discussed later in this chapter.

STABILITY OF THE AIR Air that is warmer than its surrounding environment is less dense and rises. This is called instability. Air that is colder than its surrounding environment is more dense and sinks. This is called stability. Air that is the same temperature as its surrounding environment neither rises nor sinks. It is neutral. The stability of the atmosphere depends on the relationship between the ELR and the DALR and SALR.

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Meteorology

Stability

Chapter 7

ABSOLUTE STABILITY Consider the following example. The ELR is 1°C/1000 ft. The diagram demonstrates what happens when air is forced to rise. One bubble of air is dry, one is saturated. DALR

SALR

4000 ft

3°C

9°C

3000 ft

6°C

10.5°C

2000 ft

9°C

12°C

1000 ft

12°C

ELR 1°C/1000 ft 11°C

12°C

13°C

14°C

13.5°C

15°C

15°C Dry air

Saturated air

The surface temperature is 15°C. The dry air cools at 3°C, faster than the surrounding environment is lapsing. This means that at each level the dry bubble of air is colder than the surrounding environment, and therefore more dense, so it wants to sink. The saturated air cools at 1.5°C, again faster than the lapse rate of the environment. So at each level, the saturated bubble is colder and it too wants to sink. This situation is known as absolute stability since, regardless of whether the air is saturated or not, the air is stable.

ABSOLUTE INSTABILITY Now, consider the diagram below. The ELR is 5°C/1000 ft, greater than both the DALR and the ELR. DALR

SALR ELR 5°C/1000 ft

4000 ft

8°C

14°C

3000 ft

11°C

15.5°C

2000 ft

14°C

17°C

1000 ft

17°C

18.5°C

Meteorology

5°C

10°C

15°C

20°C

20°C Dry air

0°C

Saturated air

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Chapter 7

Stability

The unsaturated air cools at 3°C/1000 ft and at each level is warmer than the surrounding environment. Thus, it’s less dense and, therefore, tends to keep rising. The saturated air cools at 1.5°C/1000 ft. At each level it is warmer than the surrounding environment, hence less dense. It too tends to keep rising. We call this situation absolute instability.

CONDITIONAL INSTABILITY Now consider a situation in which the ELR is between the SALR and the DALR, as in the diagram below. DALR

SALR ELR 2°C/1000 ft

4000 ft

8°C

14°C

3000 ft

11°C

15.5°C

2000 ft

14°C

17°C

1000 ft

17°C

18.5°C 20°C

Dry air

12°C

14°C

16°C

18°C

20°C Saturated air

The environmental temperature is lapsing at 2°C/1000 ft. The unsaturated air is cooling at 3°C/1000 ft and at each level it is cooler than the surrounding environment, so it wants to sink. The saturated air, however, is cooling at 1.5°C/1000 ft, so at each level it is warmer than the surrounding environment and it tends to rise. This situation is called conditional instability. The air is stable when unsaturated, but unstable when saturated.

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Meteorology

Stability

Chapter 7

SUMMARY OF STABILITY SALR DALR

Conditional instability Height

Absolute stability Absolute instability

Temperature

The diagram shows the stability of the air when the ELR falls in different areas of the graph. Absolute Stability

ELR < SALR < DALR

Absolute Instability

ELR > DALR > SALR

Conditional Instability

DALR > ELR > SALR

Note that in all the above cases, an initial trigger action is required to start the air rising. There are several forms that this trigger can take, which are discussed thoroughly in the chapter on Cloud Formation.

NEUTRAL STABILITY There is one more type of stability not yet mentioned. If the air is unsaturated and the ELR is exactly 3°C/1000 ft, then the rising air is cooling at the same rate that the environment is lapsing. So the air is neutrally stable. If the air was saturated, the ELR would have to be identical to the SALR for the air to be neutral.

Meteorology

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Chapter 7

Stability

CONVECTIVE OR POTENTIAL INSTABILITY Potentially unstable air occurs when horizontal air motion is present at the same time air is being lifted, such as in a low pressure centre or along a frontal surface. The air in the lower layers must be saturated and the air in the upper layers must be dry, as demonstrated in the following diagram. 25 – 24 = 1°C ELR 3.4°C/1000 ft – unstable Cools at the DALR

5000 ft

30 – 12 =18°C

8000 ft

Unsaturated air

Cools at the SALR

25°C 5000 ft

ELR 1°C/1000 ft – initially stable 30°C Saturated air

The diagram above shows that before lifting occurs, the ELR is lower than the SALR, therefore, the layer is stable. The lower air cools at the SALR as it is lifted because it is saturated. Since the air above is dry, it cools at the DALR. When the air reaches the top of the obstruction, the temperature difference between the bottom of the 5000 ft layer and the top has increased, hence the ELR has increased. It is now greater than the DALR, so the layer is unstable. In the next diagram, the lower air is dry and the upper air is saturated, so the opposite occurs. Initially the ELR is high, but as the air cools, the temperature difference decreases, lowering the ELR to below the SALR and making the layer stable.

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Meteorology

Stability

Chapter 7

14 – 12= 2°C ELR 0.8°C/1000 ft – stable Cools at the SALR

5000 ft

30 – 24 =6°C

8000 ft

Saturated air

Cools at the DALR

14°C 5000 ft

ELR 3.2°C/1000 ft – initially unstable 30°C Unsaturated air

In summary, the following processes increase stability: 1. 2. 3. 4.

Advection of cold air or other cooling at low level Advection of warm air or other heating of upper air Decreased humidity at low levels or infusion of dry air at high levels Descending air motions such as subsidence created behind mountains in high pressure centres or through divergence at low level

Factors that lead to increased instability are 1. Advection of warm air or heating of the air at low level 2. Advection of cold air or other cooling of the upper air such as night time radiation from the top of clouds 3. Increased humidity at low level 4. Enforced lifting which may lead to conditional instability (over mountains, on shore winds at coasts etc) 5. General lifting, as in low pressure centres and in the case of convergence

INVERSIONS Inversions are extremely stable, as the ELR is in fact negative. The most common inversion forms at low level during clear nights, when radiation and cooling at ground level is at its maximum. This is known as a ground inversion. When the surface is snow covered, the cooling can be intense, and surface temperature is often 10°C lower than at the level of the Stevenson Screen (1 – 2 m). From the screen upward, the air temperature rises 10 – 20°C in extreme cases.

Meteorology

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Chapter 7

Stability

In broken terrain, the cooled air at the surface drains into the lowest area of the ground, creating what is called a katabatic wind. This can lead to fog formation. This is discussed in more detail in later chapters. When winds are light and the ground is covered with snow, the inversion may be at 4000 ft to 7000 ft and dominates the weather situation. Inversions can form in the troposphere, when warm air moves over a colder layer of air, for example, with a warm front. In many cases, clouds form in the inversion but these do not have strong vertical air currents.

CLOUD FORMATION As discussed at the beginning of the chapter, you must understand the concepts of stability and adiabatics in order to understand the processes of cloud formation.

THE DRY THERMAL Consider the following hypothetical situation. The surface temperature is 10°C and the environmental temperature lapses normally until height ‘X’, then there is an inversion. The surface is heated at a particular location, which causes the temperature to rise to 20°C. The air in this region becomes less dense and starts to rise. ELR

Height

X

DALR

0

5

10

15

20

Temperature in °C

This is what is known as a “thermal.” In this case, the air is unsaturated so it is called a dry thermal. Because the air is unsaturated, it cools at the DALR — faster than the environment and hence eventually the two lines will intersect. In the hypothetical example, the two lines intersect at X, the height at which the inversion starts. If the thermal were to continue to rise it would follow the dotted line, so it would be cooler than its environment. Therefore, it will be more dense and no longer has the tendency to rise. If you were to fly below height X, you would experience turbulence due to the updrafts in the thermal. Above height X, the conditions would be smooth. 7-8

Meteorology

Stability

Chapter 7

FORMATION OF A CLOUD Dewpoint was not taken into account in the previous example, which assumed the air never reaches saturation. What would happen if, at some point in the rise of the air, it became saturated? The following diagram represents this situation. ELR

Height

X

SALR LCL DALR

DP

0

5

10

15

20

Temperature in °C

As before, the trigger action is surface insolation, which leads to the formation of a thermal that starts to rise. However, now there is a line representing the dewpoint, which has a lapse rate of 0.5°C/1000 ft. In the diagram, the DALR line intersects the dewpoint line before it intersects the ELR line. Hence the thermal has reached saturation before it has stopped rising. At this point, water vapour starts to condense to form cloud. The thermal is still warmer than the environment so it continues to rise. However, its temperature now falls at the SALR. It eventually intersects the ELR and stops rising. So, the base of the cloud is the point in which the DALR intersects the dewpoint line, known as the lifting condensation level. The top of the cloud is where the SALR intersects the ELR. Once the thermal reached saturation, the lapse decreased to the SALR. The lower the SALR the longer it will take for this line to intersect the ELR. Warm air has a higher moisture content when saturated so it has a lower SALR due to the large amounts of latent heat released. If the air is cold, the SALR is close to the DALR and the line intersects the ELR quickly. Hence, warmer air leads to a thicker cloud forming than those formed in colder air. The diagram on the next page demonstrates this scenario.

Meteorology

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Chapter 7

Stability

ELR

Cloud top for warm air Height

X

Cloud top for cold air LCL DALR Warm air SALR Cold air SALR

DP

0

5

10

15

20

Temperature in °C

CALCULATING CLOUD BASE If the dewpoint was constant, we could quite easily calculate the height that the cloud base would form. It would simply be: (T – Td) ÷ 3 × 1000 Where: T = surface temperature Td = dewpoint This would give an answer in feet. However, since the dewpoint is also lapsing, it is not quite as simple as this. The temperature to which the thermal must fall must be the same as the temperature to which the dewpoint must fall. This is referred to ‘t’: t = T – (3H ÷ 1000) but also: t = Td – (0.5H ÷ 1000) Where ‘H’ is the height of the cloud base in feet. Hence: T – 3H ÷ 1000 = Td – 0.5H ÷ 1000

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Meteorology

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Chapter 7

Rearranging the formula to make H the subject: H = (T – Td)400 Using the same process, the following formula is derived: h = (T – Td)125 Where ‘h’ is the cloud base in metres. You must memorise both formulae. The derivation, however, is for your information only. Note that the above formulae are only valid for convective clouds, that is, those formed by thermals.

FORECASTING CLOUD FORMATION When forecasters determine whether or not convective clouds are likely to form, they must initially select a representative environmental lapse rate curve for the air mass in question. The dewpoint at the ground is checked and then assumptions are made of the development of the air temperature near the ground (amount of cloud, insolation, estimated maximum temperature, etc.). The condensation level can be calculated based on the forecast temperature and current dewpoint. To forecast convection a comparison is made between the lifting (path) curve with the actual lapse rate curve. When such a comparison is made, four main types can be distinguished. The following key applies: DALR SALR ELR Dewpoint

Meteorology

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Chapter 7

1.

Stability

The condensation level is on the cold side of the lapse rate. No clouds form, dry thermals only. Rate of ascent of the thermals 0.5 – 2 m/s. Over hot (dry) surfaces, the dry thermals may be much stronger.

Height

Temperature

2.

The condensation level is on the warm side of the lapse rate. The moist adiabatic lapse rate intersects the environmental lapse rate curve rather early. Small convective clouds form. Rate of ascent 1 – 4 m/s below clouds, 5 – 10 m/s inside the clouds.

Height

Temperature

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Meteorology

Stability

3.

Chapter 7

The condensation level is on the warm side of the lapse rate curve. The moist adiabatic air does not intersect the lapse rate curve until high level. Large convective clouds form. Hail and electrical discharges may occur. Rate of ascent at tens of metres/sec in the cloud subjects the aircraft to heavy turbulence.

Height

Temperature

4.

The condensation level is on the cold side of the lapse rate and no clouds form. If the air is forced to rise, e.g. over an obstruction, temperature is forced to cool to the condensation temperature and the thermals begin to rise by themselves. This condition is called Latent Instability.

Height

Temperature

Meteorology

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Chapter 7

7-14

Stability

Meteorology

ACKNOWLEDGEMENTS Thank you to Ashley Gibbs for the use of his photographs.

INTRODUCTION Clouds are collections of water droplets, ice crystals, or a mixture of both. They provide indications of: 1. possible turbulence 2. poor visibility 3. precipitation 4. icing The average lifetime of a cloud is 15 – 20 minutes, but cumulonimbus clouds can last 2 – 3 hours. There are several different types of cloud, all with different characteristics regarding the weather factors above. Cloud formation is discussed in detail in the next chapter. This chapter focuses on defining the different cloud types and their features, with a basic mention of formation processes.

CLOUD TERMS Cirrus Cumulus Stratus Alto Nimbus Lenticularis Castellanus Mamma Fractus

Meteorology

High clouds with a feathery appearance Clouds with a flat base and a top like a cauliflower Widespread clouds of great horizontal but little vertical extension Medium level clouds Clouds with moderate precipitation Clouds with a lens like appearance Clouds with a turret like appearance Clouds with a base that has a pendulous or pouch like appearance Clouds with a broken or ragged appearance

8-1

Chapter 8

Clouds

CLOUD CLASSIFICATION The initial subdivision of clouds is into two main types: layer clouds and clouds of great vertical extension (or heap clouds).

LAYER CLOUDS These form in stable air and can be further subdivided into categories according to the height bands in which they are found. Hence there are three further subcategories as follows: High level clouds (16 500 ft to 45 000 ft) Cirrus

CI

Cirrocumulus

CC

Cirrostratus

CS

Medium level clouds (6500 ft to 23 000 ft) Altostratus

AS

Altocumulus

AC

Low level clouds (Surface to 6500 ft) Nimbostratus

NS

Stratocumulus

SC

Stratus

ST

Each cloud type has a two letter abbreviation. Notice that the medium level and the high level bands overlap. This happens because in the summer the medium level clouds can extend up to 23 000 ft, and in winter the high level clouds can come as low as 16 500 ft.

CLOUDS OF GREAT VERTICAL EXTENSION These form in unstable air and air not restricted to a particular height band like the layer clouds. Cumulus

CU

Surface to 25 000 ft

Cumulonimbus

CB

Surface to tropopause

Nimbostratus

NS

Surface to 15 000 ft

A nimbostratus cloud can be a low cloud or a cloud with vertical extension because when there is strong lifting, nimbostratus can behave like a heap cloud and extend through several height bands. The next few sections look at each of the cloud types in turn and describe the characteristics of each.

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Meteorology

Clouds

Chapter 8

LOW CLOUDS STRATUS, ST

Stratus (ST) is a layer cloud with large horizontal extent but little vertical development. It generally has a very low cloud base (below 1000 ft) and covers the whole sky. The typical depth is 1000 - 1500 ft. The base can be quite diffuse with veils hanging down beneath the cloud. It is a turbulence cloud, often found in the warm sector of polar front depressions. It can also be formed when low fog lifts. ST consists of water droplets that are sub-zero in winter but are not very dense, so light to moderate icing can be expected. Precipitation may occur as drizzle, freezing drizzle, or snow grains.

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STRATOCUMULUS, SC

A stratocumulus (SC) cloud is a stratiform cloud caused by turbulence. It can be found between heights 1000 ft and 6500 ft. Because it is formed by turbulence, you might expect light to moderate turbulence when flying in or below the cloud. Conditions are calm above the cloud. Like stratus, this cloud consists of water droplets, so light to moderate icing, drizzle, freezing drizzle, or snow grains can be expected. In addition, you can expect ice pellets and, from the thicker stratocumulus, intermittent rain or snow. Heavy snowfall can be experienced in winter.

MEDIUM CLOUDS ALTOSTRATUS, AS

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Altostratus is similar to nimbostratus but is less deep and less dense. This type of cloud can cover the whole or a major part of the sky and is an indication of the approach of a warm front. Altostratus contains water droplets and ice crystals, therefore, it can cause light to moderate icing. Light to moderate turbulence can also be expected. Precipitation can take the form of continuous or intermittent rain or snow.

ALTOCUMULUS CASTELLANUS, ACC Altocumulus castellanus gets its name for the cloud’s appearance, which is similar to castle turrets extending from the top. It develops from altocumulus when there is mid-level instability. It can therefore indicate the possibility of CBs forming. It tends to be denser than altocumulus so icing and turbulence can be moderate to severe. ALTOCUMULUS LENTICULARIS, ACL Altocumulus lenticularis is a lenticular cloud, which means it is lens-like in appearance. It is formed orographically in association with mountain waves. Icing in this cloud can be severe due to the constant replenishment of moisture by updraughts in the wave.

HIGH CLOUDS All high clouds fall within the 16 500 − 23 000 ft band. They use the prefix ‘cirr(o)’.

CIRRUS, CI

Cirrus is a thin wispy cloud. It is associated with the approach of a warm front. It can also indicate the line of a jet stream. It consists of ice crystals and does not produce icing or precipitation. Likewise, there is no turbulence. Meteorology

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CIRRO-STRATUS, CS

Cirro-stratus is a sheet-like cloud, sometimes with a wispy veil underneath. It causes a bright ring around the sun and the moon, known as the halo phenomenon. It is associated with warm fronts. Like cirrus, it consists of ice crystals and does not produce icing, precipitation, or turbulence.

CIRRO-CUMULUS, CC

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Cirro-cumulus is divided into smaller cloud elements that look like the scales of a mackerel. It is formed when there is turbulence within cirrus or cirrostratus. Cirro-cumulus consists of ice crystals and occasionally freezing water droplets. There is no icing or precipitation. There may be light turbulence.

CLOUDS WITH GREAT VERTICAL DEVELOPMENT CUMULUS

This photo features heap clouds, which are clouds that generally have greater vertical than horizontal extent. They are formed convectively and the base can be found between 3000 and 7000 ft in the summer and 700 and 4000 ft in the winter. The tops can extend to 25 000 ft. Cumulus clouds consist of water droplets, which are supercooled above the freezing level. Precipitation can be present when the cloud has a vertical extent greater than 10 000 ft. It can take the form of rain or snow showers. When the cloud becomes towering without being ‘iced’ (cirrus forming) at the top, it is called towering cumulus, TCU. Strong vertical currents can be present and larger CU should be avoided. Moderate to severe icing conditions can be encountered, but because the time taken to traverse the cloud is usually short, any ice build up tends to be small.

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CUMULONIMBUS

Cumulonimbus is a towering cumulus cloud with a top that has turned into cirrus. This is called the anvil and extends in the direction of the wind. The anvil is fibrous and diffuse in appearance. This cloud is very hazardous to aircraft. It is very dense and consists of water droplets of varying sizes, so moderate to severe icing may be expected. Moderate to severe turbulence is also likely. CB can give precipitation in the form of rain or snow showers and hail. Due to the severe weather conditions associated with this cloud, it is discussed in detail in a separate chapter on thunderstorms.

CLOUD AMOUNTS Not only is the type of cloud important, but also the amount of cloud. If half or less than half of the sky is covered with clouds, there should be little if any problem in avoiding them. If more than half the sky is covered, avoidance becomes difficult. In aviation meteorology, the sky is divided into eight equal parts called oktas. You can describe the amount of cloud as a number of oktas, for example 4 oktas. This would mean that 4/8ths, one half, of the sky is covered.

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In meteorological messages, use three letter abbreviations. These correspond to a number of oktas as specified below: SKC

Sky clear

0 oktas

FEW

Few

1 – 2 oktas

SCT

Scattered

3 – 4 oktas

BKN

Broken

5 – 7 oktas

OVC

Overcast

8 oktas

Aerodrome reports use an observation area with a radius of 5 km around the airport plus the area in the direction of approach. The exception is CBs, which are reported if they are within a 15 km radius of the airport. For airfields equipped with instrument landing systems, cloud base reports are referenced to the site of the middle marker beacon. You may also see or hear the term CAVOK in meteorological messages. This means ceiling and visibility OK. In the following conditions you can replace the visibility, weather, and cloud group in a meteorological report with the word CAVOK. 1. Visibility > 10 km. 2. No clouds occur below 5000 ft or the highest Minimum Sector Altitude, whichever is the greater. 3. No CB in the vicinity (> 15 km). 4. No precipitation (except ice crystals), thunderstorms, low snowdrift, shallow fog, low drifting dust or sand, or sand or dust storms.

CLOUD BASE In addition to the amount and type of cloud, the cloud base is also reported based on the distance from the ground to the cloud. The cloud base is the lowest zone in which the type of obscuration perceptibly changes from that corresponding to clear haze to that corresponding to water droplets or ice crystals. A METAR or MET REPORT uses 100 ft intervals for clouds up to 10 000 ft, and 1000 ft intervals for those above 10 000 ft. For example, you may receive the following in a report: FEW003 SCT010 BKN040 The numbers after the descriptive abbreviations give the cloud base in hundreds of feet, so FEW003 means 1 – 2 oktas with a cloud base of 300 ft.

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CLOUD CEILING The cloud ceiling is the height above aerodrome level of the lowest layer of cloud of more than 4 oktas.

MEASURING CLOUD BASE AIREPS There are several ways to measure the cloud base. The cheapest and easiest way is to use AIREPs (reports from the pilots of aircraft). This may not always be possible on approach and departure routes that aren’t used frequently.

HUMAN OBSERVATION The general weather service uses an imperfect method in which the observer estimates the cloud base. Estimated cloud bases can have large errors and have to be supplemented by the direct measurements to be used in aviation meteorology.

BALLOONS If the cloud base is low, as is the case of ST/SC clouds, balloons that rise at a known rate can be used to determine the cloud base. The time taken for the balloon to disappear into the cloud is measured, and the measurement is converted into a distance.

CEILOMETER Most ceilometers use a light-pulse that is reflected by the cloud. The laser reaches the higher levels without any significant scattering. The reflected light-pulse is received by a light-sensitive cell and half the time of transport gives the measurement of the cloud base. One problem of this type of ceilometer is that precipitation can also give reflecting light-pulses, which leads to the cloud base measurement being too low.

ALIDADE The Alidade is used at night. The alidade is positioned a known distance from a searchlight. The searchlight is shone on the cloud and the alidade measures the angle above the horizontal of the searchlight glow on the base of the cloud. The cloud base is calculated by trigonometry.

VERTICAL VISIBILITY If fog is thick or snowfall is heavy, the cloud base loses its importance and vertical visibility is reported. Vertical visibility indicates at what height above the ground the pilot of an aircraft should have visual contact with the ground vertically down below. An important difference between cloud base and vertical visibility is that the cloud base mostly indicates a height in which the pilot can see forward, while vertical visibility only indicates at what height the pilot can see vertically down.

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SUMMARY OF CLOUD TYPE AND CHARACTERISTICS Cloud Type

Height

Composition Turbulence

Icing

Visibility

Significance

Cirrus

CI

16 500 ft to 45 000 ft

Ice crystals

Nil

Nil

1000 m +

Found 400 to 600 nm ahead of a warm front

Cirrostratus

CS

16 500 ft to 45 000 ft

Ice crystals

Nil

Nil

1000 m +

Found 400 to 600 nm ahead of a warm front

Cirrocumulus

CC

16 500 ft to 45 000 ft

Ice crystals

Light

Nil

1000 m +

Found 400 to 600 nm ahead of a warm front when turbulence exists

Altocumulus

AC

6500 ft to 23 000 ft

Water droplets and ice crystals

Light to moderate

Light to moderate

20 to 1000 m

Turbulence cloud

Altostratus

AS

6500 ft to 23 000 ft

Water droplets and ice crystals

Light to moderate

Light to moderate

20 to 1000 m

Warm front 200 nm ahead. Merges with NS as the front is approached

Nimbostratus

NS

Ground level to 6500 ft. Can be 10 000 ft to 15 000 ft merging into AS at higher levels

Water droplets but can be ice crystals at medium levels

Moderate to severe

Moderate to severe

10 to 20 m Warm front very close

Stratocumulus

SC

1000 ft to 6500 ft

Water droplets

Light to moderate

Light to moderate

10 to 30 m Turbulence cloud

Stratus

ST

Ground level to 6500 ft

Water droplets

Nil to light

Occasion ally light to moderate

10 to 30 m Turbulence cloud

Cumulus

CU

1000 ft to 25 000 ft

Water droplets and ice crystals

Moderate to severe

Moderate to severe

Less than 20 m

Cumulonimbus

CB

1000 ft to 45 000 ft

Water droplets and ice crystals

Moderate to severe

Moderate to severe

10 to 20 m Instability cloud

Altocumulus castellanus

AC C

6500 ft to 23 000 ft

Water droplets and ice crystals

Moderate to severe

Moderate to severe

-

An indication of unstable air at mid levels; can indicate approaching CB

Altocumulus Lenticularis

AC L

6500 ft to 23 000 ft

Water droplets and ice crystals

Moderate to severe

Moderate to severe

-

Associated with mountain waves

Meteorology

Instability cloud. Large CU may develop into CB

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Meteorology

INTRODUCTION This chapter covers the formation of clouds in more depth than previous chapters. Clouds form when air rises and cools adiabatically. If rising air cools to its dewpoint, the water vapour will condense out as water droplets. The height at which this occurs is the condensation level. This is also the level the cloud base occurs. There are several different lifting processes that can lead to cloud formation. They are as follows: 1. Turbulence 2. Convection 3. Orographic uplift 4. Frontal uplift 5. Convergence

TURBULENCE CONDITIONS Turbulence clouds can form whenever there is a stable layer. Such a stable layer may occur if there is an inversion or isothermal layer above it, preventing lifting. If the wind speed is greater than about 10 kt, turbulence within the layer can lead to a steepening of the lapse rate. Note:

Although a wind speed of greater than 10 kt is necessary for turbulence clouds to form, once formed it can persist at lower speeds.

If this steepening is such that the saturation layer occurs within the turbulent layer, then turbulence clouds form.

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MECHANISM The diagrams below show what happens when there is a stable layer of 3000 ft thickness and turbulent mixing occurs within the layer.

Isothermal layer

3000 ft - 12°C

2000 ft - 13°C Stable layer – ELR of 1°C/1000 ft

1000 ft - 14°C

0 ft - 15°C

The above diagram shows the layer before turbulence commences. The layer is stable, the ELR being only 1°C/1000 ft. Surface temperature is 15°C, making the top of the layer 12°C. Above 3000 ft is an isothermal layer, where the temperature remains 12°C (although this could equally be an inversion layer).

Isothermal layer

3000 ft

6°C

12°C

2000 ft

9°C

15°C

1000 ft

12°C

18°C

0 ft

15°C

21°C

The above diagram shows the situation during turbulence. Pockets of air are circulated within the layer. Due to the nature of air as a bad conductor, the pockets cool or warm adiabatically.

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As you can see from the diagram, this means bubbles of air ascending to the top of the layer is 6°C, colder than the environmental temperature. Descending bubbles of air are 21°C when they reach the bottom of the layer, warmer than the environment.

Isothermal layer

3000 ft

2000 ft

1000 ft

0 ft

9°C

12°C

15°C

18°C

The final diagram shows the situation after turbulence. The temperature at any one level becomes the average of the temperatures of the bubbles that have ascended and those that have descended. The surface temperature has increased and the temperature at the top of the layer has decreased. Overall the ELR has increased. It is now 3°C/1000 ft. This may result in the dewpoint being reached below the top of the layer. For example, assume that the surface dewpoint is 12°C. The dewpoint lapses at 0.5°C so it would not be reached before the top of the layer in the pre-turbulence case. However, after turbulence the dewpoint would fall within the layer, hence saturation would occur and clouds would form, as shown in the next diagram.

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Isothermal layer

3000 ft

2000 ft

9°C

12°C

1000 ft

DEWPOINT 10.5°C

11°C

11.5°C 15°C

0 ft

18°C

12°C

By comparing the new environmental temperature with the dewpoint at various levels, you find that the cloud base is at 2400 ft.

CLOUD TYPES The following cloud types are formed by turbulence: 1. Stratus 2. Stratocumulus 3. Altocumulus 4. Cirrocumulus

CONVECTION Convective processes were introduced in the chapter on Stability, but the processes are recapped below.

CONDITIONS Convective clouds form when the surface is heated. This heat energy passes to the air above the surface by conduction. This air is now warmer than the surrounding environment so it starts to rise, that is, convection occurs. If the rising air reaches its dewpoint before it reaches the same temperature as the environment, condensation occurs.

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MECHANISM The following key applies to the next few diagrams: DALR SALR ELR Dewpoint

Height

Temperature

In the diagram above, the surface is heated, which starts a vertical motion of air. Initially, the air cools at the DALR until it reaches the dewpoint. Water vapour then starts to condense out as droplets and a cloud starts to form. The level at which this occurs is the condensation level and is coincident with the cloud base. The air now cools at the SALR. Lifting, and hence cloud formation, ceases when the rising air reaches the same temperature as the surrounding environment. The temperature to which the surface must be heated in order for air to be lifted to its condensation level is the critical temperature. In the diagram, the DALR intersects the dewpoint curve when the dewpoint temperature is quite close to the environmental temperature at a low height. Only a small amount of lifting occurs after this point, so the cloud form is quite small. Such small clouds are not large enough vertically to produce precipitation. They are usually isolated (forming over hot spots on the surface) and the sky is otherwise clear. They are, therefore, referred to as fair weather cumulus/cumuli. This is common on warm summer days. As temperatures fall in the evening, they tend to disappear. If fair weather cumulus form in the morning it may mean there will be large Cu or Cb later on in the day when insolation increases, for example in the next diagram.

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Height

Temperature

The relative positions of the ELR and the dewpoint curves are the same. The only difference is that the surface is heated to a much higher temperature. The DALR intersects the dewpoint curve at a greater height. After this point, there is much more lifting before the SALR intersects the ELR. So, with greater surface heating there is a much bigger cloud, but one with a higher cloud base. If this cloud exceeds 10 000 ft in height, it may produce precipitation. Another factor is the stability of the atmosphere. The steeper the environmental lapse rate, the longer it takes for the temperature of the rising air to reach the same temperature as the environment, so the larger the cloud that forms.

ADVECTION Another way for convective clouds to form is with advection. Advection is the horizontal movement of air. If cold air passes over a warm surface it becomes heated from below, starting the process of convection. Typical convective clouds such as cumulus and cumulonimbus can form. An example of this is cold air passing over a warmer sea surface such as polar air moving south over the North Atlantic.

CLOUD TYPES The following types of clouds are formed convectively: 1. Cumulus 2. Towering cumulus 3. Cumulonimbus

OROGRAPHIC UPLIFT CONDITIONS Orographic clouds form when air is forced to rise over an obstruction, such as high ground. This may occur in a stable or an unstable environment. The type of cloud that forms depends on the stability and moisture content of the atmosphere. 9-6

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MECHANISM In stable conditions, air is forced to rise over the obstruction. Initially, it cools at the DALR. Once it reaches its dewpoint, cloud starts to form. This formation is a stratiform cloud. The air cools at the SALR. As it passes over the crest of the ridge, the lifting force no longer is present so the air flows down the other side. It initially warms at the SALR. Since much of its moisture has condensed out as cloud, it becomes unsaturated again at a lower temperature than the original dewpoint. Hence the base of the cloud is higher on the leeward side than the windward side. The air then warms at the DALR. The diagram below shows the temperature at ground level on the lee side is higher than that on the windward side. This warming wind is known as the Foehn Wind.

6°C

4000 ft

6°C

7.5°C

3000 ft

7.5°C

9°C

2000 ft

10.5°C

1000 ft

13.5°C

0 ft

16.5°C

12°C

15°C

In drier conditions, the cloud base may be above the top of the ridge. If this happens, the clouds that form are altocumulus lenticularis.

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Altocumulus Lenticularis (Lenticular Cloud)

These clouds get their name from their lens shape and, generally, indicate the presence of mountain waves, which are discussed in detail in the chapter on Windshear and Turbulence. These types of clouds can cause severe turbulence. The cloud is being continuously replenished with moist air. It, therefore, contains a high concentration of supercooled droplets. Icing, therefore, can also be severe. If the conditions are unstable, the obstruction provides the initial lifting force. After the crest is reached, the air continues lifting due to the unstable nature of the air. The cloud that forms is a cumuliform rather than a stratiform. The bulk of the cloud forms on the windward side of the obstruction. Most of the precipitation falls here as well. The lee side is said to be in rain shadow.

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Cap Cloud

Another situation that causes orographic uplift is when the atmosphere is initially stable then becomes unstable. Initially, stratiform clouds form. If this is at a medium level, it becomes altocumulus. If the atmosphere then becomes unstable, this can develop into altocumulus castellanus. Stratocumulus can develop into stratocumulus castellanus but this is rare.

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CLOUD TYPES The following clouds can be formed orographically: IN UNSTABLE CONDITIONS Cumulus Cumulonimbus IN STABLE CONDITIONS Stratus Stratocumulus Altocumulus Altocumulus lenticularis WHEN ATMOSPHERE IS INITIALLY STABLE AND LATER BECOMES UNSTABLE Altocumulus castellanus

FRONTAL UPLIFT CONDITIONS A front is the boundary between two air masses, generally in motion, with different properties. Usually the comparison is made between the relative temperatures of the air masses. There are two main types of front: the warm front and the cold front. A warm front is found when warm air is replacing cold air. A cold front is found when cold air is replacing warm air. In both cases the warm air, being less dense, rises up over the cold air. Looking at it from the point of the warm front, the warm air slides up over the cold air it is replacing. From the point of view of the cold front, the cold air undercuts the warm air it is replacing. The fronts have different properties and hence the cloud types that form along them differ.

MECHANISM THE WARM FRONT The warm air rises over the cold air, forming a sloping front with a gradient of only about 1 to 150, so the lifting is very gentle and a stratiform cloud forms. From the ground up, the types of cloud that forms will be stratus, nimbostratus, altostratus, cirrostratus, and cirrus. Note that when flying toward a warm front from the cold side, you will encounter a progressively lowering cloudbase. The gradient is such that the first cloud, the high cloud cirrus, can be encountered up to 600 nm ahead of the surface position of the front.

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WARM FRONT

CI

WARM AIR CS

AS

NS

COLD AIR

ST

THE COLD FRONT Cold air pushes underneath the warm air it is replacing. The slope of the cold front is very different from that of the warm front. It averages a slope of 1 to 50, and close to the ground it can be almost vertical, sometimes forming a protruding area that looks like a nose, as shown in the next diagram. The air may be unstable, but if it is not, it can be made so by the large amount of enforced lifting. Hence the type of cloud which forms on this kind of front is generally cumuliform in type, although there can be shallow bands of stability where NS and CI can form. Since the front is steeper, the associated cloud ceases no more than about 200 nm after the passage of the surface front.

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COLD FRONT

CI

CU/CB WARM AIR COLD AIR

NS

CLOUD TYPES COLD FRONTS ONLY Cumulus Cumulonimbus WARM FRONTS ONLY Stratus Altostratus Cirrostratus MAINLY WARM FRONTS, OCCASIONALLY COLD FRONTS Nimbostratus Cirrus

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CONVERGENCE CONDITIONS Wherever there is convergence, air is forced to rise. Such convergence occurs in depressions and non-frontal troughs.

MECHANISM As air converges into the low pressure area, the air at the centre of the low, or the centre line of the trough, is forced to rise. This leads to instability and saturation, hence the formation of clouds.

CLOUD TYPES The cloud types that form are those that are associated with instability. These are cumulus, cumulonimbus, and towering cumulus.

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Meteorology

INTRODUCTION Clouds can consist of a combination of water droplets, supercooled water droplets, and ice crystals. Individual water droplets and ice crystals are very small and light, and due to upcurrents in the clouds, they do not fall as precipitation on their own. If they combine with other water droplets or ice crystals they become progressively heavier. If the upcurrents in the cloud are not strong enough to support their weight they fall as precipitation. It follows that the stronger the upcurrents are, the heavier the droplet or crystal has to be in order for precipitation to occur. So the largest droplets fall from convective clouds such as cumulus and cumulonimbus.

PRECIPITATION PROCESSES There are two theories concerning the formation of precipitation. These processes are not mutually exclusive and, given the right conditions, may both occur within the same cloud.

BERGERON THEORY (THE ICE CRYSTAL EFFECT) Where sub-zero conditions occur, both ice crystals and water droplets may be present. Water vapour may sublimate onto the ice crystals. Collision with supercooled droplets allows the crystal to grow in size. Once the crystal reaches a sufficient size, it falls as precipitation. The type of precipitation depends on the temperature of the air through which it falls. If sufficiently warm, the crystal melts and falls as a rain droplet. If not, it might fall as snow. The difference in saturation vapour pressure between ice and water is greatest at approximately -12°C, so clouds reaching this temperature produce precipitation. Snow has a relatively low rate of fall, so a cloud thickness of 1500 to 3000 ft is sufficient if the temperature at the cloud top is approximately -8°C to -12°C. If supercooled water droplets fall through colder air they might freeze and form freezing rain. This is common with nimbostratus clouds on a warm front. The droplets fall through the front into colder air. In dense clouds such as cumulonimbus, there may be a sufficient concentration of supercooled water droplets for them to freeze onto ice crystals to form a snow pellet.

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Precipitation

COALESCENCE THEORY (CAPTURE EFFECT) The Bergeron Theory requires part of the cloud to be below 0°C, so ice crystals are present. In many clouds in lower latitudes, no part of the cloud is below 0°C yet precipitation still falls. The Coalescence Theory covers this scenario. In the cloud there are water droplets of varying sizes. The larger, heavier droplets fall faster and collide with smaller droplets on their way down. When the droplets become sufficiently heavy, they fall as precipitation.

INTENSITY OF PRECIPITATION Precipitation is described by the following terms: Rainfall Rate (mm per hour) Rain

Rain/Hail Showers

Snow Accumulation (cm per hour)

Slight

< 0.5

4

10 to 50

>4

Violent

> 50

CONTINUITY OF PRECIPITATION Continuity of precipitation is described using the three terms described below. Showers Showers are of short duration and are associated only with convective clouds, that is, cumulus and cumulonimbus. Intermittent Intermittent is associated with layer clouds. Precipitation falls from time to time with short breaks. Continuous Continuous precipitation is that which falls for periods of an hour or longer without breaks. Continuous precipitation is also associated with layer clouds.

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PRECIPITATION TYPES The following table describes the different types of precipitation and the clouds they fall from. Precipitation Type Drizzle Freezing Drizzle Snow Grains

Cloud Type ST or SC

Comments Diameter: Visibility:

0.2 to 0.5 mm 500 to 3000 m

Imperceptible impact. Drizzle does not make a splash on the ground.

Rain (continuous)

Thick AS and NS

Diameter: Visibility:

0.5 to 5.5 mm 3000 to 5.5 km

1000 m in heavy rain Perceptible impact: Drops have to be large to overcome the upcurrents in the cloud in order to fall. Larger drops break up into smaller drops as the rain falls. Snow (continuous)

Thick AS and NS

Grains/Needles: Pellets:

< 1 mm diameter 2 to 5 mm diameter

Flakes: A collection of crystals greater than 4 mm in diameter. The lower the temperature the smaller the flake size. Surface temperature must be < 4°C for snow to reach the ground before melting. Hail

CB

Diameter: Weight: Height:

Rain (intermittent) Snow (intermittent)

Thick AS and SC

Rain Showers Snow Showers

Heavy CB

CU

5 to 50 mm up to 1 kg up to 48 000 ft

and A mixture of rain and snow or snow that has partially melted in the descent.

Sleet

Sleet falls when the temperature is between + 5°C to + 6°C Soft Hail, or Graupel

CB

Ice Pellets

SC

Small rounded pellets of less than 5 mm diameter Can be the early stage of hail growth Diameter:

< 5 mm

Transparent pellets either spherical or rounded.

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Precipitation

HAIL Hail forms by the ice crystal effect when there are updraughts stronger than 10 m/s. Hail can cause serious damage to an airframe, especially with larger hailstones. The table below summarises the strength of updraughts required to produce stones of various sizes and masses. Vertical Speed

Type of Hail

Diameter

Weight

10 m/s

Small Hail (Graupel)

< 5 mm

1g

20 m/s

Hail (Grêle)

2 cm

9g

30 m/s

6 cm

80 g

40 m/s

10 cm

370 g

70 m/s

14 cm

1 kg

In the UK and Northern Europe, the updraughts in thunderstorms are rarely strong enough to allow the hailstones to grow to any appreciable size. Large hailstones are more likely to be encountered in heat air mass thunderstorms in tropical locations.

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INTRODUCTION It is estimated that every day there are about 44 000 thunderstorms across the planet. Thunderstorms develop from well-developed cumulonimbus clouds. Not all cumulonimbus clouds develop into thunderstorms, however. The features described in this chapter apply to very active CBs as well as actual thunderstorms.

CONDITIONS Thunderstorms are most likely to occur with the following combination of conditions: 1. An environmental lapse rate greater than the SALR through a depth of at least 10 000 ft and extending to above the freezing level. 2. Sufficient water vapour to provide early saturation and to form and maintain the cloud. 3. A trigger action to start the lifting process. This can take several forms.

TRIGGER ACTIONS There are five different possible trigger actions: 1. Convection 2. Orographic uplift 3. Advection 4. Convergence 5. Frontal lifting (generally in association with cold fronts and occlusions)

THUNDERSTORM CLASSIFICATION Thunderstorms are generally classified as one of two types: 1. Heat or airmass — in this case the trigger action is one of the first four above. 2. Frontal — the trigger action is the fifth in the list.

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Chapter 11

Thunderstorms

HEAT/AIRMASS THUNDERSTORMS CONVECTION Although heat/airmass thunderstorms can form with one of four triggers, convection is the most likely one. Since surface heating is greater in the summer, statistically these thunderstorms are more likely in the summer. They are also more likely during the day and over land and tend to be isolated, especially if they have formed in a cold air mass. The cold air mass thunderstorms tend to dissipate in the evening. Thunderstorms that form in a warm air mass may form a multicell structure. A multicell thunderstorm is a cluster of CBs where various cells at differing stages interact. The downdraughts from dissipating and mature cells spread out as a pool of cold air along the ground surface. This forces the updraught in the front of the system to ascend providing the uplift for the formation of more CB clouds. These can persist until late into the evening.

OROGRAPHIC UPLIFT With orographic uplift, thunderstorms can occur at any time of the day or night, in summer and in winter. If the uplift is over a range of hills they may occur in a line formation. Thunderstorms are formed when the conditions are unstable or conditionally unstable. Orographic processes may enhance an existing thunderstorm that moves over the obstruction.

ADVECTION With advection, storms can occur in the day or at night, in summer or in winter. In summer, they can be caused by maritime air from a cold sea passing over the warm land and being heated from below. However, the more common case is in winter, when cold, moist air moves over a progressively warmer sea. A prime example of this would be polar maritime air moving south. The process then becomes similar to the convective case above. CONVERGENCE The fourth type of trigger is convergence. This can be in association with low pressures or nonfrontal troughs. Time of day and year depends on the type of low. The different types of lows are discussed in a later chapter. When associated with a trough, thunderstorms can form in a line along the centre line of the trough and can cause difficulties for a pilot trying to avoid them.

FRONTAL THUNDERSTORMS Frontal thunderstorms are more frequent in winter due to the increased frequency in the passage of fronts. They can form over land or sea, by day or night, and are associated with both cold fronts and occluded fronts. Because they are associated with a front, these thunderstorms tend not to be isolated but to form in a line. They can be embedded in other clouds and are difficult to identify, especially when formed on an occlusion in which there are significant layer clouds present. They are often accompanied by line squalls, which is a line of thunderstorms formed just ahead of the front. 11-2

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IDENTIFICATION OF THUNDERSTORMS A thunderstorm cloud, whether of the air mass or frontal type, usually consists of several selfcontained cells, each in a different state of development. New and growing cells can be recognised by their cumuliform shape with clear-cut outline and cauliflower top. The tops of more mature cells appear less clear-cut and are frequently surrounded by fibrous cloud. Development of cells is not always seen since other clouds may obscure the view. In frontal or orographic conditions, extensive layer cloud structures may obscure a view of the development of cumulonimbus thunderstorm cells, or ACC.

STAGES OF DEVELOPMENT There are three stages in the development of a thunderstorm, summarised in the diagram below. 40 000

Altitude (feet)

30 000

Updraught

20 000

10 000 Updraught 5000 Updraught 0

GROWTH STAGE

Downdraught

MATURE STAGE

Downdraught

DISSIPATING STAGE

GROWTH STAGE In this stage, several small cumulus clouds combine together to form a large cumulus of about 5 nm across. Strong updraughts are present, typically on the order of 1000 fpm, but can be as great as 4000 fpm. Air is drawn in from the sides and underneath the cloud, replacing the lifting air within the cloud. This stage lasts approximately 15 to 20 minutes.

MATURE STAGE The mature stage is characterised by the onset of precipitation. This precipitation is produced by the combination of ice crystals and water droplets. The precipitation causes downdraughts of approximately 2000 − 3000 fpm.

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The updraughts are still present, increasing to as much as 10 000 fpm, though 5000 fpm is a more typical figure. Tops can reach the tropopause, which can be in excess of 50 000 ft in low latitudes. Cloud tops can rise by as much as 5000 fpm. The tops of the clouds are affected by a stronger upper wind which causes it to tilt in the direction of the wind. This mixture of updraughts and downdraughts causes strong turbulence within and below the cloud. The downdraughts are colder than the surrounding air when they reach the base of the cloud, due to some water droplets evaporating and latent heat being absorbed. Once clear of the base of the cloud, they warm at the saturated adiabatic lapse rate and remain colder than the surrounding air. This combined with the absorption of latent heat intensifies the temperature difference between the downdraughts and the environment and causes the downdraught to descend even more rapidly. This strong downdraught of cold air reacts with the ground and causes a gust front extending up to 17 nm ahead of the storm. Also at this stage, there may be roll (rotor) clouds, which are stratocumulus caused by turbulence. Other hazards associated with this stage, such as microbursts and lightning, are discussed later in this chapter. The mature stage lasts approximately 20 – 30 minutes.

DISSIPATING STAGE This stage commences when the local supply of moisture is no longer sufficient to support the storm. The stage is characterised by the appearance of an anvil. This occurs when the cloud top reaches the tropopause and is spread out by the strong upper winds to form a flat-topped anvil shape. This anvil is part of a cirrus cloud. The cloud at this stage can be referred to as Cumulonimbus capillatus. Updraughts cease and the cloud starts to dissipate as the downdraughts remove the moisture from the cloud. The precipitation diminishes and the downdraughts are too strong to support roll clouds. Lightning might still occur. The dissipating stage lasts about 30 minutes but the cloud can persist for 2 to 3 hours.

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SUPERCELL THUNDERSTORMS Supercell thunderstorms are severe local storms that form when there is: ¾

Great depth of instability

¾

Strong vertical windshear

¾

A stable layer between the warm lower air and cold upper air

In the mature stage of these storms there are severe updraughts and downdraughts, which can give rise to very violent weather such as torrential rain, large hail, strong winds, and even tornadoes. The mature stage can last for several hours.

MOVEMENT OF THUNDERSTORMS Thunderstorms formed in a col or slack pressure gradient tend to move erratically, but generally thunderstorms move with the wind at the 700 hPa level, which is equivalent to approximately 10 000 ft. Supercell thunderstorms in the Northern Hemisphere tend to move 20° to the right of the 500 hPa (18 000 ft) wind.

SQUALL LINES Squall lines are usually formed in the warm air mass ahead of a cold front. Squall phenomena are most frequent during the evening and early night. They are not very common in Western Europe. Squall lines are more common over large continental areas such as Eastern Europe or, more frequently, North America. A squall line with thunderstorms also contains hail, and tornadoes can occur. Although the CB along the squall can seem very small and insignificant compared to the frontal clouds behind, in reality the most intense weather phenomena are caused by squalls.

HAZARDS TURBULENCE AND WINDSHEAR Turbulence is moderate to severe in thunderstorms, caused by updraughts and downdraughts within the cloud. Gusts associated with thunderstorms can cause vertical displacements of up to 5000 ft. The effects can be felt up to 40 miles away. Severe turbulence can be encountered several thousand feet above the cloud tops, as well as within and below the cloud. Flying within a few thousand feet of the tops of CBs should be avoided. Windshear is a more sustained change in windspeed or direction. It is, therefore, likely to be more dangerous, especially on the approach where the effect on an aircraft’s airspeed can have serious consequences. In the most extreme cases changes of as much as 80 kt in speed and 90° in direction can be experienced within a layer of only a few hundred feet.

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GUST FRONT Some thunderstorms may have a well defined area of cold air flowing out from a downdraught in all directions, but tending to lead the storm along its line of movement. A gust front might extend out 24 to 32 km from the storm centre and can be felt from the surface to about 6000 ft. The cold air undercuts warm air and windshear may be associated with it. This gust front can be quite distant from the cloud and without precipitation it does not show up on weather radar and can therefore be quite unexpected. Occasionally there may be roll cloud associated with it.

Storm Movement

Possible Roll Cloud Formation Warm Air Inflow

Turbulence

Outflow

Downdraughts

Gust Front

MICROBURSTS Microbursts are strong downdraughts of air that descend from the centre of CB clouds with speeds up to 60 kt down to levels as low as 300 ft. They are typically less than 5 km across and last from 1 to 5 minutes. As the downdraughts approach the ground, the air splays out in all directions. The following diagram shows an aircraft approaching the CB. It initially experiences a strong headwind (A), then a downdraught (B), followed by a tailwind (C).

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Microbursts are the most extreme example of windshear and can result in large airspeed changes that can result in the loss of large aircraft. There are two types of microburst: wet and dry. The wet type has large amounts of precipitation associated with it so shows up well on weather radar. In the dry type any precipitation has evaporated before reaching the ground, so is less easy to identify. Some virga may show up on radar. Dry microbursts are generally the more severe type and tend to be associated with heat airmass thunderstorms over dry near-desert regions. The evaporation of the precipitation absorbs latent heat and enhances the downdraughts.

HAIL Hail can be encountered in the cloud, below the cloud, and beneath the anvil. Since it is not possible to tell whether or not a given storm produces hail, for avoidance purposes it is safer to assume that it will. The stronger the lifting and the greater the moisture content, the greater the chance of hail. Hail can be up to 14 cm in diameter and can be encountered up to 45 000 ft, producing severe skin damage with even a short exposure.

ICING Any flight in cloud or precipitation can result in icing when the temperatures are below zero. Icing can occur down to temperatures as low as -40°C. Icing is more severe near the base of the cloud where the droplets are larger. This is discussed more thoroughly in the chapter on Icing. Carburettor icing is also a risk and can occur in the temperature range -10°C to +30°C. Meteorology

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LIGHTNING Various processes can lead to different charges separating within a CB cloud. In a CB cloud, hail can collide with water droplets and ice crystals in the cloud. This results in a net transfer of positive ions from the warmer hail to the colder supercooled water droplet or ice crystal. This results in the positively charged ice crystal/water droplet moving upward in updraughts and the negatively charged hail falling downward with gravity. As a water droplet falls within a cloud it gathers speed. Once it reaches about 9 m/s it starts to split. Larger parts of the split droplet become positive and smaller parts become negative. The small negative parts are lifted higher up the cloud than the larger positive parts. Supercooled water droplets might also freeze onto hail. Tiny splinters of ice break off, become negatively charged and ascend within the cloud. These processes result in a net charge difference within the cloud. Once this reaches a potential difference of about 3 million volts per metre over a distance of about 50 metres, a discharge of current, lightning, takes place. Most lightning occurs within 10°C (approximately 5000 ft) of the freezing level. Hazards associated with lightning are temporary blindness caused by the flash, interference with compasses and other instruments, and possible airframe damage.

STATIC Static causes interference on LF, MF, HF, and VHF radio equipment. In severe cases a visible discharge may occur, called St. Elmo’s Fire, which is a purple light around windscreen edges, wing tips, propellers, and engine nacelles. Although not dangerous in itself it is an indication that the air is highly charged and lightning is likely.

WATER INGESTION Turbine engines have a limit to the amount of water they can ingest. If the updraught velocity in the thunderstorm approaches or exceeds the terminal velocity of the falling raindrops, very high concentrations of water may occur. It is possible that these concentrations can be in excess of the quantity of water turbine engines are designed to ingest, which could result in flame-out and/or structural failure of one or more engines. To eliminate the risk of engine damage or flame-out, it is essential to avoid severe storms. During an unavoidable encounter with extreme precipitation, the recommendation is to follow the severe turbulence penetration procedure contained in the approved aircraft flight manual, with special emphasis on avoiding thrust changes unless excessive airspeed variations occur. Water can exist in large quantities at high altitudes even where the ambient temperature is as low as -30° C.

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TORNADOES Tornadoes are associated with severe thunderstorms. They form with massive convergence in a trough with sharply inclined isobars. Differing wind directions give a rotating twist and the lifted air becomes a spiral. They are very localised — less than 300 metres across — and the lifting can be so strong that it can pick up water from a sea surface or dust from the land. Wind speeds in the vortex can reach 200 kt. If the funnel does not touch the ground it is called a funnel cloud; if it does touch, it is called a tornado. Tornadoes are common in the United States but rare in the UK and Europe.

PRESSURE VARIATIONS Pressure variations can cause the given QNH/QFE to be in error, sometimes by as much as 1000 ft. Local gusts exacerbate the problem and VSIs are also subject to errors. Aircraft should be flown for attitude rather than altitude.

WEATHER RADAR Weather radar is provided to enable pilots to avoid thunderstorms and is designed to detect areas of heavy precipitation. The strength of the echo is not necessarily an indication of the strength of the associated turbulence. Radar return intensities may be misleading because of attenuation resulting from intervening heavy rain. This may lead to serious underestimation of the severity of the rainfall in a large storm, and an incorrect assumption of where the heaviest rainfall is likely to be encountered. The echo from that part of an area of rain furthest from the radar is relatively weaker and the actual position of the maximum rainfall at the far edge of the storm area is further away than indicated on the radar display, sometimes by distances up to several miles. Additionally, a storm cell beyond may be completely masked. The high rate of growth of thunderstorms and the danger of flying over or near to the tops both of the main storm and the small convective cells close to it must be considered when using weather radar for storm avoidance.

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AVOIDANCE CRITERIA When using weather radar the following avoidance criteria should be used: Echo Characteristics Flight Altitude

Shape

Intensity

Gradient of Intensity

Rate of Change

0 to 20 000 ft

Avoid by 10 nm echoes with hooks, fingers, scalloped edges or other protrusions

Avoid by 5 nm echoes with sharp edges or strong intensity

Avoid by 5 nm echoes with strong gradients of intensity

Avoid by 10 nm echoes showing rapid change of shape, height or intensity

20 to 25 000 ft

Avoid all echoes by 10 nm

25 to 30 000 ft

Avoid all echoes by 15 nm

Above 30 000 ft

Avoid all echoes by 20 nm

General rules: ¾

If a storm cloud has to be overflown, maintain at least 5000 ft vertical separation from the cloud tops.

¾

If the aircraft has no weather radar, avoid any storm cloud by 10 nm that is tall, growing rapidly, or has an anvil top.

¾

Avoid flying under a CB overhang.

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INTRODUCTION Visibility is a measurement of atmospheric clarity. Reduction in visibility can be caused by: ¾

Water droplets, such as cloud, fog, or rain.

¾

Solid particles, such as sand, dust, or smoke.

¾

Ice, such as crystals, hail, or snow.

Poor visibility is more common in stable conditions, for example, beneath an inversion. Visibility is generally better upwind of towns and industrial areas, away from the atmospheric pollutants.

TYPES OF VISIBILITY REDUCTION There are several types of visibility reduction. These are: Mist

Caused by very small water droplets in a RH of more than 95%. The visibility is between 1000 and 5000 metres.

Fog

Also water droplets. Visibility is less than 1000 metres and RH is very close to 100%.

Haze

Caused by solid particles such as sand, dust, or smoke. There is no lower or upper limit to visibility but haze is not reported above 5000 m visibility.

TYPES OF VISIBILITY METEOROLOGICAL VISIBILITY Meteorological visibility is also known as Meteorological Optical Range (MOR) and is the furthest horizontal distance on the ground that an observer with normal eyesight can recognise a dark-coloured object. At night, lights of known power are used. Readings are taken at a person’s eye level.

RUNWAY VISUAL RANGE Runway Visual Range (RVR) is the maximum distance in the direction of take-off or landing at which a pilot in the threshold area at 15 ft above ground can see marker boards by day, or runway lights by night. It is only used when the meteorological visibility is less than 1500 metres or when fog is reported or forecast.

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OBLIQUE VISIBILITY When flying at altitude, slant visibility is the maximum distance a pilot can see to a point on the ground. The oblique visibility is the distance measured along the ground from the point directly beneath the aircraft to the furthest point the pilot can see. The distinction is made in the diagram below.

SLANT VISIBILITY

DOWNWARD VISIBILITY

OBLIQUE VISIBILITY

MEASUREMENT OF VISIBILITY BY DAY Measurement by day is made by reference to suitable landmarks at known distances from the observing position.

BY NIGHT Measurement by night is done by using a suitable arrangement of lights of known power as a substitute for landmarks. If this is not possible, a Gold’s Visibility Meter can be used. A variable filter in the viewing mechanism adjusts until light is no longer seen and the reading off the meter gives an equivalent daylight visibility.

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MEASUREMENT OF RUNWAY VISUAL RANGE HUMAN OBSERVER When an observation of runway visual range is taken by a human observer, the observer is positioned 76 metres from the centreline of the runway in the touchdown area. The observer sights the number of marker boards or lights in the appropriate direction. Then, the number of observed boards or lights is converted into a distance and reported. Human reporting is inaccurate at the maximum and minimum reporting ranges and visibilities < 100 m and > 1200 m are unlikely to be reported.

INSTRUMENT REPORTING Instrument reporting is done with an instrument called a transmissometer, which consists of a projector and a receiver.

The receiver contains photoelectric cells which measure the opacity of the air and give an equivalent daytime visibility.

RVR REPORTING Three transmissometers are positioned alongside the runway giving three readings, one for touchdown, one from the mid-point, and one for the stop-end of the runway. RVR is reported in increments of 25 m up to 200 m, 50 m up to 800 m, and 100 m over 800 m. Sometimes not all three readings are transmitted. The touchdown reading is always reported but the mid-point and stop-end values may be omitted if certain conditions are met. If one reading is omitted, the second figure in the group must be specified as the mid-point or stop-end value. The conditions for the omission of midpoint and stop-end RVR values are that: a. They have equal to or greater values than the touchdown value, and. b. They are above 400 metres. E.g.

300/500/600 would be reported as R 300. 300/350/500 would be reported as R 300 mid-point 350.

OR c. E.g. Meteorology

They are 800 metres or greater. 900/850/950 would be reported as R 900. 900/850/750 would be reported as R 900 stop-end 750. 12-3

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VISIBILITY WHILE FLYING EFFECT OF SUN AND MOON Visibility is reduced looking into the sun due to the harsh glare of the strong rays. Conversely, looking into the moon may improve visibility at night as it casts a gentle light on water surfaces and other ground based features.

WITH A DEEP HAZE LAYER When flying within the layer at different heights the slant visibility stays the same. When flying higher, the vertical component of the slant visibility increases, so the horizontal component, that is oblique visibility, decreases.

Conversely, while flying above the layer flying higher increases oblique visibility.

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WITH A SHALLOW FOG LAYER If the fog is shallow the pilot may be able to see the airfield quite clearly from directly above it. Once the pilot descends and turns onto final, visibility may be much poorer looking through the horizontal extent of the fog instead of the depth. It is important, therefore, to heed the visibility readings given by the tower even if your own observations are different.

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TYPES OF FOG RADIATION FOG At night, the ground loses its heat by radiation. The ground becomes cold and cools the air in contact with it. If this lowers the air temperature below the dewpoint, water vapour condenses out as droplets, resulting in fog if there is a light wind, or dew/frost if there are calm conditions. Conditions necessary for radiation fog to form are: ¾

Clear sky which increases the rate of terrestrial radiation (fog can still form in light, high cloud cover such as scattered cirrus).

¾

High relative humidity so that only a little cooling will be required for the air to reach saturation.

¾

A light wind of 2 to 8 kt which mixes the air bringing warmer air from above to the surface to be cooled and thickening the fog.

Radiation fog is most common in autumn and winter when there is a long night giving the land time to cool. It occurs at night and early morning after a prolonged period of cooling. It doesn’t occur over the sea as the sea has insufficient diurnal variation. It forms first in the valleys due to katabatic effect and is common in anticyclones, ridges, and cols where the air remains in contact with the ground for a prolonged period. Dispersal of the fog can occur by: ¾

The increase of insolation during the course of the morning, raising the temperature above the dewpoint and evaporating the fog away from the base.

¾

The increase of thermal turbulence during the morning which lifts the fog to form low stratus.

¾

An increase of cloud cover preventing the loss of radiation from the lower atmosphere and raising the temperature of the air above the dewpoint.

¾

Replacement of the air mass with a drier air mass by advection.

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ADVECTION FOG Advection fog forms when warm moist air flows over a cold surface. It can occur over land or sea. Conditions necessary for it to form are: ¾

A wind of up to 15 kt (20 kt over the sea).

¾

A high relative humidity so little cooling is required to bring the air to saturation.

¾

The cold surface over which the air moves must have a temperature lower than the dewpoint of the warm moist moving air.

Advection fog is common over land areas in winter and early spring when the land is colder than the sea and over sea areas in late spring and early summer when the land becomes warmer than the sea.

This type of fog is much more persistent than radiation fog and can last several weeks. Examples are the coast of Newfoundland and the Kamchatka peninsula where the temperature difference between land and sea is extreme. Dispersal comes when there is a change of airmass or an increase in windspeed beyond that described in the conditions above.

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Some types of advection fog experienced in and around the UK are listed below: Thaw Fog

These fogs occur over land surfaces in winter and spring when severe frost or snowfall gives way to milder Atlantic air from the southwest.

Haar

Frequent in the spring and early summer off the Northeast coast of the UK. The sea is at its coldest having been cooled gradually through the winter months. Warm air from the continent passes over the colder sea.

Sea Fog

Common in the approaches to the English Channel during the spring and early summer when the sea is still cool. If the wind speed is over 25 kt then the fog will lift into ST.

STEAMING FOG (ARCTIC SEA SMOKE) Steaming fog occurs at very high latitudes over sea areas such as around Iceland, Greenland, and Norway. It is similar to advection fog in that the airmass is moving but in this case it is a cold moist air mass passing over a warmer sea.

Normally this would lead to convection and the formation of cumuliform cloud. However, in this case the air is too cold and stable for sufficient lifting to occur. Instead, the small amount of lifting and evaporation from the sea leads to saturation and fog formation. At such high latitudes the water content is likely to be ice crystals giving the fog a white appearance which is the reason for its nickname of Arctic Sea Smoke.

FRONTAL FOG Frontal fog is associated with warm fronts and warm occlusions. Precipitation from NS cloud above the front falls into the colder air beneath the front, saturating the colder air. Additionally, the precipitation wets the ground and the moisture then evaporates into the air just ahead of the front aiding saturation.

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This produces a band of fog up to 200 nm wide that travels just ahead of the front as shown in the diagram.

HILL FOG Hill fog is really stratiform cloud that forms when there is orographic lifting in stable conditions. The cloud stays next to the surface obscuring the tops of the hill or mountain. A nice example is the tablecloth effect on Table Mountain in Cape Town, South Africa.

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OTHER VISIBILITY REDUCERS SMOKE FOG (SMOG) Smoke fog is a combination of ordinary water droplet fog and solid particles. It occurs in industrial cities when there is an inversion layer preventing air from lifting and removing the pollutants. In addition to being visibility reducers themselves, the solid particles are hygroscopic nuclei and enhance the severity of the fog.

DUST AND SAND Dust is a solid particle less than 0.08 mm in diameter. Sand is between 0.08 mm and 0.3 mm in diameter. Winds can carry these particles aloft causing dust or sand storms. In dust storms, the wind is upwards of 15 kt and the dust can rise to up to 15 000 ft agl. In sand storms, the winds are upwards of 20 kt but these remain within a few feet of the surface due to the weight of the particles. Both types tend to be daytime phenomena as wind strengths are usually insufficient at night. Visibility in dust or sand storms is generally less than 1000 m.

PRECIPITATION Precipitation also causes reduction in visibility. Drizzle reduces visibility more than rain, as drizzle consists of large numbers of small water droplets. Drizzle can lower the visibility to 500 m. The worst type of precipitation is snow. Heavy snow can lower the visibility to 50 m and possibly even less if it is blowing or drifting. For more information, see the chapter on Precipitation.

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VISUAL ILLUSIONS SHALLOW FOG If the pilot enters a shallow fog layer on descent it can give the illusion that the aircraft has pitched up. If the pilot believes this illusion and pitches the nose down, a very dangerous situation can arise, especially if this happens on the approach to land.

RAIN SHOWERS A rain storm moving toward the aircraft can give the illusion of the horizon moving lower, causing the pilot to reduce power or lower the nose unnecessarily.

LAYER CLOUD In the absence of a well-defined horizon, the pilot may orientate himself with respect to layer clouds. If the layer clouds are not parallel to the ground, the orientation to a false horizon will cause banking.

RAIN EFFECTS Rain can have two opposing effects: 1. Rain falling between the aircraft and visual landmarks such as the runway lights will diffuse the light and make the objects or runway lights appear further away than they really are. The pilot might perceive this as being low on approach. 2. Rain on the windscreen can make runway lights bloom, making the runway appear closer than it really is. The pilot might perceive this as being high on approach and may make adjustments to the aircraft’s power and/or attitude which will result in undershooting the runway.

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Meteorology

INTRODUCTION Ice accretion can have serious implications for performance and handling of aircraft. Modern aircraft are equipped with efficient anti-icing and de-icing equipment. However, these systems may become inoperative or icing conditions may be so severe that these systems become unable to cope. Even if these systems operate perfectly there is quite a significant fuel cost in running the systems. The preferred approach would be to avoid the conditions in which severe icing may occur. It is necessary for the pilot to understand the conditions and the risks associated with icing.

CONDITIONS Ice forms on an airframe if the following three conditions are present: 1. Water is present in a liquid state. 2. The ambient air temperature is below 0°C. 3. The airframe temperature is below 0°C.

EFFECTS OF ICING The detrimental effects of icing can include the following:

AERODYNAMIC Ice forms mostly on the leading edges of the airframe and aerofoils. This spoils the aerodynamic shape of the airframe and leads to: ¾ ¾ ¾

Reduced lift (up to 30%) Increased drag (up to 40%) Increased weight

The increased weight coupled with loss of lift leads to an increased stalling speed. The added weight and increased drag results in greater fuel consumption. In addition, ice accumulation may lead to control surfaces becoming jammed, especially where ice has broken off in chunks from other surfaces and become lodged.

WEIGHT The rate of accumulation of ice is rarely constant across the airframe. This inconsistency may lead to a shifting centre of gravity which causes instability and difficulty controlling the aircraft. Uneven ice build-up on propellers can lead to severe engine vibration and possible engine damage. Meteorology 13-1

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INSTRUMENTS Ice may block the pitot and static inlets leading to gross instrument errors in the altimeter, airspeed indicator, vertical speed indicator and Machmeter. The safety implications of this are far-reaching.

OTHER EFFECTS Other miscellaneous effects include: ¾ ¾ ¾ ¾ ¾

Skin damage from chunks of ice breaking off propellers Obscuration of windscreens Increased skin friction and associated performance effects Radio interference due to ice build-up on aerials Landing gear deployment/retraction problems if ice forms in gear wells or freezes gear doors closed

ICING DEFINITIONS Any pilot encountering unforecast icing should report the time, location, level, intensity, icing type, and aircraft type to the ATS unit they are in contact with. The following definitions are the reporting definitions for levels of icing:

TRACE Ice becomes perceptible; rate of accumulation slightly greater than the rate of sublimation. It is not hazardous. De-icing/anti-icing equipment is not used unless ice is encountered for more than one hour. LIGHT The rate of accumulation might create a problem if flight in this environment exceeds one hour. Occasional use of de-icing/anti-icing equipment removes/prevents accumulation. It does not present a problem if anti-icing equipment is used. Note: The ICAO definition of light icing is: “Change of heading or altitude not considered necessary.”

MODERATE The rate of accumulation is such that even short encounters become potentially hazardous and the use of de-icing/anti-icing equipment, or diversion, is necessary. Note: The ICAO definition of moderate icing is: “Change of heading or altitude considered desirable.”

SEVERE The rate of accumulation is such that de-icing/anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary. Note: The ICAO definition of severe icing is: “Immediate change of heading and/or altitude necessary.”

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SUPERCOOLED WATER DROPLETS In order for a droplet of water to freeze, it not only must be below freezing point, but there must be a freezing nucleus present. This could take the form of salt, dust, pollen, or smoke particles. There are less freezing nuclei than condensation nuclei. Hence it is a frequent occurrence that a droplet cools to a temperature below zero but there is no freezing nucleus available. When this occurs, the droplet stays in liquid form even though it is below zero. It is then referred to as a supercooled water droplet. These droplets can exist in temperatures as low as -40°C. Most icing is caused by aircraft colliding with these droplets while in cloud or fog. As the droplet touches the airframe its surface tension breaks down and it starts to freeze.

SIZE OF SUPERCOOLED WATER DROPLETS There are two factors dictating the size of the supercooled water droplets in a cloud. First, consider the type of cloud. Layer clouds only have small water droplets, so when these become supercooled they remain small. Cumuliform clouds can have small and large water droplets, so the size of the droplets when supercooled varies. The second factor is temperature. Once the temperature drops below -20°C, large supercooled droplets freeze, regardless of the lack of a freezing nucleus. So even in cumuliform cloud, if the temperature drops below -20°C, only small supercooled droplets will be present.

LARGE SUPERCOOLED WATER DROPLETS In summary, large supercooled water droplets occur: 1. In CU and CB from 0°C to -20°C. 2. In NS at temperatures from 0°C to -10°C. 3. If the NS has been enhanced by orographic uplift, between 0°C and -20°C.

SMALL SUPERCOOLED WATER DROPLETS In summary, small supercooled water droplets occur: 1. 2. 3. 4.

In CU and CB from -20°C to -40°C. In NS at temperatures from -10°C to -40°C. If the NS has been enhanced by orographic uplift, between -20°C and -40°C. In ST, SC, AS, AC from 0 to -40°C.

Note: Supercooled water droplets do not occur in the cirriform clouds. These consist of ice crystals.

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FREEZING PROCESS When a supercooled water droplet impacts an airframe, not all of it freezes instantly. The fraction that freezes instantly depends on the temperature of the droplet. For every degree below zero, 1/80 of the droplet will freeze on impact. So if the temperature is -20°C, 1/4 will freeze on impact; if the droplet is -40°C, 1/2 will freeze on impact. So with a warmer droplet, the freezing process is slower. As a fraction of the droplet freezes, latent heat is released which delays the freezing of the remainder of the droplet. This allows the liquid part to flow over the airframe (called flowback) and freeze more gradually. Also, the size of the droplet is important. Large droplets tend to retain latent heat better, so freezing is delayed even more, allowing a greater spread of the droplet. The importance of these differences is discussed below.

TYPES OF ICING CLEAR ICE (GLAZE ICE) Clear ice, or glaze ice, forms when large supercooled droplets impact with an airframe. When the droplet impacts the airframe it does not freeze instantly. It starts to freeze and as a result some latent heat is released. This raises the temperature slightly, allowing the water to flow over the airframe before subsequently freezing. This results in a clear coating of ice which adheres strongly to the surface of the aircraft. Clear ice is a very serious form of icing which is heavy and difficult to remove. Uneven formation on propellers can lead to vibration and chunks breaking off and causing skin damage. The weight addition, which can be uneven, leads to stability and control problems and the aerofoil shape is spoiled. Because of this, clear ice is usually described as moderate to severe. Since large droplets only occur in CU, CB, and NS, this type of ice is only found in those clouds, and only in the temperature range 0°C to -20°C.

RIME ICE This forms from impact with small supercooled droplets. When the droplet impacts, most of the droplet freezes instantly with little or no flowback. Air becomes trapped between the droplets causing the ice to be opaque or cloudy. It is a granular coating which is generally easy to remove. It can cause some loss of the aerofoil shape and an increase in surface friction. It can also cause blockage of air intakes. Usually rime icing is classed as light to moderate as build up is generally light enough for antiicing measures to cope. This type of icing can occur in any cloud where there are small supercooled droplets. Hence it will occur in layer clouds at any temperature below zero (except cirriform clouds which consist of ice crystals). It will also occur in cumuliform clouds where temperatures are below -20°C. It may also occur in freezing fog. 13-4

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MIXED ICE This is a combination of clear ice and rime ice and occurs where both types of water droplets are present. This applies to clouds where the temperature is close to the transition between small and large supercooled droplets. This will be within a few degrees of: 1. -20°C for CU and CB. 2. -10°C for NS. 3. -20°C for NS enhanced by orographic uplift.

RAIN ICE This type of icing is very severe and very similar to clear ice. It is common beneath a warm front or an occlusion, when precipitation falls from NS cloud above the front. The warm rain falls into colder air and becomes supercooled. If the aircraft is above the freezing level, the airframe is below zero and the droplets strike the airframe and form ice in the same way as described above in the section on clear ice. The colder the air is below the front, the more common this type of icing becomes. Hence, it is a common occurrence over large land masses such as North America and Central Europe, but is much rarer over the UK where the temperatures are milder.

HOAR FROST This type of icing occurs when air is cooled to the temperature at which saturation occurs and the airframe is below 0°C. The frost forms by sublimation, that is, water vapour turns directly to ice without passing through the liquid state. Note that the temperature to which the air must be cooled for saturation to occur is called the frost point in this situation, rather than the dewpoint. It is a white crystalline deposit of the kind you find on your car on a cold morning. It can occur on the ground when the aircraft is parked, or during flight.

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The correct conditions for hoar frost formation occur when an aircraft takes off from an aerodrome at a sub-zero temperature and climbs through an inversion into warm moist air. Likewise, if an aircraft descends from a very cold region into a warm moist layer, the same conditions will be present. This causes similar problems to those caused by rime ice.

FACTORS AFFECTING THE SEVERITY OF ICING There are several factors which affect icing severity. These are detailed below.

SIZE OF SUPERCOOLED WATER DROPLETS As discussed above, larger supercooled droplets cause more severe icing of the clear type, and small supercooled droplets cause rime ice, which is less serious. The size of the droplets depends on the type of cloud and the ambient temperature. This was discussed above and is summarised below: Type

Severity

Conditions

Clear / Glaze Ice

Moderate to severe

Caused by large supercooled droplets, hence only found in cumuliform clouds such as CU and CB, and also in NS and ACC which have heap-type characteristics.

Rime Ice

Light to moderate

Caused by small supercooled droplets. In layer clouds from 0°C to -10°C. In cumuliform clouds from -20°C to -40°C.

Light

Caused by small supercooled droplets. In layer clouds from -10°C to -40°C.

N/A

In CI, CS and, CC (only ice crystals are present).

Nil

CONCENTRATION OF SUPERCOOLED WATER DROPLETS The higher the concentration of supercooled droplets, the more serious the icing risk. Upcurrents are stronger in the convective clouds, hence able to support a higher concentration of droplets. This increases the risk in these clouds. There is a higher concentration of droplets at the base of the cloud. This is for two reasons. First, gravity tends to increase the density lower down. Second, the base is where condensation commences, where the temperature is higher so the water content of the moist air is greater.

OROGRAPHIC UPLIFT Where clouds have formed orographically, or existing clouds have been enhanced by lifting against a hill or mountain, uplift is stronger, so the cloud can support a higher concentration of water droplets, and also a greater size of droplet. For both these reasons, icing tends to be more severe.

CLOUD BASE TEMPERATURE The higher the temperature, the greater the amount of water vapour the air can hold. If the cloud starts to form at a high temperature, the moisture content will be greater making the concentration of droplets greater. Upcurrents result in the concentration of water droplets at all levels of the cloud being greater so the icing will be more severe. 13-6

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AEROFOIL SHAPE Air flowing around thin, low-drag aerofoils tends to follow the shape quite closely, whereas air flowing around thick, high-drag aerofoils tends to be deflected away from the surface more. Hence, supercooled water droplets are more likely to adhere to the thin aerofoil shape. Aircraft with low-drag aerofoils tend to fly at a higher speed, and so they impact with more droplets in a given amount of time. This may be offset by kinetic heating effect, more details of which are given below. If the skin temperature is raised to above zero, no icing will occur.

KINETIC HEATING As an aircraft travels through the air it experiences kinetic heating of its surface which is related to its true airspeed. The formula is as follows:

Temperature Rise (°C) =

(

TAS 100

)

2

So if the true airspeed is 300 kt, the temperature rise will be 9°C. If this raises the temperature to above zero, no ice will form. However, it also has the potential to worsen the effect of icing. If the temperature were a low sub-zero temperature and was heated to a temperature which was still below zero, this may lead to increased flowback and a greater likelihood of clear ice. Hence it is important not to assume that kinetic heating will always improve the situation.

ENGINE ICING Icing can occur in both piston and turbine engines. The types of icing and conditions for formation differ between the engine types. Icing can occur to a much higher temperature in piston engines than in turbine engines. The processes involved are described below.

PISTON ENGINE ICING Several different types of icing can occur.

IMPACT ICING Impact icing occurs in the intake area of the engine. It forms by direct impact of supercooled water droplets with the surface, in much the same way as airframe icing. Temperatures need to be sub-zero for this to occur. FUEL ICING Fuel icing is caused by water in the fuel freezing in the pipes and reducing or preventing fuel flow to the engine. Again, the temperature needs to be below zero.

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CARBURETTOR ICING

ICE

FUEL

INTAKE AIR

This is the only form of icing where the ambient temperature can be above zero. It is caused by two things: 1. Latent heat being absorbed from the surroundings as fuel evaporates. 2. As air passes through the venturi its speed increases, but its pressure, and therefore its temperature, go down. The temperature reduction can be in excess of 30°C. So even at quite high temperatures the air may be cooled to a temperature below zero. If the air has sufficient moisture, content icing occurs. The effects can be more severe if a low throttle setting is used with the carburettor butterfly only partially open. A total blockage may occur. Carburettor icing is common on warm, humid days as the moisture content of the induction air is high. Indications that the conditions for carburettor ice formation may be present include wet ground or dew, reduced visibility from mist or fog, proximity to clouds, or precipitation.

JET ENGINE ICING As for piston engines, the problem of fuel icing in the supply pipes exists. Impact icing may accumulate in the intakes of a jet engine. If this breaks off, it can cause blade damage. In the early intake stages, there is a pressure reduction which can lead to adiabatic cooling on the order of 5°C. This is a particular problem if the aircraft is at high revs, such as on approach or climb-out.

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Chapter 13

In potential icing conditions, use engine igniters to help prevent failures. If there is precipitation or the outside air temperature is less than 10°C, engine anti-icing systems should be switched on.

ICE PROTECTION ANTI-ICING Anti-icing measures are designed to prevent the formation of ice. They include: ¾ ¾ ¾ ¾ ¾

Kill-frost paste applied to the leading edges. Heated windscreen and pressure head. Hot air system on leading edges and tailplane. Hot air system on engine cowling lips and spinner. Anti-icing fluids.

DE-ICING De-icing measures are designed only to remove icing after it has formed, not to prevent its formation. Examples are: ¾ ¾ ¾ ¾

Meteorology

De-icing fluids. Pulsating rubber boots. Hot air systems. Electrical heating systems.

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Meteorology

INTRODUCTION Wind is the horizontal movement of air over the surface of the Earth due to forces acting upon it. It is expressed as a wind velocity, which is a combination of direction and speed. The direction given is always that from which the wind is blowing.

Calm

20 kt, further additions up to 45 kt

1 to 2 kt

50 kt

5 kt

60 kt

10 kt

65 kt, further additions as necessary

15 kt

The wind is depicted as a straight line coming from the periphery of a circle. The examples above show a wind direction of 090°. The wind speed is normally given in knots. Other units used are kilometres per hour and metres per second. Direction is usually given in °T. Exceptions to this are in an ATIS or verbally from the control tower, where wind direction is given in °M. This is because runway direction is magnetic, enabling the pilot to calculate the wind components if the wind speed is also given in magnetic.

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TERMS ASSOCIATED WITH WIND Veer is a change of direction in a clockwise direction. Back is a change of direction in an anti-clockwise direction.

Gust is a sudden increase in wind speed lasting a few seconds. Squall is a wind speed increase of at least 16 kt to a uniform speed of at least 22 kt lasting for at least one minute. Squalls are often associated with CBs. Lull is a decrease in wind speed lasting from a few seconds to a few minutes. Gale is a mean surface wind of 34 kt or more, or gusting to 43 kt or more. Hurricane is a wind with a mean surface value of 63 kt or more. Wind Gradient is the gradual change in wind velocity between the surface and the top of the friction layer. Gust factor is calculated by the following formula: GUST FACTOR % = (MAXIMUM GUST SPEED – MINIMUM LULL SPEED) * 100% MEAN WIND SPEED

For example:

A wind averaging 35 kt with gusts to 50 kt and lulls of 20 kt would have a gust factor of: (50 – 20) × 100= 85.7% 35

FORCES ACTING UPON THE AIR There are two main forces acting upon the air. These are: 1. The Pressure Gradient Force. 2. Geostrophic Force.

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There is a third force, friction, that acts close to the surface. The thickness of the friction layer varies.

THE PRESSURE GRADIENT FORCE The Pressure Gradient Force (PGF) is the force that initiates movement of air. If there is a region of high pressure adjacent to a region of low pressure, the air flows from the high pressure to the low pressure. If there were no other forces acting, this would continue until the two pressures were equal, resulting in no more pressure gradient.

PGF

H

L

1004

1002

1000

998

The diagram shows the pressure in mb or hPa. As seen in the diagram, the PGF acts at right angles to the isobars. Calculate it using the following equation: PGF =

dp ρdn

where: dp dn ρ

= = =

the pressure difference between two points the horizontal distance between the two points air density

THE GEOSTROPHIC FORCE This is also referred to as the Coriolis Force. Geostrophic force is due to the rotation of the Earth and the law of inertia. The Earth rotates at a fixed speed. At the Equator, the line of latitude with the largest circumference, objects on the Earth move faster than those at higher latitudes, because they have to travel a longer distance in the same amount of time. In the diagram below, the thick horizontal arrows show how a position on the Earth moves in a given time at the equator and at two temperate latitudes, one in the Northern Hemisphere, one in the Southern Hemisphere.

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Four different situations are shown. A and B show movement away from the Equator in the Northern and Southern Hemispheres respectively. C and D show movement towards the Equator in the Northern and Southern Hemispheres respectively. Take, for example, situation A. A parcel of air leaves the point represented by the start of the thick horizontal arrow at the Equator and travels due north. As it travels, the point on the ground from which it left and the point on the ground for which it is aiming move due to the Earth’s rotation. You would expect the parcel of air to end up at the point of the arrow at the higher latitude, that is, the initial aiming point after following a path represented by the dashed line. However, due to inertia the parcel of air moves at the speed of objects at the Equator, so travels further east than expected, following a path represented by the thick diagonal arrow. Hence, the parcel of air appears to have turned right in the Northern Hemisphere. Now look at the Southern Hemisphere, situation B. You can see that the parcel of air appears to turn left. Now look at situations C and D. You will find that the same rule applies for movement toward the Equator.

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A

C

B

D

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Chapter 14

In summary, due to Coriolis effect, objects appear to turn right in the Northern Hemisphere, and left in the Southern Hemisphere. Geostrophic Force (GF) can be calculated using the following equation: GF = 2 Ω ρ V SIN θ

Where: Ω = THE ANGULAR ROTATION OF THE EARTH ρ = AIR DENSITY V = WINDSPEED θ = LATITUDE Note that the Pressure Gradient Force must initiate movement of a parcel of air before Geostrophic Force can come into play. Geostrophic Force has no effect on a stationary parcel of air. As the Geostrophic Force is proportional to SIN θ, it is zero at the Equator and a maximum at the poles. Within 15° of the Equator Geostrophic Force is negligible.

THE GEOSTROPHIC WIND As already discussed, movement of air is initiated by the Pressure Gradient Force. The air is then affected by the Geostrophic Force. The Geostrophic Force initially acts at right angles to the pressure gradient force, so produces a resultant wind that is at an angle between the two. However Geostrophic Force is now no longer at right angles to the wind, and so acts from the resultant wind (see diagram below). This process continues until the PGF and the GF are acting in opposite directions and are in balance, as shown in the diagram.

PGF GW 1004 GF

1012

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The resultant wind is now at right angles to the PGF. In the Northern Hemisphere it will be 90° to the right of the PGF, in the Southern Hemisphere 90° to the left. This resultant wind is called the Geostrophic Wind and flows parallel to the straight isobars as shown in the diagram. It gives rise to Buys Ballot’s law, which states: “In the Northern Hemisphere with your back to the wind, the low pressure is on your left.” Note that the opposite is true for the Southern Hemisphere. This wind does not take the third force, friction, into account and is taken to be the wind just above the friction layer. The equation for geostrophic force applies: GF = 2 Ω ρ V SIN θ

Since PGF and GF are now in equilibrium, the following is also true: PGF = 2 Ω ρ V SIN θ

The formula can be re-arranged to make V the object as follows: V=

PGF

2 Ω ρ SIN θ Hence the windspeed (V) is proportional to the PGF and inversely proportional to the latitude. Therefore as latitude decreases, the windspeed increases. This continues until about 15° of the equator, where the equation breaks down due to the negligible geostrophic force. If the windspeed at a certain latitude is known, the windspeed at another latitude, assuming the same isobar spacing, can be calculated using the relationship described above. The derived formula is as follows: VLAT A SIN LAT A = V LAT B SIN LAT B

For example:

If the geostrophic wind speed is 40 kt at 30°N, calculate the geostrophic wind speed at 60°N 40 SIN 30 = V SIN 60 V = 23 KT

In summary, the conditions for the geostrophic wind are: ¾

Above the friction layer.

¾

Greater than 15°N/S.

¾

A pressure system that is not changing rapidly.

¾

Straight and parallel isobars.

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Chapter 14

THE GEOSTROPHIC WIND SCALE Consider the formula for windspeed again: V=

PGF

2 Ω ρ SIN θ Weather charts are usually for a limited latitude range and altitude. The angular rotation of the Earth is constant so the denominators of the equation can be replaced by a constant: V=

PGF K

This simple relationship means that windspeed can be determined from the pressure gradient force, which in turn comes from the isobar spacing. A scale called the Geostrophic Wind Scale is printed on the chart. An example is shown: 100 50

30

20

15

10

5 kt

As you can see, the relationship is not linear but logarithmic. To find the windspeed for a given point, measure the distance between successive isobars passing through that point, and compare this to the scale. Align your measured distance with the left end of the scale but read the speed off from the right. In the example above the isobars are well spaced, giving a speed of about 18 kt. The closer the isobars, the stronger the wind.

THE GRADIENT WIND The Geostrophic Wind only applies to straight parallel isobars. When dealing with curved isobars the situation becomes slightly more complicated. Consider circular pressure systems. In the Northern Hemisphere the pressure gradient force and geostrophic force act opposite to each other, and the resultant wind is 90° to the right of the PGF. However, the wind follows the curved isobars so the air starts to rotate around the centre of the system. This rotation brings an additional force into play, called centrifugal force. This is a force acting outwards from the centre of the system. In the next two diagrams, the following key applies: CF — Centrifugal Force PGF — Pressure Gradient Force GF — Geostrophic Force GW — Gradient Wind

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GW

CF

L

PGF

GF

In the case of a low pressure system, centrifugal force opposes the pressure gradient force, hence the resultant wind speed is lower than the geostrophic wind for the same isobar spacing. This is termed sub-geostrophic. If a geostrophic wind scale is used it will over-read. The resultant wind is called the Gradient Wind, and blows anti-clockwise around a low pressure system in the Northern Hemisphere.

H

GF

PGF CF

GW

In the case of a high pressure system, centrifugal force supports the pressure gradient force, hence the resultant wind speed is higher than the geostrophic wind for the same isobar spacing. This is termed super-geostrophic. If a geostrophic wind scale is used it will under-read. The resultant Gradient Wind blows clockwise around a high pressure system in the Northern Hemisphere.

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WINDS NEAR THE EQUATOR At latitudes less than 15° the formula for geostrophic wind breaks down due to the low value of the geostrophic force. With straight isobars the wind tends to flow across the isobars from high to low pressure. However, with curved isobars the situation is different. In some situations the centrifugal force becomes so large that it balances the pressure gradient force. When this happens, the wind is said to be cyclostrophic. Examples are in a tropical revolving storm or a tornado.

THE SURFACE WIND Both the Geostrophic and the Gradient wind act above the friction layer. The third force, friction, must be taken into account in this layer. The strength of the frictional force depends on the following factors: ¾

The roughness of the landscape – the rougher the landscape, the greater the friction;

¾

Stability of the air – an unstable air mass creates thermal turbulence. This causes the slow surface wind to interact with faster higher winds, resulting in increased wind speed at the surface;

¾

Season – in summer the turbulence layer is thicker over land due to surface heating. The same effect will be seen as above;

¾

Type of system – the layer is thicker in low pressure than in high;

¾

Windspeed – the higher the windspeed, the greater the resulting frictional effect.

Friction between the moving air and the surface slows the air down. Therefore V, the windspeed, decreases. Any decrease in V leads to a decrease in geostrophic force, according to the geostrophic wind equation discussed above. PGF

PGF

SW GW

FF

GF GF

2000 ft wind

Surface wind

In the above diagram GW is Geostrophic Wind, SW is Surface Wind, and FF is Friction Force.

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If the geostrophic force reduces then PGF and GF will no longer be in balance. PGF dominates so the surface wind deflects toward the PGF, that is, deflected toward the low pressure. As seen in the diagram, this will be a back in the Northern Hemisphere. In the Southern Hemisphere it will be a veer. In both cases the surface wind will be slower than the wind above the friction layer. Note: The above process applies equally to the wind around curved isobars. The number of degrees of deflection and the reduction in windspeed for different situations are shown in the table. Deflection of Surface Wind from 2000 ft wind

Speed of Surface Wind as a % of the 2000 ft wind

15° 30° 45°

75% 50% 25%

Over the Sea Over the Land by Day Over the Land by Night

DIURNAL VARIATION OF THE SURFACE WIND The following paragraphs describe the diurnal variation of winds at different heights. Note that these are for the Northern Hemisphere. In the Southern Hemisphere the speed changes are the same but changes of direction are opposite.

SURFACE WIND During the day, surface heating causes turbulent mixing and an increase in wind speed at the surface. During night the air cools down, turbulence ceases, and the friction has full effect. Night to day

Veer and increase

Day to night

Back and decrease

Over land from night to day the surface wind approximately doubles and veers by about 15°. Windspeeds are highest at around 1500 hours as this is when there is greatest surface heating. Windspeeds are lowest at around 0600 hours when temperatures are lowest.

1500 FT WIND By day 1500 ft lies within the friction layer, hence is affected by friction. By night it lies above the layer so is not affected. Night to day

Back and decrease

Day to night

Veer and increase

2000 FT WIND 2000 ft is generally above the friction layer by day and by night, hence experiences little diurnal variation. Night to day Day to night

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Meteorology

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Chapter 14

MEASUREMENT OF SURFACE WIND At an airport, wind is measured by placing the sensors 10 metres above an even-ground surface. This prevents false readings caused by surges due to ground obstacles or uneven ground. The wind vane gives direction as shown in the simple version below.

Wind Vane 270° 360°

180° 90°

The most common wind velocity sensor is the cup anemometer, shown below. Pressure tube anemometers may also be used. The cup anemometer tends to under-read the value of gusts and over-read the average wind speed due to its inertia.

3-CUP ANEMOMETER

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ISALLOBARIC EFFECT If the pressure gradient changes, the three forces of PGF, GF, and centrifugal force are temporarily out of balance. The wind tends to flow across the isobars from high to low until balance is restored. An Isallobar is a line joining places that have an equal rate of pressure change, hence the term Isallobaric Effect. When air blows toward an area of falling low pressure, this is called convergence. When air flows outwards from an area of increasing high pressure this is called divergence.

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Meteorology

INTRODUCTION The previous chapter explored lower winds which come about as a result of pressure differences on a large scale. In this chapter more localised wind effects will be explored. These tend to become apparent when the pressure gradient is slack or when the same air mass remains in contact with the ground for an extended period, such as in a stable high pressure system.

LAND AND SEA BREEZES These winds are common when there is an anticyclone with a light pressure gradient on a clear sunny day.

SEA BREEZE During the day, the land heats up more quickly than the sea. The air in contact with the land heats up and rises by the process of convection which leads to a decrease in pressure at the surface and an increase in pressure at approximately 1000 — 2000 ft agl. This causes air at that height to move over the sea. Air then descends over the sea causing an increased pressure at the surface of the sea. Air then flows from the slightly higher pressure over the sea surface to the lower pressure over the land surface and creates the sea breeze. The circulation is shown in the diagram below.

Return Flow

Warm

Sea Breeze

L

Meteorology

Cool

H

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Sea breezes are typically 10 kt in temperate latitudes and extend to about 10 nm either side of the coastline. In tropical areas they can be 15 kt and extend to 40 or 50 nm inland. Initially the wind will be at right angles to the coastline but as insolation increases throughout the day the wind will extend further from the coast and due to this longer fetch Coriolis effect comes into play. This causes a veer in the Northern Hemisphere and a back in the Southern Hemisphere.

LAND BREEZE After sunset the land starts to cool down much more rapidly than the sea. This leads to a reversal of the above situation. The sea surface experiences a lower pressure and the land a higher pressure as shown in the diagram. The wind now blows from the land to the sea.

Return Flow

Cool

Warm

H

L

The temperature difference between land and sea is less at night so the land breeze is weaker than the sea breeze – typically half the speed (5 kt in temperate latitudes) – and only extends to about 5 nm out to sea.

OPERATIONAL IMPLICATIONS OF THE LAND AND SEA BREEZES At coastal airfields, the landing and take-off direction is reversed from day to night if the runway is at right angles to the coast. During the day landing/take-off will be towards the sea and at night towards the land. Coastal airfields with runways running parallel to the coast experience crosswinds when the sea and land breeze are well-established. Fog off the coast can be blown inland during the day reducing visibility at coastal airfields. Lifting of air over land by the sea breeze can cause small cumulus clouds to form which assist pilots in the identification of coastlines.

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KATABATIC AND ANABATIC WINDS These winds occur on hillsides and valley sides and tend to form in slack pressure gradients.

KATABATIC WIND During the night a hillside cools down rapidly. The air in contact with it is cooled by conduction and becomes more dense than the free air next to it. It therefore flows down the hillside.

The katabatic wind is more apparent if the sky is clear as radiation is greater. If the slope is snow covered this also assists. The air remains in contact with the ground at all times and does not warm adiabatically. The average speed is 10 kt. If this wind occurs in a valley cold air collects at the bottom increasing the likelihood of fog or frost.

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ANABATIC WIND Anabatic wind is the opposite of the Katabatic wind and occurs during the day on slopes which are subject to direct sunlight. As insolation increases, the air in contact with the land warms up, becomes less dense and flows up the slope. The Anabatic wind is typically weaker than the Katabatic (about 5 kt) since it flows against the force of gravity.

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FOEHN WIND/EFFECT This topic was already mentioned in the chapter on Cloud Formation. The Foehn Wind was named for a warm dry wind that occurs in the Alps. There are several other winds in other parts of the world which are caused by the same effect, such as the Chinook, which flows down the east side of the Rocky Mountains. The Foehn Wind occurs when air is forced to rise up a mountain side in stable conditions. It cools initially at the DALR until it reaches saturation. At this point, cloud starts to form and the air continues to rise, but now cools at the SALR. Once it reaches the top of the mountain it starts to flow down the other side. Initially it warms at the SALR but quickly becomes unsaturated as much of its moisture has already been lost. It then warms at the DALR. Since the cloud base is higher on the lee side, the air at the base on that side will be warmer than on the windward side. The difference can be as much as 10°C (20°C with the Chinook).

8000 ft - 0°C

0°C

6000 ft - 3°C

3°C

4000 ft - 6°C

9°C

2000 ft - 9°C

15°C

0 ft - 15°C

21°C

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Local Winds

VALLEY/RAVINE WIND When wind blows against a mountain barrier it finds its progress impeded. If there is a gap or valley it is forced to flow through this. The restriction acts like a venturi and the wind speeds up. Wind speeds of 70 kt can be experienced. The combination of high wind speeds and rough terrain can result in turbulence at low level. An additional hazard results from the fact that small changes in the general direction of the wind can lead to sudden reversals in direction of the ravine wind.

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Chapter 15

HEADLAND EFFECT Where the 2000 ft wind blows parallel to the coast around a headland or cape the isobars push together causing an increase in pressure gradient and hence an increase in wind speed.

LOW-LEVEL JET A Low Level Jet (LLJ) is defined as a narrow, horizontal band of relatively strong wind (usually between 20 and 80 kt) located between 500 to 5000 feet AGL. They are often several hundred miles long and a few hundred miles wide. There are four common types of LLJ.

NOCTURNAL JET When the ground cools quickly, an inversion may build, and the wind quickly slows along the surface by friction. However above the inversion, the wind is not affected by friction, and the cold calm air along the ground serves as a gliding layer. The result is a strong wind, just above the inversion. Maximum wind speed is usually attained about 4 − 8 hours after sunset, the time depending on the latitude. The wind abates when insolation and convection destroys the inversion layer.

VALLEY INVERSION Often accentuated in mountainous regions where cold air drains into the bottom of a valley, valley inversions create an elevated stable layer and surface inversion. Wind speeds of more than 50 kt are sometimes reported above such inversions.

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Local Winds

COASTAL JET Water temperature differentials along many coasts around the world create elevated inversions or shallow frontal zones where low level jet (LLJ) phenomena occur. These LLJ can persist both day and night for as long as the temperature differentials last.

LOW LEVEL JET IN FRONT OF AN EXTRA-TROPICAL COLD FRONT A large temperature contrast across a cold front can create a similar wind phenomenon as the shallow coastal front and a pronounced LLJ forms ahead of a cold front inside the warm air mass.

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INTRODUCTION Air masses are large volumes of air with properties of humidity and temperature which remain almost constant in the horizontal. This phenomenon of more or less constant properties arises from the fact that the air in air masses remains stationary over its source for an extended period of time. This essentially means that air masses originate only in high pressure areas, as low pressures tend to be temporary features.

ORIGIN AND CLASSIFICATION Air masses are initially classified by the latitude from which they originate. This gives us three main types: ¾

Tropical

¾

Polar

¾

Arctic

They are further subdivided depending on whether they originate over sea or land: ¾

Maritime

¾

Continental

This gives us five main air masses: 1. continental Tropical (cT) 2. maritime Tropical (mT) 3. continental Polar (cP) 4. maritime Polar (mP) 5. Arctic (not subdivided) (A)

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maritime Arctic (mA)

COLD

continental Polar (cP) maritime Polar (mP)

continental Tropical (cT)

maritime Tropical (mT)

WARM Tropical air originates in the sub-tropical high pressure zones. An example of continental tropical air would be the air mass which originates in North Africa. Maritime tropical air originates in the permanent high pressures over the oceans. In the North Atlantic this is the Azores high. There is an equivalent high pressure in the North Pacific. Continental polar air originates in the high pressures over large land masses, hence this air mass is mainly a winter phenomenon. Examples of sources are Siberia and North America. Maritime polar air originates in the north of the North Atlantic and North Pacific. Arctic air originates over the North Polar ice cap. Since the region is ice covered, arctic air is not subdivided into continental and maritime. In the Southern Hemisphere there is an Antarctic air mass originating over the South Polar ice cap.

MODIFICATION OF AIR MASSES As the air masses pass over other regions as they travel away from their sources, their properties alter. In general, the following rules apply: ¾

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An air mass passing over a warmer area: •

Becomes warmer.

•

Becomes more unstable.

•

Experiences a reduction in relative humidity.

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Air Masses

¾

Chapter 16

An air mass passing over a colder area: •

Becomes colder.

•

Becomes more stable.

•

Experiences an increase in relative humidity.

AIR MASSES AFFECTING EUROPE We have introduced the various types of air masses. The next sections go into more detail about the kind of weather conditions that these air masses bring to Europe.

ARCTIC Originating over the North Polar ice cap, the arctic air mass is very cold and stable at the source. It has a low absolute humidity and low relative humidity. It is more common in the winter and moves south if there is a high pressure to the west of the UK and a low pressure to the east.

H

L

As an arctic air mass moves south toward Scotland, it becomes warmer and more unstable. It also picks up moisture from the sea to the north of Scotland. Over land, large cumulus will form bringing very cold weather, snow showers, and possible blizzards. If it occurs in summer, there will be rain showers and the region will experience a marked drop in temperature.

POLAR MARITIME POLAR A maritime polar air mass is cold and stable at its source, with a low absolute humidity but a high relative humidity. The air mass which comes to the UK originates in the far North Atlantic in the Greenland/Iceland areas. As it moves south over the sea it becomes heated in the lower layers and becomes unstable. It also picks up moisture.

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Once it reaches the UK it produces unstable weather with cumulus, cumulonimbus with heavy showers, and sometimes thunderstorms and hail. Visibility is generally good outside of cloud and showers. At night in winter the clouds clear and radiation can lead to an inversion and radiation fog.

RETURNING MARITIME POLAR This is maritime polar air that has reached the UK via an indirect route. It occurs when the air gets deflected by a low pressure system in the North Atlantic. This results in the air first travelling to the south of the North Atlantic before changing direction and approaching the UK from the south-west. The result is that the air becomes unstable as it travels south. Once it has turned north the lower layers become stable, but the upper layers remain unstable. In summer, convection can break through the lower stable layer resulting in Cu, Cb, and thunderstorm activity, with hail and heavy showers.

CONTINENTAL POLAR A continental polar air mass is mainly a winter phenomenon which originates in Siberia. It is very cold, stable, and dry. It brings a cold easterly wind to the UK, with mainly good visibility except for some occasional industrial smoke from Northern Europe. If the air mass originates from further north it may pass over the North Sea on its way to the UK. In this case it will become unstable and increasingly moist, resulting in cumulus clouds and heavy showers on the east coast of England and Scotland. The conditions are not as severe as those associated with maritime polar as the air mass has a much shorter sea passage. In the summer, the high pressure over Siberia replaces low pressure as the land mass heats up. Air originating in this area is then generally referred to as continental tropical. Occasionally there may be a high pressure over Scandinavia. This results in an air mass passing over the North Sea. This sea will now be colder than the surrounding land areas, so the air mass will become cooled and more stable. It will absorb moisture as it passes over the sea. This results in what is referred to as Haar conditions on the coast of east Scotland and north-east England. These conditions are very low stratus with drizzle, advection fog, and bad visibility. In the northeast of England, these conditions are colloquially termed Sea Fret.

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TROPICAL MARITIME TROPICAL A maritime tropical air mass originates in the Azores high in the south of the North Atlantic. It is warm and stable with a high absolute humidity and a moderate relative humidity. As it moves northeast, it cools and becomes more stable with increased relative humidity. On reaching the south-west coast of the UK it produces low stratus and stratocumulus with drizzle and poor visibility. Advection fog occurs over the land areas in winter and early spring and sea areas in late spring and early summer. In summer the increased insolation and convection clears the low cloud resulting in clear skies and good visibility, with occasional fair weather cumulus.

CONTINENTAL TROPICAL A continental tropical air mass originates in North Africa and south-east Europe, plus Siberia in the summer. It is a warm dry air mass which brings clear dry weather with generally good visibility. Occasionally, some dust haze comes north from the Sahara region. Occasionally the air mass picks up some moisture over the Mediterranean and becomes unstable but this moisture is lost as showers over France.

AIR MASS SUMMARY ARCTIC Normal Winter Only Source Region North Polar Ice Cap

Conditions at Source Temperature

Cold Relative Humidity Low Absolute Humidity Low

Modifications

Weather

Moves south and is heated from below, becoming unstable

Arrives over Europe as extremely cold, moist and unstable

Evaporation from sea causes increased dewpoint and RH

CU or CB give heavy snow showers, possibly TS on north and north-east facing coasts Inland clear and cold

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MARITIME POLAR Summer Source Region Sea areas around Iceland and Greenland

Conditions at Source

Modifications

Cold

The air mass is heated as it moves south-east

Relative Humidity High

It becomes unstable over a great depth

Absolute Humidity Low

Moisture evaporates from the ocean so RH increases

Temperature

Weather Widespread CU and CB activity overland with moderate to heavy showers of rain or hail Moderate to severe icing and turbulence in cloud Visibility good outside cloud

Winter Source Region Sea areas around Iceland and Greenland

Conditions at Source Temperature

Cold Relative Humidity High Absolute Humidity Low

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Modifications

Weather

The air mass is heated as it moves south-east to a greater extent than in summer

Day – as above but more severe. Strong gusts and squalls common

Becomes unstable over a great depth

Night – skies clear with possible radiation fog

Moisture evaporates from the ocean so RH increases

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RETURNING MARITIME POLAR Summer Source Region Sea areas around Iceland and Greenland, with a low pressure to the west of Ireland

Conditions at Source Temperature

Cold Relative Humidity High Absolute Humidity Low

Modifications As the air moves south it becomes unstable over a great depth Continuous evaporation raises the dew point and the RH remains high The depression west of Ireland drags the air mass in an anti-clockwise direction towards Europe The movement into colder regions stabilises the air mass in the lower layers and leaves the upper layers unstable Near the surface the air mass has similar characteristics to the mT

Weather Day Warmer than average temperatures with a relatively high RH Insolation heats up the land surfaces As air moves over the heated surfaces the lower layer becomes unstable leading to the development of CU and CB CB produce widespread TS and showers which are most marked in the afternoon Visibility moderate to good Night Convective activity dies out as surface temperatures fall CU may spread into SC Visibility moderate

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RETURNING MARITIME POLAR Winter Source Region Sea areas around Iceland and Greenland, with a low pressure to the west of Ireland

Conditions at Source Temperature

Cold Relative Humidity High Absolute Humidity Low

Modifications

Weather

As the air moves south it becomes unstable over a great depth

As for the mT although medium level instability may be encountered

Continuous evaporation raises the dew point and the RH remains high

CB formation over mountains

The depression west of Ireland drags the air mass in an anti-clockwise direction toward Europe

Medium level ACC may be apparent

The movement into colder regions stabilises the air mass in the lower layers and leaves the upper layers unstable Near the surface the air mass has similar characteristics to the mT

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CONTINENTAL POLAR Normal Winter Only Source Region Siberia, Northern Europe and Scandinavia

Conditions at Source Temperature

Cold Relative Humidity Low Absolute Humidity Low

Modifications

Weather

Moves over the cold winter land of Europe and remains cold, dry, and stable

If the airflow is from the east via continental Europe, the weather is very cold and very dry with no precipitation

If the air passes over the relatively warm North Sea the air is heated from below and the absolute humidity increases

If the airflow is over the North Sea, Then CU and CB can give showers on the east coast of UK (In summer the air mass is rare. With a high pressure over Scandinavia in early summer a North Easterly flow occurs. The air is dry, warm, and stable, leading to Haar conditions)

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MARITIME TROPICAL Summer Source Region The Azores high

Conditions at Source Temperature

Warm Relative Humidity Mod Absolute Humidity High

Modifications As it moves north-east toward Europe, the air is cooled from below, increasing stability Continual evaporation gives a high dew point and high RH

Weather Advection fog likely over the sea Warm moist conditions with some ST or SC, visibility moderate or poor

Winter Source Region The Azores high

Conditions at Source Temperature

Warm Relative Humidity Mod Absolute Humidity High

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Modifications As it moves north-east toward Europe the air is cooled from below, increasing stability Continual evaporation gives a high dew point and high RH

Weather Extensive low SC giving continuous drizzle or light rain Temperatures above the seasonal average, with moderate to poor visibility Advection fog forms if the air flows over a snow covered surface. This flow can also cause a general thaw

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Chapter 16

CONTINENTAL TROPICAL All seasons, but more common in summer Source Region North Africa and South East Europe

Conditions at Source Temperature

Warm Relative Humidity Low Absolute Humidity Low

Meteorology

Modifications Moves north and is cooled from below becoming more stable Movement overland keeps the humidity low

Weather Hot, dry conditions Sometimes hazy with dust from the Sahara Some cloud and precipitation over France if the air mass picks up moisture and becomes unstable over the Mediterranean

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Meteorology

INTRODUCTION The previous chapter discussed air masses, where the properties of temperature and humidity are relatively constant in the horizontal throughout the air mass. Also discussed was how the properties of air masses differ from those of other air masses. The boundary between two air masses with different properties is called a front. Fronts can produce quite active weather. This chapter discusses the characteristics of various types of front.

TYPES OF FRONT Where two air masses meet, the warmer air is less dense and rises up over the colder air. This gives a sloping frontal surface. Initially this chapter explores the three main types of front.

WARM FRONT Where warm air replaces cold air, as shown below, it is called a warm front. Also shown below is the symbol used on synoptic charts to represent the warm front.

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COLD FRONT Where cold air replaces warm air, as shown below, it is called a cold front. Also shown below is the symbol used on synoptic charts to represent the cold front.

QUASI-STATIONARY FRONT Where there is little frontal movement, and neither air mass can be said to be replacing the other, it is termed a quasi-stationary front. A diagram representing this situation is shown below along with the chart symbol for the quasi-stationary front.

PRESSURE SITUATION AT A FRONT As an aircraft flies from a warm air mass into a cold air mass across a front, if it maintains the same true altitude then the colder air means higher density and hence higher pressure. Shown below is the view from above as an aircraft flies along an isobar towards the front. Once it crosses the front, the pressure increase means that the isobars have changed orientation. They bend towards the low pressure. The greater the temperature change at the front, the greater the change in direction of the isobars. As the isobars determine the direction of the wind, one would expect stronger windshear when the temperature change is greater.

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Chapter 17

SEMI-PERMANENT FRONTS OF THE WORLD In both hemispheres there are several semi-permanent or quasi-stationary fronts.

ARCTIC FRONT This is the boundary between arctic and polar air and is found at latitudes above 65°.

POLAR FRONT A polar front is the boundary between polar and tropical air. It is found between latitudes 35° and 65° in the Northern Hemisphere and at around 50° in the Southern Hemisphere. In winter the polar front stretches from Florida to south-west UK. In the summer it retreats north, stretching from Newfoundland to the north of Scotland. In this region, a phenomenon called the Polar Front Depression arises. This is the major factor in the weather patterns found in the UK and Europe and will be discussed later in this chapter.

MEDITERRANEAN FRONT This front only exists in winter when there is low pressure in the Mediterranean. It is the boundary between polar continental or maritime air and tropical continental air.

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INTER-TROPICAL CONVERGENCE ZONE (ITCZ) Originally called the inter-tropical front, the inter-tropical convergence zone was renamed since it is not really a front. It is a boundary zone around 300 nm wide between tropical air masses on either side of the heat equator. Since both masses are tropical, the word ‘front’ is misleading hence the name change. It is also sometimes referred to as the Equatorial Trough or just the Heat Equator. The ITCZ is discussed in considerably more detail in the chapters on Climatology.

CHARACTERISTICS OF FRONTS This section explores characteristics of the warm front and the cold front, including the likely kinds of weather to expect.

WARM FRONT A warm front occurs when warm air replaces cold air. It rides up over the cold air forming a sloping frontal surface with an average gradient of about 1:150. Since warm air is less dense, its progress is retarded by the cold dense air ahead of it. The front therefore travels at about 2/3 of the geostrophic wind speed that would otherwise be expected from the isobar interval along the front.

The gentle slope of the front means that lifting will not be strong enough to form cumuliform cloud. Instead, layer cloud will form. Approaching the front from the cold air side layer clouds appear in the following order: Ci, Cs, As, Ns. A progressively lowering cloud base results. The cirrus cloud will be seen up to 600 nm in advance of the surface position of the front.

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No precipitation will be experienced prior to reaching the altostratus. where you will see virga – precipitation that doesn’t reach the ground. As you approach the nimbostratus the rain will become continuous moderate or heavy.

As the front approaches, the pressure drops, but once it passes the fall will be arrested. However, since the air behind the front is warmer, it settles to a lower value than that preceding the front. The wind veers, but since the passage of the system is quite slow, this change tends to be gradual and doesn’t usually result in problematic windshear.

COLD FRONT A cold front occurs when cold air replaces warm air. The cold air undercuts the warm air because it is more dense and its progress is not impeded by the warm air it replaces. It therefore moves at the geostrophic wind speed. The cold front is much steeper, averaging about 1:50. Sometimes it becomes vertical and even bulges out into the warm air forming a nose-like protrudence. Cold front lifting is much greater hence this front produces cumuliform cloud such as Cu and Cb and possible thunderstorm activity. There may be shelves of nimbostratus or cirrus cloud extending into the cold air when there is a stable layer. Since the slope is much steeper than that of the warm front, the band of associated cloud only spans up to about 200 nm.

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COLD FRONT

CI

CU/CB WARM AIR COLD AIR

NS

As the front approaches, the pressure drops due to the rising air, but after its passage it rises again and settles at a greater value than that preceding the front since the air is now colder. Wind direction changes over a much shorter passage of time than that of the warm front. Hence strong windshear tends to be associated with active cold fronts.

POLAR FRONT DEPRESSIONS These form on the polar front – the boundary between polar and tropical air. At the front the pressure is lower as the warm air rises up over the cold air. Moving away from the front on either side the pressure increases. Obeying Buys Ballots Law the wind flows along the isobars with the low pressure to the left. As the diagram below illustrates the wind on either side of the front flows in opposite directions.

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The above situation causes friction which leads to the formation of waves or ripples along the front. As the size of the ripples increases with increasing wind speed, the warm air bulges into the cold air as shown below.

More warm air flows into the depression, causing the depression to deepen.

The result is a system shaped like a shark fin, with a warm front followed by a cold front. The tip of the shark fin is a low pressure centre.

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Growth of a polar front depression takes about four days. The depression dies away as it fills which typically takes ten days.

The system moves in an easterly direction under the influence of the westerly upper winds, forming an overall picture like that shown on the synoptic chart below. This is known as a westerly wave.

WEATHER ASSOCIATED WITH THE POLAR FRONT DEPRESSION INTRODUCTION As a polar front depression passes over a point, the first weather experienced will be that associated with a warm front before the cold front arrives. The weather in this sector will depend on the stability of the air in this sector, as described below. After the warm front comes the cold front, bringing with it the expected cold front weather. After the cold front passes there will be a period of cold clear weather before the arrival of the next polar front depression. A typical picture is shown in the next figure:

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WARM FRONT The weather associated with the passage of the warm front is summarised in the table below: Warm Front In Advance

At the Passage

In the Rear

Pressure

Steady fall

Fall arrested

Little change or slow fall

Wind

Backing slightly and increasing

Veer and decrease

Steady direction

Temperature

Steady or slow rise

Rise

Little change

Dewpoint

Rise in precipitation

Rise

Steady

Relative Humidity

Rise in precipitation

May rise further if not already saturated

Little change, may be saturated

Cloud

CI, CS, AS, NS in succession, increasing to 8 oktas

Low ST

ST, SC may persist perhaps some CI

Weather

Light continuous from AS becoming moderate continuous from NS

Precipitation eases or stops

Dry or intermittent rain or snow

Visibility

Good except in precipitation

Poor, often mist or fog

Moderate or poor, mist or fog may persist

WARM SECTOR The weather in the warm sector depends on the stability of the air. If the air is stable it is called a kata front. The clouds will be mainly stratiform.

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Unstable air produces cumuliform cloud, with the possibility of embedded CBs.

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COLD FRONT The weather associated with the passage of the cold front is summarised in the table below: Cold Front In Advance

At the Passage

In the Rear

Pressure

Fall

Sudden rise

Rise continues more slowly

Wind

Backing and increasingly becoming squally

Sudden veer, perhaps squall

Further squalls before settling

Temperature

Steady, but falling in pre-frontal rain

Sudden fall

Little change, variable in showers

Dewpoint

Little change

Sudden fall

Little change

Relative Humidity

Rise in pre-frontal precipitation

Remains high in precipitation

Rapid fall as precipitation ceases, variable in showers

Cloud

ST or SC, AC, AS then CB

CB, CU sometimes NS and CI

Lifting rapidly

Weather

Some rain, perhaps thunder

Heavy rain or snow, perhaps hail and thunder

Heavy rain or snow for usually a short period, sometimes more persistent, then fine

Visibility

Moderate or poor, perhaps fog

Good except in showers

Becomes excellent well behind the front

OCCLUSIONS

Consider the polar front depression. The warm front is followed by a cold front. As previously mentioned, the cold front moves at a speed equivalent to the geostrophic wind speed expected by measuring the isobar spacing at the front.

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However, the warm front is moving at only 2/3 this speed. Hence as the polar front depression travels east across the North Atlantic, the cold front gains on the warm front, progressively narrowing the warm sector between the two fronts. Eventually it catches up with the warm front, as shown in the diagram below.

A B

This occurrence is called an occlusion. The two types of occlusions are warm and cold. Which type of occlusion occurs depends on the relative temperatures of the air masses ahead of the warm front (A) and behind the cold front (B). If the air at A is colder, it is termed a warm occlusion; if the air at B is colder, it is a cold occlusion. Both air masses are in fact part of the same air mass, the polar air. However, as an air mass travels, its characteristics change according to the surface over which it passes. In the UK in summer, the most common type of occlusion is the cold occlusion. This is because the air ahead of the warm front has spent a greater length of time over the warmer land, but the air behind the cold front has much more recently been over the cold sea. Conversely, in winter, the sea is warmer than the land; hence the common occlusion type is the warm occlusion.

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Occlusions are shown as below on the synoptic chart. As you can see, for a warm occlusion, it is the warm front which continues along the same line. For a cold occlusion, it is the cold front that continues.

WARM OCCLUSION

As shown above, in a warm occlusion the air behind the cold front is less cold than the air ahead of the warm front. Hence it rides up over the air in front. The warm front extends down to the surface, but the cold front doesn’t. The warm sector is never in contact with the ground. The expected cloud types are the same as with a warm front initially, with cumuliform cloud coming at around the same time as the later warm front clouds. Most of the weather will be experienced before the passage of the surface front.

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COLD OCCLUSION

In the case of a cold occlusion, the air behind the cold front is colder, so it undercuts the air in front of the warm front. The cold front extends to the surface but the warm front does not. Again, the warm sector is no longer in contact with the ground. Expect the same pattern of clouds as for the warm occlusion, but more of the cloud occurs after the passage of the surface front.

BACK BENT OCCLUSION As the polar front depression travels, the occluded section can lag behind, in which case it may bend back on itself. This can give a region of intense weather at the two occluded sections and the low pressure between them.

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INTRODUCTION The chapter on Wind introduced the concepts of the geostrophic wind. The formula for geostrophic wind speed is given again here: V=

PGF

2Ωρ

SIN θ

Just like lower winds, the upper winds are caused by the same forces: pressure gradient force, geostrophic force, and cyclostrophic force. This means that the geostrophic wind formula above also applies to upper winds. Since the wind speed is inversely proportional to air density, wind speed would be expected to increase as height increases and density decreases. For example, the density at 20 000 ft is approximately half that at the surface, thus doubling the wind speed.

THERMAL WIND COMPONENT INTRODUCTION The following diagram shows two columns of air, one cold, and one warm. The surface pressure is the same in both cases – in the example we have used 1020 hPa. Pressure falls more quickly over cold air and less quickly over warm air, so the air pressure over the cold air would be expected to be lower than that at the same height over the warm air.

Meteorology

1010 hPa

1009 hPa

1020 hPa

1020 hPa

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The wind must obey Buys Ballot’s Law: “In the Northern Hemisphere with your back to the wind, the low pressure is on your left.” Hence, in this example, the wind must be blowing off the page. This gives a new law similar to Buys Ballot’s Law: “In the Northern Hemisphere with your back to the upper wind, the cold air is on your left.” In the Southern Hemisphere the cold air is on your right.

CALCULATING THE THERMAL WIND COMPONENT To calculate the thermal wind component use the following formula:

THERMAL WIND SPEED = TEMP GRADIENT PER 100 NM X

ALTITUDE DIFF (FT)

1000

8000 ft 24°C

18°C

200 nm

For example, to calculate the thermal wind component for the above picture case, determine that the temperature gradient is 3°C/100 nm and the thickness of the layer is 8000 ft. Hence the thermal wind component is:

3X

8000 1000

= 24 KT

The direction of the wind depends on the relative positions of the cold and warm air masses. Note:

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This formula is only valid for the 50° latitude. For other latitudes, multiply the answer by sin 50 ÷ sin Latitude. Meteorology

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Chapter 18

UPPER WIND If the geostrophic wind was calm, the upper wind at any level would simply be the thermal wind component over the layer between that level and 2000 ft. Thermal wind component

Geostrophic wind

Upper wind

If there is a geostrophic wind, then the upper wind will be the vector sum of the geostrophic wind and the thermal wind component. Resolve this graphically or by using the CRP-5. Continuing on from the previous example, assume a geostrophic wind of 040/20 with cold air to the north, in the Northern Hemisphere. The following steps show how to calculate the upper wind for 10 000 ft using a CRP-5. STEP 1

The cold temperature is to the north. Using Buys Ballot’s Law with the wind behind, the low temperature is on the left. The wind direction must be from 270°.

STEP 2

Having already calculated the thermal wind speed at 24 kt, the thermal wind component is 270/24 kt.

STEP 3

Set the 2000 ft wind velocity using the zero line.

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STEP 4

Set the TWC. The origin of the TWC is the end of the geostrophic wind component.

STEP 5

Move the end of the TWC component to the centre line and read off the upper wind at 10 000 ft — 325/20.

Note:

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If the geostrophic and the thermal wind component are in opposite directions, the wind first decreases in speed as height increases, becoming calm before reversing in direction and increasing in speed.

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GLOBAL UPPER WINDS The diagram below shows the Earth with warm air over the equator and a decreasing temperature as we move towards the poles. In the Northern Hemisphere the wind keeps the low temperature to its left, in the Southern Hemisphere it keeps the low temperature to its right. In both cases this gives a westerly wind.

Exceptions to the rule occur in the tropics and over the poles, where the upper winds are easterly.

JET STREAMS INTRODUCTION A jet stream is a wind greater than 60 kt in speed, which manifests itself as a long corridor of wind with typical dimensions of 1500 nm in length, 200 nm in width and 12 000 ft in depth. They are caused by large temperature differences in the horizontal.

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The wind speed is fastest at the core and decreases with movement away from the core. 60 kt

80 kt 100 kt 120 kt

Speeds in excess of 100 kt are quite common, but it is rare for jet streams to be faster than 200 kt. However, jets of 300 kt have been reported on occasion. These extreme examples tend to occur in the east Asia/Japan area.

COMMON JET STREAMS The table below shows the common global jet streams. Latitude

Pressure Level

Polar front jet stream

45° to 65° N/S

300 hPa – 30 000 ft

Sub-tropical jet stream

20° to 40° N/S

200 hPa – 45 000 ft

Equatorial jet stream

10° to 15° N/S

100 hPa – 55 000 ft

Polar jet stream

70° to 80° N/S

50 hPa – 75 000 ft

Details of the equatorial and the polar jet stream are not required for the course, but this chapter goes into more detail about the Sub-tropical and the Polar Front jet streams.

SUB-TROPICAL JET STREAM These occur above the sub-tropical anti-cyclones and are caused by the circulation of the Hadley cells. The Hadley cells are a circulation which starts with lifting over the heat equator due to surface heating. When the air reaches the tropopause it flows away from the equator to higher latitudes. At approximately 30° latitude, the air is cooled such that it starts to descend. Where it reaches the surface it forms the sub-tropical anti-cyclones. It then flows into the low pressure at the heat equator. The following diagram shows the circulation of air on the Earth.

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Polar cell Ferrel cell

Hadley cell

The sub-tropical jet stream forms when air from the Hadley cells meet air from higher latitudes. Due to the large amount of air, not all of it descends; some of it is forced to flow horizontally. In both the Northern and Southern Hemispheres geostrophic force turns it to the right. In both cases this results in a westerly jet. PGF

GF

PLAN VIEW

NH

PGF GF

Heat equator

Heat equator

SH

The sub-tropical jet streams exist all year round but move as the heat equator moves. In winter they exist in the latitude band 25° — 40° and in the summer are found in the latitude band 40° to 45°.

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Upper Winds

POLAR-FRONT JET STREAM Like the name suggests, a polar-front jet stream occurs on the Polar Front. The following diagram shows the position of the jet stream in cross section and in plan view.

The diagram shows that the jet stream forms in the warm (tropical air) just below the warm air tropopause. In the plan view the jet stream appears to be in cold sector. However, it is the surface position of the fronts that is shown. The fronts slope so in fact the jet is in the warm air. Unlike the sub-tropical jet stream, the polar front jet stream is not in a constant westerly direction. It follows the patterns of the polar front depressions and forms a zig-zag shape which is westerly on average. They are less permanent than the sub-tropical jets, tending to die out a bit in summer. Average speeds in summer are 60 kt; in winter, 80 kt. Like the sub-tropical jet stream, the polar front jet streams change position with the movement of the heat equator. Approximate positions are between 40°N and 65°N and at around 50°S.

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Upper Winds

Chapter 18

WINDS AROUND A POLAR FRONT DEPRESSION This chapter has explored the polar front depression and the pattern of isobars around it. The 2000 ft wind follows the isobars in an anti-clockwise direction around the low pressure centre, as shown in the diagram.

Super-imposed onto this is the polar front jet stream, which obeys the rule of always keeping the cold air to its left. As a result, the 2000 ft wind and the upper wind often come from different directions. This is summarised below: Position

2000 ft wind

Upper wind

Trend

Ahead of the warm front

South-westerly

North-westerly

Veer and increase

In the warm sector

Westerly

Westerly

Increase

Behind the cold front

North-westerly

South-westerly

Back and increase

CLEAR AIR TURBULENCE The windshear within and around jet streams leads to friction within the atmosphere. This causes turbulence known as clear air turbulence (CAT), due to the fact that it is not caused by clouds or by proximity to the ground. The most severe CAT is found level with the core of the jet on the cold air side. A secondary area of severe CAT is found above the core, above the warm air tropopause.

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Upper Winds

If CAT associated with a polar front jet stream in the Northern Hemisphere is experienced, descend and turn to the south. This brings the aircraft into the warm air and away from the strongest turbulence.

IDENTIFICATION OF JET STREAMS It is usually impossible to identify a jet stream visually. However, if the air is moist, there may be a trail of cirrus cloud associated with the jet stream, as shown below.

This cirrus is caused by a lowering of pressure and temperature around the jet stream, due to the high velocity of the air. This cools the air to its dewpoint causing some water vapour to sublimate out as ice crystals. Another way to identify a jet stream is by looking at meteorological charts, like those discussed below. Other important charts are discussed in the chapter on Upper Air Charts.

CONTOUR CHARTS For lower winds, use synoptic charts. These show isobars (lines of constant pressure) and from this the direction of the wind is predictable. For upper winds a different system is used. Rather than using a chart for a given height above mean sea level and showing the different pressures on the chart, charts with constant pressure are used and the lines drawn join places of constant height above mean sea level at which that pressure occurs. This is useful for high altitude flying as flights are conducted at flight levels/pressure altitudes, that is, the aircraft flies along a line of constant pressure. Common charts in use are as follows: Pressure (hPa)

Equivalent Pressure Altitude (feet)

700 500 300 250 200 150

10 000 18 000 30 000 34 000 39 000 53 000

The lines joining places of equal height are called contour lines and the heights are expressed in one of two ways. The number may represent the height in 100s of feet or the height in decametres (10s of metres). 18-10

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Chapter 18

A line with a low value means that the pressure for which the chart is produced is found at a lower height, whereas a high value means the pressure is found at a greater height. As can be seen from the following diagram, this means that areas of low contour heights are areas of low pressure. 31 000 ft

30 000 ft

< 300 hPa

> 300 hPa

300 hPa 29 000 ft

Since the wind follows Buys Ballot’s law, it flows with the low contour lines to its left in the Northern Hemisphere. As for a synoptic chart, the closer the contour lines, the stronger wind.

THICKNESS CHARTS Another chart used to discern wind direction is the thickness chart which shows the thickness of the layer between two given pressure values. As shown in the diagram below, a low thickness value is associated with cold air and a high value is associated with warm air. Lines of constant thickness are called isopleths. 500 hPa

Cold air – low thickness value

Warm air – high thickness value

1000 hPa

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Upper Winds

In the Northern Hemisphere the thermal wind keeps the cold air to its left, hence it travels parallel to the isopleths keeping the low value isopleths to its left. In the Southern Hemisphere the low value isopleths are to the right. If the isopleths are closely spaced, this indicates a steep temperature gradient and hence stronger winds.

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Meteorology

WINDSHEAR The following meteorological factors can cause windshear: 1. 2. 3. 4. 5. 6. 7. 8.

Inversions Mountain waves and rotors Katabatic winds (fall winds) Sea breeze fronts Air mass fronts CB cloud Low level jet Jet streams

DEFINITIONS AND THE METEOROLOGICAL BACKGROUND In discussing windshear it is not easy to find a definition which satisfies both meteorologist and pilot. At its simplest, windshear is a change in wind direction and/or speed in space, including updraughts and downdraughts. Despite the emphasis on the windshear hazard in recent years, there are still some who argue that aviators have lived with windshear since the dawn of aviation, seeing it as an extreme form of wind gradient, which would itself fit this definition.

DEFINITION Variations in vector wind along the aircraft flight path of a pattern, intensity, and duration so as to displace an aircraft abruptly from its intended path requiring substantial control action.

LOW ALTITUDE WINDSHEAR Low altitude windshear is windshear along the final approach path or along the runway and along the takeoff and initial climb out flight path. Further refinement offers: ¾ ¾

¾

Vertical windshear as the change of horizontal wind vector with height, as might be determined by two or more anemometers at different heights on a mast; Horizontal windshear as the change of horizontal wind vector with horizontal distance as might be determined by two or more anemometers mounted at the same height at different points along a runway; Updraught/downdraught shear as changes in the vertical component of wind with horizontal distance.

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Windshear and Turbulence

Setting aside the basic windshear definition above, the other definitions allow for changes of vector wind from the relatively minor event upwards. The essence of the windshear with which this chapter is concerned is spelt out by the basic definition with its emphasis on abrupt displacement from the flight path and the need for substantial control action to counteract it. A windshear encounter is a highly dynamic and potentially uncomfortable event; to think of windshear as an aggravated form of wind gradient is unwise. Windshear can strike suddenly and with devastating effect, sometimes beyond the recovery powers of experienced pilots flying modern and powerful aircraft. An encounter may cause alarm, a damaged landing gear, or a total catastrophe. The first and most vital defence is avoidance.

METEOROLOGICAL FEATURES The most potent examples of windshear are associated with thunderstorms (cumulonimbus clouds), but windshear can also be experienced in association with other meteorological features such as the passage of a front, a marked temperature inversion, a low-level wind maximum, or a turbulent boundary layer. Topography or buildings can exacerbate the situation; particularly in a strong wind.

THUNDERSTORMS The chapter on Meteorological Notes describes thunderstorm formation and how the wind flows in and around the thunderstorm which causes the most severe windshears. Diagrams do no justice to the violence of totally dynamic and unpredictable thunderstorms with turbulence, hail, windshear, and lightning as separate or joint hazards. Shears and draughts may strike from all angles and are certainly not limited to the horizontal or vertical; an assessment of the aircraft’s actual angle of attack relative to some thunderstorm wind flows is difficult to make, which in turn makes the risk of a stall harder to gauge. This is significant if a thunderstorm is encountered on the approach or following take-off.

FRONTAL PASSAGE Fronts, whether warm, cold, or occluded, vary in strength. It is only well developed active fronts, with narrow surface frontal zones and with marked temperature differences between the two air masses, which are likely to carry a risk of windshear. Warning signs to look out for include sharp changes in wind direction indicated on the weather charts by an acute angle of the isobars as they cross the front, a temperature difference of 5º C or more across the frontal zone, and the speed of movement of the front, especially if 30 kt or more. It should be mentioned that windshear is possible in fronts which are slow moving, stationary or even reversing direction. The passage of a vigorous cold front poses the greater risk though, relative to a warm front, as the period of windshear probability is likely to be much shorter and occurs just after the surface passage of the front. With a warm front, the effect precedes the passage and is more prolonged. To illustrate the potential severity of frontal windshear, there is the case of a twin jet aircraft caught by the passage of a cold front while flaring to land. Within about ten seconds, the wind shifted from 230/10 kt to 340/16 kt, so that a 10 kt crosswind from the left and slight tail wind changed to an 8 kt crosswind from the right with 14 kt headwind. The pilot, finding directional control for landing to be difficult, wisely carried out a missed approach from a very low level.

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Chapter 19

This is a classic case of horizontal windshear. A sea-breeze front may occasionally present a hazard; for example if it impinges on a thunderstorm it may significantly alter the outflow from the storm; a catastrophic accident in the USA in 1975 involved such a feature.

INVERSIONS Vertical windshear is nearly always present in the boundary layer, but this normally involves a gradual change in the wind with which pilots are well familiar. A hazard exists, however, when an unexpectedly strong vertical shear develops. This can occur broadly in two situations: ¾

¾

A low-level jet (more accurately referred to as a low-level wind maximum) can form just below the top of, or sometimes within, a strong radiation inversion which may develop at night under clear skies. Other low-level jets may develop in association with a surface front, particularly ahead of cold fronts; On occasions, low-level inversions develop and decouple a relatively strong upper flow from layers of stagnant or slow moving air near the surface. Windshear may be pronounced across the interface.

TURBULENT BOUNDARY LAYER Within the boundary layer, turbulence becomes a windshear hazard in two different situations: ¾ ¾

Strong surface winds are generally accompanied by large gusts and lulls (horizontal windshear). Roughly speaking, the stronger the mean wind, the greater the gust or lull. Thermal turbulence (updraughts and downdraughts) is caused by intense solar heating of the ground, which is more common in hot countries, but can occur anywhere on a hot sunny day.

TOPOGRAPHICAL WINDSHEARS Either natural or man-made features affect the steady state wind flow and cause windshears of varying severity. The strength and direction of the wind relative to the obstacle are significant and a change of direction of relatively few degrees may appreciably alter the residual effect. The flow of wind across a mountain range is a simple large scale example, with waves and possibly a rotor forming on the leeside. Wind blowing between two hills or along a valley, or even between two large buildings may be funnelled, thus changing direction and increasing in speed, or a strong flow may be heavily damped. Either way, this creates the possibility for shear, with sudden changes of wind vector becoming a hazard. Usually local effects become well known and predictable, with warnings given on aerodrome approach plates (e.g. Gibraltar). Large airport buildings adjacent to busy runways can create hazardous local effects and typical windshear problems, such as loss of airspeed and abrupt crosswind changes, causing upsets to airliner-size aircraft which have been near to major accidents. On smaller aerodromes, lines of trees can mask the wind and cause problems at a late stage in the approach. These incidents usually contribute to a pilot’s experience, but damaged landing gear can result from wind effects of greater significance than a steep wind gradient or low-level turbulence alone. Meteorology

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Chapter 19

Windshear and Turbulence

THE EFFECTS OF WINDSHEAR ON AN AIRCRAFT IN FLIGHT Windshear affects aircraft in many different ways and during an encounter the situation is constantly changing, especially during the more dynamic thunderstorm windshears. Particular types of aircraft vary in their reaction to a given shear; a light high-wing piston-engined aircraft may react in a totally different way to a swept-wing four-engine jet. It is not easy to describe the effects in general terms, as they do not apply universally. The notes which follow only attempt to describe stylised windshears and their progressive effects. Windshear can, of course, be encountered at any height and the effects will be similar. The windshear encounter at low level which is a great hazard; it is this which must be borne in mind when the effects are described. An understanding of windshear is difficult, unless the relationship of an aeroplane in a moving air mass to its two reference points is appreciated. One reference is the air mass itself, the other is the ground. In a windshear encounter it is not only the magnitude of the change of wind vector that counts but the rate at which it happens. For example, an aeroplane at 1000 ft agl may have a headwind component of 30 kt, but the surface wind report shows that the headwind is only 10 kt on the runway. That 20 kt difference may taper off evenly with the effect of a reasonable wind gradient. However, it may be noticed that the 20 kt differential still exists at 300 ft and the change, when it comes, will clearly be far more sudden and its effects more marked. Shear implies a narrow borderline and the 20 kt of wind speed may well be lost over a vertical distance of 100 ft as the aircraft descends from 300 to 200 ft.

If the pilot wanted a stabilised approach speed of 130 kt, the power would be set according to conditions, providing the required airspeed and rate of descent. On passing through the shear line, the loss of airspeed is sudden, but the inertia of the aircraft at first keeps it at its original groundspeed of 100 kt and power is needed to accelerate the aircraft back to its original airspeed. This takes time; meanwhile the aircraft having lost 20 kt of airspeed, sinks faster as a substantial amount of lift has also been lost. 19-4

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The headwind was a form of energy and when it dropped 20 kt, an equivalent amount of energy loss occurred. One source available to balance that loss is engine power; this arrests the increased rate of descent and starts the process of accelerating back to the approach reference speed.

The opposite effect can be illustrated using similar conditions, but seen from the point of view of an aeroplane taking off. Initially take-off along the runway and into the second segment of the climb, with a 10 kt headwind, the wind becomes a 30 kt headwind after encountering the shear between 200 and 300 ft. Assuming a target climbing speed of 120 kt, the effect of a sudden transition through the shear line into a 20 kt increase of headwind, increases the lAS by the same amount until the momentum of the groundspeed is lost. This is a case of temporary energy gain, with lift added so the aircraft climbs more rapidly. This example shows the windshear as being positively beneficial and it is true to say that a rapid increase in headwind (or loss of tailwind), because they are “energy gains,” temporarily enhances performance. It may help with understanding windshear to see it in terms of energy changes, when it is readily apparent that the windshear which causes temporary loss of energy (sudden drop of headwind or increase in tailwind, and downdraughts) is the main danger at low altitude. The effect of a downdraught is not always easy to visualise, as we normally think of the aeroplane in relation to airflow along the flight path even when climbing or descending. It is now necessary to envisage flying suddenly from a horizontal flow into air with a vertical component. In turbulent conditions, air in motion may strike the aeroplane from an angle and the situation may be constantly changing. However, in thunderstorms, substantial shafts of air which can be moving either up or down may be encountered with no warning; such shafts may be virtually side by side and the shear very marked and violent. Entering a vertical updraught or downdraught from a horizontal airflow, the aeroplane's momentum at first keeps it on its original path relative to the new direction of flow. In addition to a loss of airspeed, also realise that the shift of relative airflow affects the angle of attack of the wing, Meteorology

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Chapter 19

Windshear and Turbulence

which may result in either increased or decreased lift. A slight increase of angle may not cause much concern. However, if the aircraft is already on the approach with a high angle of attack, an increase might put the wing near the stall and any decrease will bring about a loss of lift. Normally, below 1000 ft, the risk of a downdraught is more likely than an updraught.

Having described the combination of increasing headwind followed by downdraught followed by increasing tailwind consider, that this is the sequence which might be encountered in a microburst on the approach or following a take-off. This may be a rare occurrence in the United Kingdom or Europe, but it needs to be appreciated by those flying to the USA. Even on this side of the Atlantic, an encounter with a downburst, a headwind followed by downdraught, or a downdraught followed by tailwind is possible and may cause problems.

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Windshear and Turbulence

A

B

C

Chapter 19

Energy gain Increasing headwind Airspeed rising Rate of descent reduced Tendency to go high on glide path Energy loss Reducing headwind and downdraught Airspeed falling Rate of descent increased Tendency to go low on glide path Energy loss Increasing tailwind Airspeed still falling Rate of descent checked by missed approach Success depends upon power, height and speed reserves available

An aircraft, approaching on a 3° ILS glidepath, might see ahead an area of heavy rain. Ideally this might alert the pilot to possible danger, and a missed approach could be executed in good time, though even this might take the aircraft into the microburst. Then, however, the aircraft will have gained precious extra height. Given that the approach continues towards the microburst, the leading edge can produce a rapidly increasing headwind; the airspeed increases and the aircraft goes high on the glidepath. The likely reaction is to reduce power to increase the rate of descent and adjust attitude to reduce airspeed. Then comes the downdraught when the rate of descent increases rapidly and the aircraft passes through and below the glidepath, still possibly with the nose high and the power low. Power is re-applied, but it takes time to spool up the engines, meanwhile the aircraft passes from downdraughts to increasing tail wind with the airspeed dropping. The rate of descent is not checked and the nose is high while power increases. No figures are attached to this description, merely the likely sequence of events. A very strong microburst has a more pronounced effect on the rise and fall of airspeed and extremes of rate of descent. The power reserves available and the rate at which they can be applied and built up to give maximum thrust, determine the aircraft’s ability to counteract the energy loss of downdraught and increasing tailwind. Strong wind buffeting, the lashing of rain, and possibly blinding flashes of lightning may accompany this dynamic sequence of events. If this is a black picture, it matches the descriptions of those that have flown through a microburst and would probably be echoed by some who have tried but failed to fly through one. The aim must be to avoid severe windshear at all costs. It might be thought that an encounter with windshear from a microburst after take-off is likely to be less hazardous than when approaching to land. The aircraft is at high power and is not constrained by the need to hold a precise glide path. The temporary energy gain from meeting the increasing headwind, with a burst of higher air-speed and rate of climb may seem positively beneficial.

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Windshear and Turbulence

The transition to downdraught soon kills any rise in airspeed; it may even drop. The rate of climb may lessen or even show a rate of descent enhanced by the shift to increasing tailwind, when the airspeed (with the aircraft close to the ground) may drop further. Any benefits of high power may be balanced by higher aircraft weight. There may be a small power reserve in hand and this may, or may not, be sufficient to enable the aircraft to fly through the microburst or downburst, together with other measures described later.

TECHNIQUES TO COUNTER THE EFFECTS OF WINDSHEAR Windshear can vary enormously in its impact and effect. There is as yet no international agreement on definitions for grading windshear, but clearly some shears are more severe and consequently more dangerous than others. In discussing guidance on countering the effects of windshear, one must inevitably deal with the “worse case” situation. If the golden rule of avoidance fails for whatever reason, it is impossible to predict at the first stages of a windshear encounter how severe it will be and it is not bad advice to suggest that recovery action should anticipate the worst. No pilot who studies the meteorological situation carefully in advance and updates his knowledge with the latest reports during flight should be taken totally by surprise by windshear. If thunderstorms are forecast in the vicinity of the planned destination and then are reported as being active and are seen on the weather radar or visually, then a mental Windshear Alert should register. At this stage, depending on the evidence, a diversion might be considered, as windshear avoidance is the safest course. If it is decided to continue to the destination, then the crew should consider a few basic measures to anticipate a possible windshear encounter. One of these is to increase the airspeed on the approach. The amount of airspeed increase to be recommended is less easy to assess, as what might be suitable for a light twin-piston engined aeroplane might be quite inappropriate for a swept-wing jet. Rule of thumb guidance includes adding half the headwind component of the reported surface wind to VAT, or, half the mean wind speed plus half the gust factor, in each case up to a maximum of 20 kt. This may be satisfactory for a strong but turbulent wind, but may not meet the thunderstorm case, where it is not uncommon for light and variable winds to precede the onslaught of a gust front or downburst. The unpredictability of windshear is such that, if it does not materialise, the aircraft can arrive at threshold with excessive speed to be shed and that could be embarrassing on a short runway. Because the amount of airspeed “margin” is related to the aircraft's acceleration potential, the relatively slow propeller driven aircraft is probably at an advantage over a faster jet aircraft. Remember that the rate of shear is important and the aircraft which penetrates the shear zone slower experiences a lower rate of shear — the rapid response of propeller driven airflow over a wing also helps. The windshear encounter which produces a sudden increase in airspeed (temporary energy gain) on the approach destabilises it to a greater or lesser extent, which calls for some control adjustment. The normal reaction to the rise above the glidepath is to reduce power to regain the glidepath and as the deviation was sudden, the power reduction will probably be more than just a slight one. The pilot must then be alert to the need to re-appIy power in good time to avoid dropping below the glidepath. If the wind component then stabilises, leaving the aircraft merely with a stronger headwind, a further power adjustment will be needed to a higher setting than the initial one which had given a stable airspeed and rate of descent.

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Chapter 19

When an aircraft on the glidepath in the later stages of an approach runs into an “energy loss” windshear, it can be much more hazardous. A building or line of trees obstructing the windflow might cause the shear, and the resulting drop in the wind speed might bring about a very sudden drop in airspeed with a consequent increase in the rate of descent. To avoid a heavy and premature landing, a rapid and positive increase in power is needed. Another likely effect is for the nose to drop initially, requiring a check with an increase in pitch attitude - but not so much that this causes a further loss of airspeed; as always power and attitude adjustments must be coordinated. These actions may enable the aircraft to regain the glidepath and continue the approach. Anticipate the power reduction to avoid flying through the glidepath and expect to set slightly less power than that originally used, to continue the approach. If the approach has been badly destabilised, full missed approach action may be the wiser and safer option, with a second approach made with an airspeed “margin” to counter the anticipated windshear effect. Vital Actions to counter loss of airspeed caused by windshear near the ground: ¾ ¾ ¾ ¾

Briskly increase power (full go round power if necessary) Raise the nose to check descent Co-ordinate power and pitch Be prepared to carry out a missed approach rather than risk landing from a destabilised approach

To counter the effect of a downburst or microburst on an approach or take-off calls for more stringent measures. It must be stressed that any well-founded report of either phenomenon must be treated seriously and the approach or take-off delayed until the danger has passed. If there is an inadvertent encounter, the aircraft may be affected by wind from any flank by the descending and outflowing column of air, but again the worst case will be considered - entry on one side, through the centre and exit through the other side. It will be a turbulent and unpleasant experience which can tax the abilities of the most skillful pilots. The presence of thunderstorms should be known and obvious, so the increase in speed caused by the rising headwind should be seen as the forerunner of a downburst or microburst; any hope of a stabilised approach is abandoned and a missed approach is the only safe course of action the technique is to make it as safe as possible. The initial rise in airspeed and rise above the approach path should be seen as a bonus and capitalised. Without hesitation, increase to go-around power, being prepared to go to maximum power if necessary, select a pitch angle consistent with a missed approach, typically about 15° and hold it against turbulence and buffeting. The next phase may well see the initial advantages of increased airspeed and rate of climb rapidly eroded. The downdraught now strikes, airspeed may be lost and the aircraft may start to descend despite the high power and pitch angle. It will be impossible to gauge the true angle of attack, so there is a possibility that the stick shaker (if fitted) may be triggered; only then should the attempt to hold the pitch angle normally be relaxed.

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Windshear and Turbulence

The point at which downdraught begins to change to increasing tailwind may well be the most critical period. The rate of descent may lessen, but the airspeed may still continue to fall; the height loss may have cut seriously into ground obstacle clearance margins. Given that maximum thrust is already applied, as an extreme measure if the risk of striking the ground or an obstacle still exists, it may be necessary to increase the pitch angle further and deliberately raise the nose until stick shaker is felt. Then an easing forward of the control column to try and hold this higher pitch angle should be made, until the situation eases with the aircraft beginning to escape from the effects of the microburst. When there is an indefinite risk of shear, it may be possible to use a longer runway, or one that points away from an area of potential threat. It may also be an option to rotate at a slightly higher speed, provided this does not cause undue tyre stress or any handling problems. The high power setting and high pitch angle after rotate have already put the aircraft into a good configuration should a microburst then be encountered. The aircraft is, however, very low where there is little safety margin and the ride can be rough. If there is still extra power available, it should be used without hesitation. Ignore noise abatement procedures and maintain the high pitch angles, watching out for stick shaker indications as a signal to ease the controls forward. In both approach and take-off cases, vital actions are: ¾ ¾ ¾

Use the maximum power available as soon as possible. Adopt a pitch angle of around 15° and try and hold that attitude. Do not chase airspeed. Be guided by stick shaker indications when holding or increasing pitch attitude, easing the back pressure as required to attain and hold a slightly lower attitude.

Windshear warning can be provided in several ways: ¾ ¾ ¾ ¾ ¾

Meteorological warning ATS warning Pilot warning On board pre-encounter warning On board encounter warning and/or guidance

ICAO DEFINITIONS The following windshear reporting system is used to give pilots a common understanding of the problem of windshear:

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Intensity of windshear

Vertical windshear/100 ft

Horizontal windshear/2000 ft

Up or down draught

Effect on flight altitude

Light Moderate Strong Severe

0 – 4 kt 4 – 8 kt 8 – 12 kt > 12 kt

0 – 4 kt 4 – 8 kt 8 – 12 kt > 12 kt

0 – 4 kt 4 – 8 kt 8 – 12 kt > 12 kt

Small Significant Hazardous Highly dangerous

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NATURE OF TURBULENCE The small-scale vortices that constitute turbulence, form: ¾ ¾ ¾

When the air-flow is disturbed by an obstruction, (e.g. the ground surface). When two air-flows of different direction and/or speed adjoin each other. When the speed of the air changes rapidly within the same air-flow.

Turbulence transfers momentum from one volume of air to another by exchanging small amounts of air. The wind speed, for example, can be accelerated or retarded. To describe this we use the words gust and lull. Gust is an increase of the wind speed of short duration. Lull is a short-lived decrease of the wind speed.

TURBULENCE, METEOROLOGICAL FACTORS Windshear caused by ascending and subsiding thermals, convection, results in the aircraft bouncing along through the thermals, which creates thermal turbulence.

THERMAL TURBULENCE Thermal turbulence is generated by heated thermals ascending through the air, causing a return flow at the sides. During a flight, this causes severe bumps, and during the landing phase the upand downdrafts may disturb the approach. Thermal turbulence is marked over warm surfaces, such as tarmac, concrete, mountains, sand, or dark ground surfaces. As a matter of fact, it is often a question of a combination of up- and down-winds with a clear local character. Thermal turbulence occurs: ¾ Above land in the daytime and generally in association with convective clouds. ¾ In the autumn/winter above seas by day and night. Except during the landing phase thermal turbulence does not constitute any major problem in Northern Europe. In extreme cases, however, the aircraft can be bumped into exceptional flight attitudes, and it may be rather uncomfortable to fly in areas with severe thermal turbulence

MECHANICAL/ FRICTIONAL TURBULENCE Windshear and turbulence occur because of friction against the ground surface at high wind speeds. The mechanical effect depends on the structure of the surface and the wind speed, see the table below. The consequence is very uncomfortable flight up to 2000 — 3000 ft above the terrain with the aircraft being subjected to accelerations of several “g”.

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Windshear and Turbulence

Criteria of mechanical turbulence: Surface

Wind< 15 kt

15-30 kt

>30 kt

Sea

Light

Moderate

Mod/severe

Plain

Only light

Moderate

Severe

Broken terrain

Light -moderate

Severe

Extreme

Mechanical turbulence occurs throughout the year, when the prevailing wind is high. The more unstable the air, the more severe the turbulence - this applies to both thermal and mechanical turbulence.

MOUNTAIN WAVES FLIGHT OVER AND IN THE VICINITY OF HIGH GROUND Air flow is more disturbed and turbulent over high ground than over level country and the forced ascent of air over high ground often leads to the formation of cloud on or near the surface. This sometimes extends through a substantial part of the troposphere if the air is moist enough. Forced ascent also increases instability so that thunderstorms embedded in widespread layer cloud may occur over high ground, even when no convective clouds form over low ground. When the air is generally unstable, cloud development is greater, icing in the clouds is more severe and turbulence in the friction layer and in cloud is intensified over high ground. The air flowing over high ground may be so dry that, even when it is forced to rise, little or no cloud is formed. The absence of cloud over high ground does not imply the absence of vertical air currents and turbulence. Strong down currents are caused by the air descending the lee slope and it is, therefore, especially hazardous to fly towards high ground when experiencing a headwind. On some occasions, the disturbance of a transverse airflow by high ground creates an organised flow pattern of waves and large scale eddies in which strong up-draughts and downdraughts and turbulence frequently occur. These organised flow patterns are usually called mountain waves but may also be referred to as lee waves or standing waves. These can be associated with relatively low hills and ridges as well as with high mountains.

CONDITIONS Conditions favourable for the formation of mountain waves are: ¾ ¾ ¾ ¾

A wind blowing within about 30° of a direction at right angles to a substantial ridge. The wind must increase with height with little change in direction (strong waves are often associated with jet streams). A wind speed of more than 15 kt at the crest of the ridge is also usually necessary. A marked stable layer (approaching isothermal, or an inversion), with less stable air above and below, between crest level and a few thousand feet above.

Mountain wave systems may extend for many miles downwind of the initiating high ground. Satellite photographs have shown wave clouds extending more than 250 nm from the Pennines in the UK; 50 to 100 nm is a more usual extent of wave systems in most areas. Wave systems, on occasion, extend well into the stratosphere. 19-12

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The average wavelength of mountain waves in the troposphere is about 15 miles, but much longer waves occur. Derive a good estimate of the wave length using the following formula: Wavelength = mean troposphere wind ÷ 7 Disturbances in the stratosphere are often irregular features located very near or just over the initiating mountains. When waves to the lee of the high ground are evident, their length is usually greater than in the troposphere. A typical wavelength is 15 nm, but wavelengths of 60 nm have been measured. The amplitude of waves is much more difficult to determine. In general, the higher the mountain and the stronger the airflow, the greater the resulting disturbance. The most severe conditions occur when the natural frequency of the waves is tuned to the ground profile. In the troposphere, the double amplitude (peak-to-trough) of waves is commonly 1500 ft with vertical velocities about 1000 ft/min. However, double amplitudes of about 20 000 ft and vertical velocities aver 5000 fpm have been measured.

VISUAL DETECTION OF MOUNTAIN WAVES The clouds which owe their appearance to the nature of wave flow are a valuable indicator of the existence of wave formation. Provided there is sufficient moisture available, the ascent of air leads to condensation and formation of characteristic clouds. These clouds form in the crest of standing waves and therefore remain more or less stationary. They occur at all heights from the surface to cirrus level.

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Windshear and Turbulence

Lenticular Clouds provide the most unmistakable evidence of the existence of mountain waves. They form within stable layers in the crusts of standing waves. Air streams through them, the clouds forming at the up-wind edges and dissipating downwind. They have a characteristically smooth, lens-shaped outline and may appear at several levels, sometimes resulting in an appearance reminiscent of a stack of inverted saucers. Lenticular clouds usually appear up to a few thousand feet above the mountain crests, but are also seen at any level up to the tropopause and even above. Mother-of-pearl clouds, seen on rare occasions over mountains, are a form of wave-cloud at an altitude of 80 000 ft. Air flow through these clouds is usually smooth unless the edges of the cloud take on a ragged appearance, which is an indication of turbulence. Rotor Clouds, or roll clouds, appear as ragged cumulus or stratocumulus parallel to and downwind of the ridge. On closer inspection, these clouds rotate about a horizontal axis. Rotor clouds are produced by local breakdown of the flow into violent turbulence. They occur under the crests of strong waves beneath the stable layers associated with the waves. The strongest rotor normally forms in the first wave downwind of the ridge and is usually near or somewhat above the level of the ridge crest. There are usually no more than one or two rotor clouds in the lee of the ridge. Cap Clouds form on the ridge crest. Strong surface winds which are commonly found sweeping down the lee slope may extend the cap cloud down the slope. Although cloud often provides the most useful visible evidence of disturbances to the airflow, other cloud systems, particularly frontal cloud, sometimes obscures the characteristic cloud types.

TURBULENCE TURBULENCE AT LOW AND MEDIUM LEVELS A strong wind over irregular terrain produces low-level turbulence which increases in depth and intensity with increasing wind speed and terrain irregularity. In a well developed wave system, the rotor zone and the area below are strongly turbulent and reversed flow is often observed at the surface. Strong winds confined to the lower troposphere, with reversed or no flow in the middle and higher troposphere, produce the most turbulent conditions at low levels. These are sometimes accompanied by rotor streaming, comprised of violent rotors which are generated intermittently near lee slopes and move downwind. These lowlevel travelling rotors are distinct from the stationary rotors which form at higher levels in association with strong mountain waves.

TURBULENCE IN THE ROTOR ZONE Rotors lie beneath the crests of lee waves and are often marked by roll-cloud. The most powerful rotor lies beneath the first wave crest down-stream of the mountains. Rotors give rise to the most severe turbulence found in the air flow over high ground. On occasions it may be as violent as that in the worst thunderstorms.

TURBULENCE IN WAVES Although flight in waves is often remarkably smooth, severe turbulence can occur. The transition from smooth to bumpy flight can be abrupt. Very occasionally, violent turbulence may result, sometimes attributed to the wave breaking.

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Chapter 19

TURBULENCE AT HIGH LEVELS (NEAR AND ABOVE THE TROPOPAUSE) TURBULENCE NEAR THE JET STREAM Turbulence in jet streams is frequently greatly increased in extent and intensity over high ground. Strong vertical windshears are often concentrated in a few stable layers just above and below the core of the jet stream. Distortion of these layers when the jet stream flows over high ground, particularly when mountain waves form, can produce local enhancements of the shears so that the flow in those regions breaks down into turbulence. Usually the cold side of the jet stream is more prone to turbulence, but mountain waves may be more pronounced on the warm side. TURBULENCE IN THE STRATOSPHERE Flight experience shows that in the stratosphere, moderate or severe turbulence is encountered over high ground about four times more frequently than over plains and about seven times more frequently than over the oceans.

DOWNDRAUGHTS Whether or not a well developed wave system exists, if the air is stable a strong surface air flow over high ground produces a substantial and sustained downdraught and/or turbulence on the lee side. Such downdraughts may, on occasion, be strong enough to defeat the rate of climb capability of some aircraft. In a wave system, a series of downdraughts and updraughts exists, the most powerful being those nearest the high ground.

ICING Adiabatic cooling caused by the forced ascent of air over high ground generally results in a lowering of the freezing level and an increase of liquid water concentration in clouds. Thus, when extensive cloud is present, airframe icing is likely to be more severe than at the same altitude over lower ground. This hazard is at a maximum a few thousand feet above the freezing level, but in general is unlikely to be serious at altitudes above 20 000 ft except in cumulonimbus clouds.

FLYING ASPECTS The effects of the airflow over high ground on aircraft in flight depends on the magnitude of the disturbance to the airflow; in other words, the altitude and the aircraft’s speed and direction in relation to the wave system. A broad distinction may be made between low-level hazards (below about 20 000 ft) and high-level hazards (above 20 000 ft).

LOW ALTITUDE FLIGHT The main hazards arise in low altitude flight from severe turbulence in the rotor zone, from downdraughts and from icing. The presence of roll clouds in the rotor zone may warn pilots of the region of most severe turbulence, but characteristic cloud formations are not always present or, if they are present, may lose definition in other clouds. Similarly, the updraughts and downdraughts are, in general, not visible. If an aircraft remains for any length of time in a downdraught, which may be remarkably smooth (e.g. by flying parallel to the mountains in the descending portion of the wave), serious loss of height may occur. During upwind flight, the aircraft’s height variations are normally out of phase with the waves; the aircraft is, therefore, liable to be at its lowest height when over the highest ground. The aircraft may also be driven down into a roll-cloud over which ample height clearance previously appeared to be available.

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Downwind flight may be safer. Height variations are usually in phase with waves, but it must be appreciated that the relative speed of an accidental entry into the rotor zone is greater than in upwind flight because the rotor zone is stationary with regard to the ground. Thus, the structural loads imposed on the airframe when gusts are encountered are likely to be greater, and there will probably be less warning of possible handling difficulties.

HIGH ALTITUDE FLIGHT The primary danger at high altitude is that of a sudden encounter with localised disturbances (i.e. turbulence or sudden large wind and temperature changes) at high penetration speeds. This is particularly relevant at cruising levels above FL 300 where the buffet-free margin between the Mach number for 1g buffet and the stall is restricted. In this respect, flight downwind is likely to be more critical than flight up-wind, especially when the wind is strong. As in the case of low altitude flight, the waves are stationary relative to the ground. The higher the relative speed on accidentally encountering a standing wave while flying downwind, the greater the likelihood of greater loads on the airframe. There is often no advance warning of wave activity from preliminary variations in flight instrument readings, or from turbulence. Although downdraughts are present, they are probably not hazardous and icing and rotor zone turbulence are unlikely.

INVERSIONS Inversions on the leeward side of a mountain range can prevent the down-slope wind from reaching the ground. A very powerful shear is generated from about 300 ft up to 1500 ft above the ground. When the downdraught moves over the inversion, a low level jet forms. Fresh winds over a mountain but light winds at the airport on the leeward side of the mountain indicate strong low-level windshear.

MARKED TEMPERATURE INVERSION The marked temperature inversion occurs during cloudless nights due to terrestrial radiation. The situation is enhanced if the aerodrome is situated in a valley. A pocket of cold air is trapped under higher warm air. A low level jet can form just below the top of a strong radiation inversion on clear nights. At certain airfields, a warning of marked temperature inversion is issued when a temperature difference of 10°C or more exists between the surface and any point up to 1000 ft above the surface.

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Chapter 19

REPORTING TURBULENCE CAT remains an important operational factor at all levels of flying but particularly above FL 150. Pilots encountering CAT are requested to report time, location, level and intensity, and aircraft type to the appropriate ATS unit. This is done as a Special Observations Report. The criteria required are: INCIDENCE ¾ OCCASIONAL — less than 1/3 of the time. ¾ INTERMITTENT — 1/3 to 2/3. ¾ CONTINUOUS — more than 2/3. INTENSITY LIGHT ¾ Light Turbulence — IAS fluctuates 5 - 15 kt, turbulence that momentarily causes slight erratic changes in attitude and/or altitude. ¾ Light Chop — Turbulence that causes slight rapid rhythmic bumping without appreciable changes in altitude or attitude. No IAS fluctuations. ¾ Reaction Inside Aircraft — Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may be conducted and little or no difficulty is encountered when walking. MODERATE ¾ Moderate Turbulence — IAS fluctuates 15 - 25 kt, turbulence that is similar to light turbulence but of greater intensity. Changes in altitude and/or attitude can occur but the aircraft remains in positive control at all times. ¾ Moderate Chop — Turbulence that is similar to light chop but of greater intensity. Rapid bumps or jolts without appreciable changes in altitude or attitude. IAS may fluctuate slightly. ¾ Reaction Inside Aircraft — Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult. SEVERE ¾ Severe Turbulence — IAS fluctuates more than 25 kt; turbulence that causes large, abrupt changes in altitude and/or attitude. The aircraft may be momentarily out of control. ¾ Reaction Inside Aircraft — Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food service and walking impossible.

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Chapter 19

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Windshear and Turbulence

Meteorology

INTRODUCTION Air pressure varies considerably between positions on the Earth’s surface. These pressure differences are important to the Earth’s weather and winds. On the meteorological charts the pressure pattern is shown by isobars, enclosing areas of different pressure.

LOW, CYCLONE OR DEPRESSION, AND TROUGH For a low pressure system, the isobars are generally closely spaced which results in windy weather. The centre of the low pressure system experiences calm winds. Convergence occurs and air is forced upward and cools adiabatically. If the air is humid, condensation occurs and clouds form.

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In the Northern Hemisphere, the wind follows a left-hand circuit parallel to the isobars. Friction acts as a brake on the wind in the friction layer, and the wind blows in toward the low pressure centre. The result is a general lifting of the air within the low pressure area. For the Southern Hemisphere the rotation is in the opposite direction. In a low-pressure area, convective movement is strengthened, and CB are likely to form if the air is unstable. If the air is stable but humid, clouds form. In this, extensive stratiform cloud layers form. Visibility at low levels is generally better than in an anticyclone, due to a stronger mixing of the air.

LOW PRESSURE TYPES As in the case of the high pressures, there are two basic types of lows, warm and cold. Dynamic Low Thermal Low

Cold low, the low deepens at altitude and the winds are increasing. Warm low, the low weakens aloft and turns into a high pressure.

SECONDARY DEPRESSION The secondary depression forms in the circulation of a larger primary depression. The secondary depression can be frontal or non-frontal depending upon how it forms. Secondary depression movement depends upon the hemisphere. ¾ ¾

In the Northern Hemisphere, a secondary depression moves anti-clockwise around the primary depression. In the Southern Hemisphere, a secondary depression moves clockwise around the primary depression.

The secondary depression can form: ¾ ¾ ¾

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On the tip of an occlusion. On unstable waves on a trailing cold front. Inside the primary depression circulatory system.

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Chapter 20

The life cycle and weather patterns associated with secondary depressions are similar to those of a primary depression. As the secondary depression deepens, the depression may become the dominant feature. In this case, the old primary depression becomes the secondary depression and starts circulating around the new primary depression. This process is known as “dumb belling.” The weather in a secondary depression is often more severe than in a primary depression. The worst weather associated with a non-frontal secondary depression usually occurs on the side of the secondary furthest from the primary.

ICELANDIC LOW The Icelandic low, as shown on the January and July mean value surface pressure charts below, is a dynamic system. The adiabatic cooling (due to the expansion of the air) leads to extensive clouds in the low pressure area. In temperate latitudes there is a transport of unstable cold air in the northern and western areas of the low, while there is an airflow of more stable warm air in the southern and eastern areas. Showers are more frequent in the north-western parts of the low. Apart from the showers, visibility is good. If the lifted air is humid, extended AS and AC with embedded areas of light rain can form.

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Non-Frontal Pressure Systems

JANUARY

JULY

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Chapter 20

THE ORIGIN OF LOW PRESSURES AND WEATHER In aviation, dynamic and thermal classifications are rarely used. Normally depressions are classes as Non-Frontal or Frontal: Examples of non-frontal depressions are: ¾ ¾ ¾ ¾ ¾ ¾ ¾

Orographic lows Thermal lows Summer lows over land or Monsoon low Equatorial low or trough Instability lows Winter lows over sea

¾ ¾ ¾ ¾ ¾ ¾ ¾

Mediterranean low Polar low Baltic Sea low Cold air pool Tropical revolving storm Easterly waves Whirlwind or Tornado

The last three items are discussed in Chapter 27 — Tropical Storms and Tornadoes.

OROGRAPHIC OR LEE SIDE LOWS OR TROUGHS If a current of air flows perpendicular to a mountain range, the barrier will force the air to compress on the windward side and over the mountain. The air on the leeward side of the mountain seems to “stretch.” There will be a tendency for anticyclonic curvature over the mountain with closely spaced pressure surfaces, and on the lee-side there will be a clearly visible cyclonic curvature. Falling air pressure on the leeward side forms a depression. This is known as a lee-depression or a lee-trough. The lee-trough is usually stationary if the airflow remains the same and no deepening low forms.

The lee-low causes the pressure surfaces to slope down towards the mountain and become closely packed over the mountain. 666

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Chapter 20

Non-Frontal Pressure Systems

On the leeward side, foehn winds prevail, and the weather typically is fine. Humid air may be sucked into a lee trough giving clouds and sometimes precipitation. When a cold front encloses warmer air on the leeward side of the mountain, rapid development of the system occurs. The low deepens and intense cumulonimbus clouds form.

A cold front may be activated/intensified when passing a range of mountains. When the cold air sweeps around the sides of the mountain and across it, the warm air on the leeward side acts as a warm sector and a wave forms on the front. This wave normally develops rapidly which leads to an occlusion-like process, and storms move away from the mountain. The most severe Orographic lows that form over north-west Italy and affect the Mediterranean form when the western Alps stop cold fronts. ¾ ¾ ¾ ¾

The lower portion of the cold front is slowed by the mountains. The slope of the frontal surface increases. Eventually the cold air spills over into the warm air on the lee side of the Alps. The warm air is undercut by the cold air causing severe instability.

Similar phenomena appear on the Skagerak when the Scandinavian mountains impede cold fronts. In this particular case, air sweeps around the southern edge of the mountains, giving strong winds. Humidity and temperature increase in the air that travels around the mountain at low levels. Travelling over a relatively warmer water surface and causing increased instability, rapid cyclonic development on the lee side occurs, often giving clusters of showers.

THERMAL DEPRESSIONS Thermal depressions form over warm surfaces. The heated air rises through convection and turbulence. A high pressure aloft is formed causing an outflow of air at height. The air pressure at the surface begins to decrease and a circulation similar to the sea breeze occurs.

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Chapter 20

An influx of air occurs at the surface low and an ascending motion is generated, strengthening the convective clouds in the area, if any. The most predominant thermal depressions are: ¾ ¾ ¾ ¾

The monsoon low pressures in Asia. The Equatorial low pressure belt. The summer lows in south-western USA. The lows of north-east Africa.

Less intense thermal lows are common on the weather charts in the summer, especially over France and Spain. These smaller cyclones are shallow and do not affect weather to any greater extent. In the winter, thermal depressions can form over “warm” water surfaces such as the Baltic Sea, the Skagerak, the Black Sea, and the Mediterranean. These are referred to as instability lows. If the air is dry, thermal lows bring good flying weather with some cloud and moderate to good visibility. If the air is humid, however, convective CB are likely to form, and heat thunderstorms or squalls will also appear. This is a common feature in France and on the Iberian Peninsula. Thermal lows generated in these areas may drift towards north-western Europe and Scandinavia.

INSTABILITY LOWS If large scale organised convection occurs in an area where there is already a lee low, a development may take place that looks similar to a thermal low. This is an instability low. The same process that created the thermal low also influences the instability low. A significant amount of the energy is derived from the released latent heat of the condensation process. According to the hydrostatic equation, heating causes the distance between two pressure surfaces to increase. As a result a high pressure is generated aloft resulting in an outflow of air and falling pressure at the ground. If divergence already exists at height, the effect will be strengthened and a rapid pressure fall can occur at the surface level. This generates a spiral flow in toward the centre. Instability lows can be very intense, particularly in the Tropics. In mid-latitudes the humidity content is low and the lows are thus less intense.

MEDITERRANEAN LOW A typical winter low that forms over the sea when cold polar air reaches the warm Mediterranean water. A separate low forms in which clusters of convective cloud are found.

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POLAR LOWS Instability lows often form when cold polar or arctic air moves south over a gradually warmer sea or major water area. Common from November to March in the Northern Hemisphere sea areas. The air transforms due to an intense heating and vapour increase in the lower levels resulting in intense convection caused by the southerly travel of the airmass. Between the two highs there is a tendency to a cyclonic airflow and the formation of lee-lows off the south-eastern coast of Norway. When the cold air reaches the warmer water, small intense instability lows develop. A similar type of instability low forms in the winter on the Bay of Genoa, generated when the cold Mistral wind sweeps down over the warm Mediterranean.

BALTIC SEA CYCLONES If energy is released in an area where a low pressure already exists, the low deepens or intensifies. This is common in the autumn on the Baltic Sea, when lows from the continent in the south and east move out over relatively warm water. It can also occur when a low has passed Scandinavia from the west.

More precipitation and lower cloud bases than predicted in a forecast affect the Baltic Sea isles and coasts. Heavy northerly squalls may develop.

CELLS OF COLD AIR ALOFT (COLD POOLS) The general theories about the long waves encircling our globe, separating the cold polar from the warm tropical air, are discussed in Climatology. The waves are usually zonal moving from west to east along a latitude line. Cold air outbreaks can cut off from the main stream air and generate a pool of cold air at height in a position south of the normal Polar front. This cold pool can remain for several days constituting a potential area of instability at height. In the summer, thermal lows form over the continents and may develop into instability lows. This happens when cold air is carried in over the low (by the upper airflow) or when a cold pool already exists at height. In these conditions, the atmosphere becomes unstable, and a major area of thunderstorms may develop. 20-8

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Chapter 20

ANTICYCLONE OR HIGH, AND RIDGE OR WEDGE A high pressure system is an area enclosed by isobars that decrease in value with distance from the centre.

NATURE OF A HIGH Isobars are normally well spaced resulting in light winds. Where a high is adjacent to a low the pressure gradient can become steep, leading to moderate or strong winds. In a high pressure cell, mass convergence at height and divergence at low levels creates subsidence within the core of the anticyclone with an outflow at low level. The subsidence is checked above the ground, due to the thermal mixing in the surface layer and a subsidence inversion is formed. The height of the subsidence inversion depends on the intensity of the anticyclone, the degree of thermal mixing, and the distance from the core. Inversions form from 2000 to 5000 ft in cold anticyclones and up to FL 100 in warm anticyclones. Above the friction layer, in the Northern Hemisphere, the wind blows in a right-hand circuit parallel to the isobars. In the friction layer, friction slows the wind and it blows at an angle out from the centre. The outflow at the bottom of the high leads to a sinking motion of air, which is compressed and adiabatically heated. The subsidence inversion forms, the temperature rises significantly and the humidity decreases. In the Southern Hemisphere, the rotation is reversed. The air above the inversion is dry, while the air below may or may not be dry depending on the circumstances that prevail. Air pollution collects below the inversion, and this leads to a drop in visibility at the lowest levels. If the inversion persists, clouds can form in the inversion. At high latitudes, the increased loss of terrestrial radiation due to the drying at height creates nocturnal inversions at the surface. Large areas with SC and ST may form in maritime air masses. In the winter these clouds can persist for several days. Where the humidity is high and the lower levels are cold, fog forms below the subsidence inversion. In the summer, or at lower latitudes, SC often dissipates during the day and returns at night. If the air below the subsidence inversion is unstable or conditionally unstable, CU may form below the inversion during the day. In continental air masses, the humidity content is low, but visibility is still limited below the inversion. If the air passes over a major water feature, moisture is rapidly absorbed and cloud forms. Maritime airmasses dry out with an extensive passage over a major land surface. The weather above the subsidence inversion is normally fine; cloudless with good visibility.

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Chapter 20

Non-Frontal Pressure Systems

HIGH PRESSURE SYSTEMS There are two main high pressure systems, depending on whether they consist of warm or cold air.

SUBTROPICAL HIGHS (WARM ANTICYCLONES) Subtropical highs are formed by air from the equatorial regions travelling away from the equator at altitudes around the Equatorial tropopause. They are deflected by Coriolis which generates a subtropical jet stream and an accumulation of air around the 30º latitude. At low levels the air pressure increases and there is an outflow of air from the system. In the subtropical high, subsidence from aloft occurs. The subsidence inversion in these cells is sometimes called a Trade Inversion. These anticyclones are often stationary or move in a seasonal manner and are therefore referred to as permanent highs. Europe’s nearest subtropical anticyclone is the Azores High, which is the source region of maritime tropical air. The air below the subsidence inversion is humid and unstable; above it is dry and stable. CU dominates the weather below the inversion. The height to the inversion varies within the high pressure cell. The highest values are found in the western areas nearest the Equator (5000 − 7000 ft) and the lowest in the north-eastern areas (1500 − 2000 ft). Tropical showers are more likely to develop in the western part of an ocean than in the eastern. As the low level air travels away from the equator, the humidity increases. The sea temperature decreases and the air is cooled from below. In winter, this frequently leads to vast areas of low clouds, drizzle, and fog over NW Europe. In summer, the anticyclone occasionally intensifies over the North Atlantic. This causes lows and the associated rain areas to move in a wide arc north of Scandinavia, forming a blockage (a “blocking high”) with dry and sunny weather over western Europe.

CONTINENTAL HIGHS (COLD ANTICYCLONES) Consisting of polar air, the cold anticyclone forms over the cold continents in the winter. They seldom reach higher than 700 hPa (FL 100), but the horizontal extension may be considerable. Thermal highs are not as stable as dynamic highs, and travelling depressions can break them down. The Siberian and the Canadian highs consist of, and are the source regions of, continental polar air. In midwinter they also constitute the source region of arctic air from within the Arctic and Antarctic permanent cold anticyclones. If the pressure system spreads over a coastal area, there will be convection and snow showers over the open water surface with fog and mist below the inversion inland. If the air is dry and there is no advection from open water, the weather can be cold, bright, and cloudless. In clear and extremely cold areas, ice fog or diamond dust may form. 20-10

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Chapter 20

HIGH PRESSURES AND HIGH PRESSURE RIDGES (OR WEDGES) IN SERIES OF TRAVELLING DEPRESSIONS The third type of high pressure forms between the lows of a family of depressions. The ridges, or temporary high, forms as cold air sweeps behind a frontal low. This type of high is thermal, and as a consequence it is not visible on an upper air chart. High pressure ridges follow low pressure systems in their movements and constitute a break in the storms associated with the frontal systems of the lows. The ridge can be subdivided into three weather zones: ¾ ¾ ¾

Ahead of the axis of the ridge (just behind the cold front) Along the axis of the ridge Behind the ridge (in front of the next warm front)

There is a high risk of showers, often troughs with low pressure and line squall showers/thunderstorms well ahead of the ridge axis. CB turns to CU and SC closer to the ridge axis. In winter, terrestrial radiation from the Earth is high, and nocturnal radiation fog is likely to form if the wind is light. ST or SC form if the wind is stronger at the border of the ridge/cold high. When the ridge passes, the air is humidified in the prevailing SW wind, which again leads to increased cloud with CU and SC at lower levels while the frontal cloud deck thickens at height.

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Chapter 20

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Non-Frontal Pressure Systems

Meteorology

TYPES OF SERVICE PRE-FLIGHT BRIEFING The primary method of meteorological briefing for flight crews is self briefing. An alternate method for obtaining information is from the Meteorological Information Self Briefing Terminal (MIST). Where the primary method is not available, then special forecasts are often provided. If the personal advice of a forecaster is required, then information is only given on the understanding that full use is made of all available information. Note: Meteorological observations and forecasts have certain expected tolerances of accuracy.

METEOROLOGICAL CHARTS Meteorological information is available on various charts which are routinely transmitted over the METFAX network to major aerodromes. They provide information under the following headings: Low and medium level flights within the UK and to near Europe Surface Weather Chart for: Surface − 15 000 ft amsl. (Form 215) Spot Wind/ Temperature Chart for 1000 ft − 24 000 ft amsl, (Form 214) Medium and high level flights to Europe and the Mediterranean Significant Weather/Tropopause/Maximum Wind Charts for FL 100 − FL 450. Upper Wind and Temperature Charts for FLs: 50, 100, 180, 240, 300, 340, 390 and 450. High level flights to North America Significant Weather/Tropopause/MaximumWind Chart for FL 250 − FL 630. Upper Wind and Temperature chart for FLs: 180, 240, 300, 340, 390, 450 and 530.

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Meteorological Observations and Meteorological Services

High level flights to Middle/Far East Significant Weather/Tropopause/MaximumWind Chart for FL 250 − FL 450. Upper Wind and Temperature chart for FLs: 180, 240, 300, 340, 390, 450 and 530. High level flights to Africa, The Caribbean and South America Significant Weather/Tropopause/MaximumWind Chart for FL 250 and above. Upper Wind and Temperature charts for FLs: 180, 240, 300, 340, 390, 450 and 530. Other area charts or additional Flight Level information, which are not routinely available by METFAX, are requested from the LONDON/Heathrow forecast office, subject to available METFAX capacity. Amended charts are issued when forecast conditions change significantly.

BROADCAST TEXT METEOROLOGICAL INFORMATION The following reports are broadcast by teleprinter: METAR Aerodrome meteorological reports. METARs are routinely broadcast every ½ hour during aerodrome opening hours. Exceptionally they are sometimes broadcast every 1 hour. TAF Aerodrome Forecasts. FC denotes a TAF valid for a period less than 12 hours, usually 9 hours, which is issued every 3 hours. FT denotes a TAF valid between 12 and 24 hours which is issued every 6 hours. Amendments are broadcast between routine times as required. SIGMET Warnings of weather significant to flight safety these are available for areas within 1000 nm of the UK. The following are additions that may be added to the METAR: ¾ ¾

Short term landing forecasts (TREND), which are valid for 2 hours. Information on runway state when weather conditions require and continue until conditions cease. Special Aerodrome Meteorological Reports are issued when conditions change through specific limits.

SPECIAL AERODROME METEOROLOGICAL REPORTS (SPECI) Special Aerodrome Meteorological Reports are issued when conditions change significantly. Selected Special Reports (SPECI) are defined as Special Reports disseminated beyond the aerodrome of origin. The UK does not normally issue Selected Special Reports.

TERMINAL AERODROMES FORECAST (TAF) TAFs are normally provided only for those aerodromes where official meteorological observations are made. For other aerodromes, Local Area Forecasts are made. Amended TAFs or Local Area Forecasts are issued when forecast conditions change significantly.

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Chapter 21

SPECIAL FORECASTS AND SPECIALISED INFORMATION For departures from an aerodrome where the standard pre-flight meteorological briefing is inadequate for the intended flight, a special forecast may be issued. Normally a Special Flight Forecast is supplied from the last UK departure point to the first transit aerodrome outside the coverage of standard documentation. By prior arrangement, forecasts are provided for other legs if the initial ETD to final ETA does not exceed 6 hours and no stops longer than 60 minutes are planned. Forecast offices normally require prior notification for special forecasts. For flights up to 500 nm at least 2 hours is required before the time of collection. For flights over 500 nm at least 4 hours is required before the time of collection.

SIGMET SERVICE Aircraft can be supplied with information in flight. MWOs are responsible for the preparation and issue of SIGMETs to the appropriate ATC unit. Aircraft in flight are warned of the occurrence or expectation of one or more of the following SIGMET phenomena for the route ahead, for up to 500 nm or 2 hours flying time: a.

At Subsonic Cruising Levels (SIGMET) i. Thunderstorm (See Note) ii. Heavy hail (See Note) iii. Tropical cyclone iv. Freezing rain v. Severe turbulence (not associated with convective cloud) vi. Severe icing (not associated with convective cloud) vii. Severe mountain waves viii. Heavy sand/dust storms ix. Volcanic ash cloud

Note: Thunderstorm does not refer to isolated or occasional thunderstorms not embedded in cloud layers or concealed by haze. This refers only to thunderstorms widespread, including if necessary CB which is not accompanied by a TS, within an area: With little or no separation Along a line with little or no separation Embedded in cloud layers Or concealed in cloud layers or concealed by haze

FRQ SQL EMBD OBSC

TS and tropical cyclones each imply: Moderate or severe turbulence Moderate or severe icing and hail Heavy hail HVYGR is used as a further description of the TS as necessary b.

Meteorology

At Transonic and Supersonic Cruising Levels (SIGMET SST) i. Moderate or severe turbulence ii. Cumulonimbus cloud iii. Hail iv. Volcanic ash cloud 21-3

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In general SIGMET messages are identified by the letters WS at the beginning of the header line. Tropical Cyclones and Volcanic Ash will be identified by WC and WV respectively.

AIRCRAFT REPORTS SIGMETs are not usually valid for more than 4 hours, except volcanic ash clouds where the period is upwards of 12 hours. SIGMETs are sequentially numbered through the day Flight levels for SIGMET SST are as follows: ¾ FL 250 − FL 600 London and Scottish UIRs ¾ FL 400 − FL 600 Shanwick OCA

ROUTINE AIRCRAFT OBSERVATIONS Routine aircraft observations are not required in the London/Scottish FIR/UIR. In the Shanwick OCA, aircraft are to conform with the requirements laid out in the ENR section or applicable NOTAM.

SPECIAL AIRCRAFT OBSERVATIONS Special observations are required in any UK FIR/UIR/OCA when: a. Severe turbulence or severe icing is encountered. or b. Moderate turbulence, hail or cumulonimbus clouds are encountered during transonic or supersonic flight. or c. Any factors which a pilot believes affects the safety of flight are encountered. or d. When requested by the meteorological office. or e. When there is an agreement between the meteorological office and the aircraft operator.

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Chapter 21

CLEAR AIR TURBULENCE (CAT) CAT remains an important operational factor at all levels of flying but particularly above FL 150. Pilots encountering CAT are requested to report time, location, level, intensity, and aircraft type to the ATS unit they are operating with. This is done as a Special Observations Report. The criteria required are: INCIDENCE ¾ OCCASIONAL — less than 1/3 of the time. ¾ INTERMITTENT — 1/3 to 2/3. ¾ CONTINUOUS — more than 2/3. INTENSITY LIGHT ¾ Light Turbulence — IAS fluctuates 5 − 15 kt, turbulence that momentarily causes slight erratic changes in attitude and/or altitude. ¾ Light Chop — Turbulence that causes slight rapid rhythmic bumping without appreciable changes in altitude or attitude. No IAS fluctuations. ¾ Reaction Inside Aircraft — Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may be conducted and little or no difficulty is encountered when walking. MODERATE ¾ Moderate Turbulence — IAS fluctuates 15 − 25 kt, turbulence that is similar to light turbulence but of greater intensity. Changes in altitude and/or attitude can occur but the aircraft remains in positive control at all times. ¾ Moderate Chop — Turbulence that is similar to light chop but of greater intensity. Rapid bumps or jolts without appreciable changes in altitude or attitude. IAS may fluctuate slightly. ¾ Reaction Inside Aircraft — Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult. SEVERE ¾ Severe Turbulence — IAS fluctuates more than 25 kt; turbulence that causes large, abrupt changes in altitude and/or attitude. The aircraft may be momentarily out of control. ¾ Reaction Inside Aircraft — Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food service and walking impossible.

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AIRFRAME ICING Any pilot encountering unforecast icing is requested to report the time, location, level, intensity, icing type, and aircraft type to the ATS unit they are operating with. The following are reporting definitions, and are not necessarily forecasting definitions. Trace Ice becomes perceptible. Rate of accumulation slightly greater than the rate of sublimation. It is not hazardous even though de-icing/anti-icing equipment is not used unless ice is encountered for more than one hour. Light The rate of accumulation might create a problem if flight in this environment exceeds 1 hour. Occasional use of de-icing/anti-icing equipment removes/prevents accumulation. It does not present a problem if anti-icing equipment is used. Moderate The rate of accumulation is such that even short encounters become potentially hazardous and the use of de-icing/anti-icing equipment, or diversion, is necessary. Severe The rate of accumulation is such that de-icing/anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary.

AERODROME CLOSURE The term SNOCLO is added to the end of an aerodrome report in a VOLMET radio broadcast when it is unusable for take-off or landing due to heavy snow on the runways, or the runway is blocked for snow clearance.

IN-FLIGHT PROCEDURES An in-flight enroute service is available in exceptional circumstances by prior arrangement with the meteorological office. Make applications for this service in advance stating: 1. 2. 3. 4.

The flight levels and route sector required. The period of validity required. The approximate time and position the request will be made. The ATS unit the aircraft expects to be in contact.

Aircraft can obtain aerodrome weather information from any of the following sources: ¾ ¾ ¾ ¾

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VOLMET broadcasts. Automatic Terminal Information Service (ATIS) broadcasts as described in the GEN section. By request to an ATC unit. If an aircraft proposes to divert to an aerodrome for which no forecast is provided, the commander may request the relevant information from the ATS unit serving the aircraft.

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Chapter 21

ACCURACY OF METEOROLOGICAL MEASUREMENT OR OBSERVATION The accuracies listed refer to assessment by instruments (except cloud amount). They are not usually attainable in observations made without instruments. Element

Accuracy of Measurement or Observation

Mean Surface Wind

Direction: ± 5° Speed: ± 1 kt up to 20 kt, ± 5% above 20 kt

Variations from the mean surface wind

± 1 kt

Visibility

± 50 m up to 500 m ± 10% between 500 m and 2000 m ± 20% above 2000 m up to 10 km

RVR

± 25 m up to 150 m ± 50 m between 150 m and 500 m ± 10% above 500 m up to 2000 m

Cloud amount

± 1 okta in daylight, worse in darkness and during atmospheric obscuration

Cloud height

± 33 ft up to 3300 ft ± 100 ft above 3300 ft up to 10 000 ft

Air temperature temperature

and

dew

Pressure value (QFE, QNH)

point ± 0.2° C ± 0.3 mb

MARKED TEMPERATURE INVERSION At certain aerodromes a Warning of Marked Temperature Inversion is issued whenever a temperature difference of 10° C or more exists between the surface and any point up to 1000 ft above the aerodrome. This warning is broadcast on departure and arrival ATIS at aerodromes so equipped, or in the absence of ATIS passed by radio to departing aircraft before take-off, and to arriving aircraft as part of the report of aerodrome meteorological conditions.

AERODROME WARNINGS Aerodrome warnings are issued as appropriate when one or more of the following occurs or is expected to occur: a. b. c.

d.

e. f. g. Meteorology

Gales or strong winds agreed to locally agreed criteria (Gales — mean surface wind >33 kt or gusts >42 kt). Squalls, hail or thunderstorms. Snow, including the expected time of beginning, duration and intensity of fall; the expected depth of accumulated snow, and the time of expected thaw. Amendments or cancellations are issued as necessary. Frost warnings when any of the following are expected to exist. i. A ground frost with air temperatures not below freezing point. ii. The air temperature above the surface is below freezing (Air frost). iii. Hoar frost, rime or glaze deposited on parked aircraft. Fog (normally when visibility is expected to fall below 600 m). Rising dust or sand. Freezing precipitation. 21-7

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SPECIAL FACILITIES WINDSHEAR ALERTING Forecasters at airports review the weather conditions on an hourly basis and monitor any aircraft reports of windshear experienced on the approach or climb-out. Whenever a potential low level windshear condition exists, a Windshear alert, based on one or more of following criteria, is issued: a. Mean surface wind speed at least 20 kt. b. The magnitude of the vector difference between the mean surface wind and the gradient wind (an estimate of the 2000 ft wind) of at least 40 kt. c. Thunderstorm(s) or heavy shower(s) within approximately 5 nm of the Airport. Note: Alerts are also issued based on recent pilot reports of windshear on the approach or climb-out. The Alert message is given in the arrival and departure ATIS in one of three formats: a. Windshear Forecast (WSF) When the meteorological conditions indicate that low level windshear on the approach or climb-out (below 2000 ft) might be encountered. b. Windshear Forecast and Reported (WSFR) As above, supported by a report from at least one aircraft of windshear on the approach or climb-out within the last hour. c. Windshear Reported (WSR) When an aircraft reports windshear on the approach or climb-out within the last hour, but insufficient meteorological evidence exists for the issue of a forecast of windshear.

WINDSHEAR REPORTING CRITERIA Pilots using navigation systems providing direct wind velocity readout should report the wind and altitude/height above and below the shear layer, and its location. Other pilots should report the loss or gain of airspeed and/or the presence of up or down draughts or a significant change in crosswind effect, the altitude/height and location, their phase of flight, and aircraft type. Pilots not able to report windshear in these specific terms are to do so in terms of its effects on the aircraft, the altitude/height and location and aircraft type (e.g. Abrupt windshear at 500 ft on finals, maximum thrust required, B747). Pilots encountering windshear are requested to make a report even if windshear was previously forecast or reported.

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Chapter 21

OBSERVING SYSTEMS AND OPERATING PROCEDURES — SURFACE WIND Surface wind sensors are positioned to give the best practical indication of the winds which an aircraft encounters during take-off and landing within the layer between 6 and 10 m above the runway. The surface wind reported for take-off and landing by ATS units supporting operations by aircraft whose MTWA is less than 5700 Kg is usually an instantaneous wind measurement with direction referenced to Magnetic North. At other designated airports the wind reports for take-off and landing are averaged over the previous 2 minutes. Variations in the wind direction are given when the total variation is 60° or more and the mean speed above 3 kt. The directional variations are expressed as the two extreme directions between which the wind has varied in the past 10 minutes. In reports for take-off, surface winds of 3 kt or less include a range of wind directions whenever possible if the total variation is 60° or more. Variations from the mean wind speed (gusts) during the past 10 minutes are only reported when the variation from the mean speed exceeds 10 kt. Variations are expressed as the maximum and minimum speeds attained. Note: Surface wind measurement in a METAR and SPECI are referenced to true north.

CLOUD HEIGHT Information on cloud height is obtained by the use of: a. b. c. d. e.

Ceilometers Cloud searchlights Alidades Balloons Pilot reports or observer estimation

At some aerodromes an additional cloud ceilometer is installed on the approach. The cloud heights reported from an approach ceilometer are: a. The most frequently occurring value during the past 10 minutes if the value is 1000 ft or less. b. If cloud is indicated at heights 100 ft or more below that indicated at (a) above then the height of the lowest cloud is reported, prefaced by OCNL. c. If the most frequently occurring value is above 1000 ft but the lowest value is 1000 ft or below, then only the lowest value is reported.

TEMPERATURE Temperature is reported in whole degrees Celsius, M indicates a negative value.

HORIZONTAL SURFACE VISIBILITY Horizontal surface visibility is assessed by human observer. Visibility is reported in increments of: a. 50 m up to 500 m b. 100 m up to 5000 m In METAR, SPECI or TAF the maximum value is "10 km or more."

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When the lowest visibility is less than 1500 m and visibility in another direction is more than 5000 m, additionally, the maximum visibility and the direction in which it occurs is reported.

RUNWAY VISUAL RANGE (RVR) RVR assessment is made by either human observer or by an Instrument RVR system (IRVR). For the UK the standard RVR reporting increments are: a. 25 m between 0 and 200 m. b. 50 m between 200 and 800 m. c. 100 m from 800 m. Assessment and reporting in RVR begin when the horizontal visibility, or the RVR, is observed at less than 1500 m. RVR is passed to aircraft before take-off and during the approach to landing. Changes to the RVR are passed throughout an aircraft's approach. Where multi-site IRVR systems are installed, the procedure is for touchdown, mid-point and stop end values of RVR to be given (e.g. RVR 600, 500, 550). Where only touchdown and one other value is given, the RVR is given as RVR 500 stop end 500. Where a single transmitter fails and the remainder of the system is serviceable RVR readings are not suppressed (e.g. RVR touchdown missing, 600, 500). If two out of the transmissions fails, then the remaining value is given provided that it is not the stop end value. Aerodromes suppress mid-point and/or stop-end values when: a. They are equal to or higher than the touchdown zone value unless they are less than 400 m. Example: 300 350 350 All values are reported. or b. They are 800 m or more. Example: 1000 900 900 Only the touchdown value is reported.

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INTRODUCTION A variety of weather messages are originated by Meteorological Observers at aerodromes. These are collated and broadcast in text form to stations around the world. Aviators are usually able to distinguish between the various message types and their uses.

AERODROME METEOROLOGICAL REPORT Aerodrome Meteorological Reports (METAR) contain observations on the conditions that actually exist at a station and are made every 30 minutes throughout the day. ¾ ¾

Short term landing forecasts, valid for two hours (TREND), may be added to METARS. Information on runway condition is added to METAR when appropriate, until these conditions cease.

SPECIAL AERODROME METEOROLOGICAL REPORTS Special Aerodrome Meteorological Reports (SPECI) are issued when conditions change significantly. Selected Special Reports (SPECI) are defined as Special Reports disseminated beyond the aerodrome of origin.

TERMINAL AERODROME FORECASTS Terminal Aerodrome Forecasts (TAF) are normally provided only for those aerodromes where official meteorological observations are made. For other aerodromes, Local Area Forecasts are made. Amended TAFs or Local Area Forecasts are issued when forecast conditions change significantly.

ACTUAL WEATHER CODES The content and format of an actual weather report are shown in the following table. Report Type

Location Identifier

Date/Time

Wind

Visibility

RVR

METAR

EGSS

291250Z

31015G30KT

1400SW 6000N

R24/P1500

Present Weather

Cloud

Temp/ Dew Pt

QNH

Recent Weather

Wind Shear

Trend

Rwy State

SHRA

FEW005 SCT010CB BKN025

10/05

Q0999

RETS

WS RWY25

NOSIG

88290592

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Meteorological Messages

IDENTIFIER The identifier has three components: Report Type: ICAO Indicator: Date/Time UTC:

Example:

METAR or SPECI. This is a four-letter group indicating the airfield (e.g. EGPL, LFPB). In a METAR or SPECI this is the date and time of the observation in hours and minutes UTC (e.g. 091250Z). METAR EGDL 211020Z.

Note: If a meteorological bulletin consists of a set of reports from one or more airfields the codename METAR or SPECI is replaced by SA (Actual Report), or SP (Special Report) followed by a bulletin identifier, date, and time of the observation.

SURFACE WIND VELOCITY The first 3 figures indicate the wind direction (T) to the nearest 10°, followed by two figures (exceptionally 3 figures) giving the mean windspeed during the previous 10 minutes. The permitted units of speed are: ¾ ¾ ¾

KT indicating knots KMH for kilometres per hour, or MPS for metres per second. Example: 30015KT.

These may be followed by a letter G and two more figures if the maximum gust speed exceeds the average speed by 10 kt or more. Example: 30015G30KT. Variations in wind direction of 60° or more in the 10 minutes preceding the observation are shown as 3 figures then the letter V followed by another 3 figures, but only if the speed is more than 3 kt. Example: 270V330 meaning, the wind is varying in direction between 270°T and 330°T. 00000 indicates calm conditions, a variable wind direction is shown by VRB followed by the speed.

HORIZONTAL VISIBILITY When there is no marked variation in direction the minimum visibility is given in metres. The minimum visibility with the direction is given when there is a marked variation with direction. Example: 2000NE.

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Chapter 22

When the minimum visibility is less than 1500 metres and the visibility in any other direction is greater than 5000 metres the maximum visibility and its direction is also shown. Example: 1200NE 6000SW. 9999 indicates a visibility of 10 kilometres or more, 0000 indicates a visibility of less than 50 metres.

RUNWAY VISUAL RANGE (RVR) Runway Visual Range is reported when the meteorological visibility falls below 1500 m. It has the form R, followed by the runway designator, a diagonal and then the Touchdown RVR. If more than one runway is in use, the RVR group is repeated. Parallel runways are distinguished by adding C, L, or R to the runway designator. Example: R24L/1200R24R/1100. When RVR is greater than the maximum assessable value the prefix P is added followed by the maximum value. Example: R15/P1500. The prefix M indicates the RVR is less than the minimum value that can be assessed. Example: R15/M0050. Tendencies are indicated by U for up, D for down, or N for no change. They show a significant change (100 m or more) from the first 5 minutes to the second 5 minutes in the 10 minute period prior to the observation. Example: R25/1000D. Variations are reported if the RVR changes minute by minute during the 10 minute period prior to the report. The 1 minute minimum and maximum separated by V are reported instead of the 10 minute mean. Example: R15L/0850V1000.

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Meteorological Messages

WEATHER Each weather group may consist of the appropriate intensity indicators and abbreviations, making groups of two to nine characters from the table below.

SIGNIFICANT PRESENT AND FORECAST WEATHER CODES QUALIFIER

WEATHER PHENOMENA

Intensity or Proximity

Descriptor

Precipitation

Obscuration

Other

1

2

3

4

5

- Light

MI Shallow

DZ Drizzle

BR Mist

PO Dust/sand whirls

Moderate (no qualifier)

BC Patches

RA Rain

FG Fog

PR Partial (Covering part of Aerodrome)

SN Snow

FU Smoke

SQ Squalls

+ Heavy 'Well developed in the case of FC and PO

DR Drifting

SG Snow Grains

VA Volcanic Ash

FC Funnel Cloud(s) (tornado or water-spout)

VC In the vicinity (within 8 km of aerodrome perimeter but not at aerodrome)

BL Blowing

IC Ice Crystals (Diamond Dust)

DU Widespread Dust

SS Sandstorm

SH Shower(s)

PE Ice-Pellets

SA Sand

DS Duststorm

TS Thunderstorm

GR Hail

HZ Haze

FZ Freezing Super-Cooled

GS Small hail (