Jeppesen Meteorology

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ii ISBN 0-88487-350-1

PREFACE_______________________

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The next three pages contain a list and content description of all the volumes in the ATPL series.

iii

ATPL Series

Meteorology (JAR Ref 050)

• The Atmosphere • Air Masses and Fronts
• Wind • Pressure System
• Thermodynamics • Climatology
• Clouds and Fog • Flight Hazards
• Precipitation • Meteorological Information

General Navigation (JAR Ref 061)

• Basics of Navigation • Dead Reckoning Navigation
• Magnetism • In-Flight Navigation
• Compasses • Inertial Navigation Systems
• Charts

Radio Navigation (JAR Ref 062)

• Radio Aids • Basic Radar Principles
• Area Navigation Systems
• Self-contained and • Basic Radio Propagation Theory

External-Referenced
Navigation Systems

Airframes and Systems (JAR Ref 021 01)

• Fuselage • Hydraulics
• Windows • Pneumatic Systems
• Wings • Air Conditioning System
• Stabilising Surfaces • Pressurisation
• Landing Gear • De-Ice / Anti-Ice Systems
• Flight Controls • Fuel Systems

Powerplant (JAR Ref 021 03)

• Piston Engine • Engine Systems
• Turbine Engine • Auxiliary Power Unit (APU)
• Engine Construction

Electrics (JAR Ref 021 02)

• Direct Current • Generator / Alternator
• Alternating Current • Semiconductors
• Batteries • Circuits
• Magnetism

iv

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 • Boundary Layer
• Aerofoil Airflow • High Speed Flight
• Aeroplane Airflow • Stability
• Lift Coefficient • Flying Controls
• Total Drag • Adverse Weather Conditions
• Ground Effect • Propellers
• Stall • Operating Limitations
• CLMAX Augmentation • Flight Mechanics
• Lift Coefficient and Speed

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 • Meteorological Messages
Flights • Point of Equal Time
• Point of Safe Return
• ICAO ATC Flight Planning • Medium Range Jet Transport
• IFR (Airways) Flight Planning
• Jeppesen Airway Manual Planning

Air Law (JAR Ref 010)

• International Agreements • Air Traffic Services
and Organisations • Aerodromes
• Facilitation
• Annex 8 – Airworthiness of • Search and Rescue
Aircraft • Security
• Aircraft Accident Investigation
• Annex 7 – Aircraft Nationality • JAR-FCL
and Registration Marks • National Law

• Annex 1 – Licensing
• Rules of the Air
• Procedures for Air Navigation

v

Human Performance and
Limitations (JAR Ref 040)

• Human Factors
• Aviation Physiology and Health Maintenance
• Aviation Psychology

Operational Procedures (JAR Ref 070)

• Operator • Low Visibility Operations
• Air Operations Certificate • Special Operational Procedures
• Flight Operations
• Aerodrome Operating Minima and Hazards
• Transoceanic and Polar Flight

Communications (JAR Ref 090)

• Definitions • Distress and Urgency

• General Operation Procedures Procedures

• Relevant Weather Information • Aerodrome Control

• Communication Failure • Approach Control

• VHF Propagation • Area Control

• Allocation of Frequencies

vi

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

xiv Meteorology

Table of Contents

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

Meteorology xv

Table of Contents

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

xvi Meteorology

Table of Contents

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

Meteorology xvii

Table of Contents

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

xviii Meteorology

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 Pressure
¾ Temperature
¾ Density
¾ 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 1-1

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: 78.09%
Oxygen: 20.95%
Argon: 0.93%
Carbon Dioxide: 0.03%

The graph below represents this composition:

Other trace elements include:

¾ Neon
¾ Helium
¾ Krypton
¾ Xenon
¾ Hydrogen
¾ Methane
¾ Iodine
¾ Nitrous Oxide
¾ Ozone
¾ Sulphur Dioxide
¾ Nitrogen Dioxide
¾ Ammonia
¾ 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.

1-2 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 1-3

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.

1-4 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.

Meteorology 1-5

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 Winter Summer
0° 56 000 ft 55 000 ft
55 000 ft 52 000 ft
10° 52 000 ft 51 000 ft
20° 45 000 ft 47 000 ft
30° 38 000 ft 43 000 ft
40° 35 000 ft 38 000 ft
50° 33 000 ft 35 000 ft
60° 29 000 ft 31 000 ft
70° 25 000 ft 29 000 ft
80°

1-6 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.

Meteorology 1-7

Chapter 1 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 15°C
Pressure
From MSL to 11 km Density 1013.25 hPa
(36 090 ft) 1225 g/m3
From 11 km to 20 km
(65 617 ft) Temperature decreases at 1.98°C
From 20 km to 32 km per 1000 ft (6.5°C per km)
(104 987 ft)
Temperature constant at –56.5°C

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

1-8 Meteorology

The Atmosphere 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 ISA ISA Deviation
10 000 temperature (°C) Temperature (°C)
17 000 -12
-10 -34.5 +8
38 000 -7
8000 -59.32 -48.36 +10

-15.84 +22
-18
-32.7

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 An ISA temperature deviation of –8.5°C
¾ A JSA temperature deviation of 0°C

Meteorology 1-9

Chapter 1 The Atmosphere

ANSWERS TO ISA DEVIATION QUESTIONS

Height (ft) Ambient ISA ISA Deviation
temperature (°C) Temperature (°C)
10 000 -5.2
17 000 -10 -4.8 -12
25 000 -30.66 -18.66 +8
34 000 -26.5 -34.5 -7
38 000 -59.32 -52.32 +10
8000 -46.5 -56.5 -15
32 000 -15.84 -0.84 +22
15 000 -26.36 -48.36 -18
-32.7 -14.7

1-10 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 s2
h1

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 2-1

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.

2-2 Meteorology

Pressure and Pressure Systems 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 2-3

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/sub- Temperate
tropical latitudes latitudes –
– typically 3 hPa 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.

2-4 Meteorology

Pressure and Pressure Systems Chapter 2

Pressure (hPa) Approximate Height (ft)

850 5 000 amsl
700 10 000 amsl
500 18 000 amsl
400 24 000 amsl
300 30 000 amsl
200 40 000 amsl
100 53 000 amsl
50 68 000 amsl

Be sure to learn the figures in the above table.

THE RELATIONSHIP BETWEEN PRESSURE AND
TEMPERATURE

120 ft

ISA + ISA 120 ft
1°C
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 2-5

Chapter 2 Pressure and Pressure Systems

Note: 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 27 ft
20 000 amsl 30 ft
40 000 amsl 50 ft
100 ft

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

2-6 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 height required
H1 known height
T the mean temperature of the column of air in K
P1 pressure at h1
P2 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 2-7

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.

2-8 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 up to 1500 km in diameter
Tropical Revolving Storms approximately 300 km in diameter
Tornado 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 2-9

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 Chapter 2

1004 L
1006
1008 30 – 35 000 ft
1010

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 2-11

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.

1002 1004

996 998 1000

Centre line

TROUGH WEATHER

Cloud For frontal troughs, the cloud types depend on the type of front. With
cold fronts clouds with a large vertical development are expected.
Precipitation With warm fronts, layer clouds are more likely.
Visibility For non-frontal troughs, CB and CU can be expected.
Winds
Showers, thunderstorms, hail with either cold frontal or non-frontal
systems. Light to moderate rain and drizzle with warm fronts.

Good except in precipitation.

Moderate with possibility of gusts and squalls.

2-12 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.

1006 1004

1010 1008

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

HL

COL 1004
1008
1000
996 H

L

Meteorology 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 3-1

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 Chapter 3

Example 2: 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 – FL 350 35 000 + FL 350
1003 hPa
(27 × 10) = (27 × 10) =
34 730 ft 1013 hPa 35 270 ft

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

1020 hPa 500 – 270 = 230 ft
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 3-5

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!’

3-6 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 <20 hPa ISA 20 hPa <ISA >20 hPa
change change change

QFF <1018 QFF = QNH 1018 QFF >1018 Sea level

The diagram shows an aerodrome above sea level. The QFE is 998 hPa and the elevation is 540
ft above sea level. QNH is always calculated using ISA conditions, so over 540 ft there is a 20
hPa change, making the QNH 20 hPa greater than the QFE, that is, 1018 hPa.

QFF is calculated using actual conditions. The left hand column demonstrates what happens
when it is warmer than ISA. The air is less dense so the pressure change is less over the same
height change. So the change is less than 20 hPa, making the QFF <1018.

The right hand column demonstrates what happens when it is colder than ISA. The air is denser
so the pressure change is more over the same height change. So the change is more than 20
hPa, making the QFF >1018.

3-8 Meteorology