U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Aviation Weather Date: 8/23/16 AC No: 00-6B Initiated by: AFS-400 Change:
This advisory circular (AC) was published by the Federal Aviation Administration (FAA) Flight Standards Service (AFS), with contributions from the National Weather Service (NWS).
The publication began in 1943 as CAA Bulletin No. 25, Meteorology for Pilots, which at the time contained weather knowledge considered essential for most pilots. As aircraft flew farther, faster, and higher, and as meteorological knowledge grew, the bulletin became obsolete. It was revised in 1954 under a new title, The Pilots’ Weather Handbook, and updated again in 1965.
In 1975 it was revised under its current title.
Previous editions have suffered one common problem—they dealt in part with weather services that continually change, in keeping with current techniques and service demands. As a result, each edition was somewhat outdated almost as soon as it was published, its obsolescence growing throughout the period it remained in print.
In 1975, in order to alleviate this problem, the authors completely rewrote the AC. They streamlined it into a clear, concise, readable book, and omitted all reference to specific weather services.
The 1975 text remained valid and adequate for many years. Its companion manual, the current edition of AC 00-45, Aviation Weather Services, supplements this AC. In 2015, this supplement was updated concurrently with this text. This was done to reflect changes brought about by new products and services, particularly since this information is now available through the Internet.
The companion AC describes current weather services and formats, and uses real world examples of weather graphics and text products.
The two manuals can be downloaded for free via the Internet in PDF format. Print versions are also sold separately at nominal cost, allowing pilots the opportunity to own a reference copy of the supplement to keep current with aviation weather services.
New scientific capabilities now necessitate an update to this AC. In 1975, aviation users were not directly touched by radar and satellite weather. In 2016, much of what airmen understand about the current atmosphere comes from these important data sources. This AC is intended to provide basic weather information that all airmen must know. This document is intended to be used as a resource for pilot and dispatcher training programs.
This AC cancels AC 00-6A, Aviation Weather for Pilots and Flight Operations Personnel.
ORIGINAL SIGNED by /s/ John Barbagallo Deputy Director, Flight Standards Service
iii
CONTENTS Paragraph Page
Chapter 1. The Earth’s Atmosphere1-1
1.1 Introduction1-1
1.2 Composition1-1
1.2.1 Air Parcel1-2
1.3 Vertical Structure1-2
1.3.1 Troposphere1-2
1.3.2 Stratosphere1-3
1.3.3 Mesosphere1-4
1.3.4 Thermosphere1-5
1.3.5 Exosphere1-5
1.4 The Standard Atmosphere1-5
Chapter 2. Heat and Temperature2-1
2.1 Introduction2-1
2.2 Matter2-1
2.3 Energy2-1
2.4 Heat2-1
2.5 Temperature2-1
2.5.1 Temperature Measurement2-1
2.5.2 Temperature Scales2-1
2.6 Heat Transfer2-3
2.6.1 Radiation2-3
2.6.2 Conduction2-6
2.6.3 Convection2-7
2.7 Thermal Response2-8
2.8 Temperature Variations with Altitude2-11
2.8.1 Atmospheric Sounding2-12
2.8.2 Isothermal Layer2-12
2.8.3 Temperature Inversion2-12
iv
Chapter 3. Water Vapor3-1
3.1 Introduction3-1
3.2 The Hydrologic Cycle3-1
3.2.1 Evaporation3-2
3.2.2 Transpiration3-2
3.2.3 Sublimation3-2
3.2.4 Condensation3-2
3.2.5 Transportation3-2
3.2.6 Precipitation3-2
3.2.7 Runoff3-2
3.2.8 Infiltration3-2
3.2.9 Groundwater Flow3-3
3.2.10 Plant Uptake3-3
3.3 Saturation3-3
3.4 Relative Humidity3-3
3.5 Dewpoint3-4
3.6 Temperature-Dewpoint Spread (Dewpoint Depression)3-4
3.7 Change of Phase3-5
3.7.1 Latent Heat3-5
Chapter 4. Earth-Atmosphere Heat Imbalances4-1
4.1 Introduction4-1
4.2 The Earth-Atmosphere Energy Balance4-1
4.3 Heat Imbalances Between Earth’s Surface and the Atmosphere4-3
4.3.1 Sensible Heating4-3
4.3.2 Latent Heat4-4
4.4 Heat Imbalance Variations with Latitude4-4
4.5 Seasons4-5
4.6 Diurnal Temperature Variation4-7
Chapter 5. Atmospheric Pressure and Altimetry5-1
5.1 Introduction5-1
v
5.2 Atmospheric Pressure5-1
5.2.1 Barometer5-2
5.2.2 Atmospheric Pressure Units5-2
5.2.3 Station Pressure5-3
5.2.4 Pressure Variation5-3
5.2.5 Sea Level Pressure5-5
5.2.6 Constant Pressure Surface5-7
5.3 Density5-10
5.3.1 Volume Effects on Density5-10
5.3.2 Changes in Density5-11
5.3.3 Density Effects on Pressure5-11
5.3.4 Temperature Effects on Density5-12
5.3.5 Water Vapor Effects on Density5-12
5.4 Altimetry5-13
5.4.1 Altitude5-13
5.5 Density Altitude5-16
Chapter 6. Weather Charts6-1
6.1 Introduction6-1
6.2 Weather Observation Sources6-1
6.3 Analysis6-2
6.3.1 Analysis Procedure6-2
6.4 Surface Chart6-6
6.5 Constant Pressure Chart6-7
Chapter 7. Wind7-1
7.1 Introduction7-1
7.2 Naming of the Wind7-1
7.3 Forces That Affect the Wind7-1
7.3.1 Pressure Gradient Force (PGF)7-1
7.3.2 Coriolis Force7-2
7.3.3 Friction Force7-4
7.4 Upper Air Wind7-5
7.5 Surface Wind7-6
vi
Chapter 8. Global Circulations and Jet Streams8-1
8.1 Non-Rotating Earth Circulation System8-1
8.2 Rotating Earth Circulation System8-1
8.3 Jet Streams8-2
8.3.1 Introduction8-2
8.3.2 Direction of Wind Flow8-3
8.3.3 Location8-4
Chapter 9. Local Winds9-1
9.1 Description9-1
9.2 Hazards9-2
9.3 Sea Breeze9-2
9.3.1 Sea Breeze Front9-3
9.3.2 Effects of Coastline Shape9-3
9.4 Land Breeze9-4
9.5 Lake Breeze9-5
9.6 Valley Breeze9-7
9.7 Mountain-Plains Wind System9-8
9.8 Mountain Breeze9-9
Chapter 10. Air Masses, Fronts, and the Wave Cyclone Model10-1
10.1 Air Masses10-1
10.1.1 Air Mass Classification10-1
10.1.2 Air Mass Modification10-2
10.2 Fronts10-3
10.3 The Wave Cyclone Model10-7
10.4 Dryline10-9
Chapter 11. Vertical Motion and Cloud Formation11-1
11.1 Introduction11-1
11.2 Vertical Motion Effects on an Unsaturated Air Parcel11-1
11.3 Vertical Motion Effects on a Saturated Air Parcel11-2
11.4 Common Sources of Vertical Motion11-5
11.4.1 Orographic Effects11-5
11.4.2 Frictional Effects11-6
vii
11.4.3 Frontal Lift11-6
11.4.4 Buoyancy11-7
Chapter 12. Atmospheric Stability12-1
12.1 Introduction12-1
12.2 Using a Parcel as a Tool to Evaluate Stability12-1
12.3 Stability Types12-1
12.3.1 Absolute Stability12-1
12.3.2 Neutral Stability12-2
12.3.3 Absolute Instability12-3
12.3.4 Conditional Instability12-4
12.3.5 Summary of Stability Types12-6
12.4 Processes that Change Atmospheric Stability12-7
12.4.1 Wind Effects on Stability12-7
12.4.2 Vertical Air Motion Effects on Stability12-7
12.4.3 Diurnal Temperature Variation Effects on Stability12-8
12.5 Measurements of Stability12-9
12.5.1 Lifted Index12-9
12.5.2 Convective Available Potential Energy12-10
12.6 Summary12-11
Chapter 13. Clouds13-1
13.1 Introduction13-1
13.2 Cloud Forms13-1
13.3 Cloud Levels13-2
13.4 Cloud Types13-2
13.4.1 High Clouds13-2
13.4.2 Middle Clouds13-5
13.4.3 Low Clouds13-9
Chapter 14. Precipitation14-1
14.1 Introduction14-1
14.2 Necessary Ingredients for Formation14-1
14.3 Growth Process14-1
14.4 Precipitation Types14-3
viii
Chapter 15. Adverse Wind15-1
15.1 Introduction15-1
15.2 Crosswind15-1
15.3 Gust15-1
15.4 Tailwind15-2
15.5 Variable Wind/Sudden Wind Shift15-2
15.6 Wind Shear15-2
Chapter 16. Weather, Obstructions to Visibility, Low Ceiling, and Mountain Obscuration16-1
16.1 Weather and Obstructions to Visibility16-1
16.1.1 Fog16-1
16.1.2 Mist16-5
16.1.3 Haze16-5
16.1.4 Smoke16-5
16.1.5 Precipitation16-5
16.1.6 Blowing Snow16-6
16.1.7 Dust Storm16-6
16.1.8 Sandstorm16-6
16.1.9 Volcanic Ash16-7
16.2 Low Ceiling and Mountain Obscuration16-8
16.2.1 Low Ceiling16-8
16.2.2 Mountain Obscuration16-9
Chapter 17. Turbulence17-1
17.1 Introduction17-1
17.2 Causes of Turbulence17-1
17.2.1 Convective Turbulence17-1
17.2.2 Mechanical Turbulence17-3
17.2.3 Wind Shear Turbulence17-5
17.3 Turbulence Factors17-6
Chapter 18. Icing18-1
18.1 Introduction18-1
18.2 Supercooled Water18-1
ix
18.3 Structural Icing18-1
18.3.1 Rime Icing18-1
18.3.2 Clear Icing18-1
18.3.3 Mixed Icing18-2
18.3.4 Icing Factors18-2
18.3.5 Icing in Stratiform Clouds18-4
18.3.6 Icing in Cumuliform Clouds18-4
18.3.7 Icing with Fronts18-4
18.3.8 Icing with Mountains18-5
18.3.9 Icing Hazards18-6
18.4 Engine Icing18-7
18.4.1 Carburetor Icing18-7
18.4.2 High Ice Water Content (HIWC)18-7
Chapter 19. Thunderstorms19-1
19.1 Introduction19-1
19.2 Necessary Ingredients for Thunderstorm Cell Formation19-1
19.3 Thunderstorm Cell Life Cycle19-1
19.4 Thunderstorm Types19-2
19.5 Factors that Influence Thunderstorm Motion19-5
19.6 Hazards19-6
19.6.1 Lightning19-6
19.6.2 Adverse Wind19-6
19.6.3 Downburst19-6
19.6.4 Turbulence19-8
19.6.5 Icing19-8
19.6.6 Hail19-9
19.6.7 Rapid Altimeter Changes19-10
19.6.8 Static Electricity19-10
19.6.9 Tornado19-10
x
Chapter 20. Weather Radar20-1
20.1 Principles of Weather Radar20-1
20.1.1 Antenna20-1
20.1.2 Backscattered Energy20-1
20.1.3 Power Output20-2
20.1.4 Wavelengths20-2
20.1.5 Attenuation20-3
20.1.6 Resolution20-4
20.1.7 Wave Propagation20-7
20.1.8 Intensity of Precipitation20-9
Chapter 21. Tropical Weather21-1
21.1 Circulation21-1
21.1.1 Subtropical High Pressure Belts21-1
21.1.2 Trade Wind Belts21-3
21.1.3 The Intertropical Convergence Zone (ITCZ)21-5
21.1.4 Monsoon21-5
21.2 Transitory Systems21-7
21.2.1 Remnants of Polar Fronts and Shear Lines21-7
21.2.2 Tropical Upper Tropospheric Trough (TUTT)21-8
21.2.3 Tropical Wave21-9
21.2.4 West African Disturbance Line (WADL)21-10
21.2.5 Tropical Cyclones21-11
Chapter 22. Arctic Weather22-1
22.1 Introduction22-1
22.2 Climate, Air Masses, and Fronts22-1
22.2.1 Long Days and Nights22-2
22.2.2 Land and Water22-2
22.2.3 Temperature22-2
22.2.4 Clouds and Precipitation22-2
22.2.5 Wind22-3
22.2.6 Air Masses—Winter22-3
xi
22.2.7 Air Masses—Summer22-3
22.2.8 Fronts22-3
22.3 Arctic Peculiarities22-3
22.3.1 Effects of Temperature Inversion22-3
22.3.2 Light Reflection by Snow-Covered Surfaces22-3
22.3.3 Light from Celestial Bodies22-3
22.4 Weather Hazards22-4
22.4.1 Fog and Ice Fog22-4
22.4.2 Blowing and Drifting Snow22-4
22.4.3 Frost22-4
22.4.4 Whiteout22-4
Chapter 23. Space Weather23-1
23.1 The Sun—Prime Source of Space Weather23-1
23.2 The Sun’s Energy Output and Variability23-1
23.3 Sunspots and the Solar Cycle23-1
23.4 Solar Wind23-1
23.5 Solar Eruptive Activity23-2
23.6 Geospace23-2
23.7 Galactic Cosmic Radiation23-3
23.8 Geomagnetic Storms23-4
23.9 Solar Radiation Storms23-4
23.10 Ionospheric Storms23-5
23.11 Solar Flare Radio Blackouts23-5
23.12 Effects of Space Weather on Aircraft Operations23-6
23.12.1 Communications23-6
23.12.2 Navigation and Global Positioning System (GPS)23-6
23.12.3 Radiation Exposure to Flight Crews and Passengers23-6
23.12.4 Radiation Effects on Avionics23-6
xii List of Figures
Figure 1-1. Vertical Structure of the Atmosphere1-4
Figure 1-2. U.S. Standard Atmosphere within the Troposphere1-6
Figure 2-1. Comparison of Kelvin, Celsius, and Fahrenheit Temperature Scales2-3
Figure 2-2. Radiation Example2-4
Figure 2-3. Temperature’s Effect on Radiation Wavelength2-5
Figure 2-4. Solar Zenith Angle2-6
Figure 2-5. Heat Transfer Examples2-8
Figure 2-6. Specific Heat Capacity: Water Versus Sand2-10
Figure 2-7. Variation of Mean Daily Temperatures for San Francisco (Maritime)
and Kansas City (Continental)2-11
Figure 2-8. Sounding with an Isothermal Layer2-12
Figure 2-9. Sounding with a Temperature Inversion2-13
Figure 3-1. The Hydrologic Cycle3-1
Figure 3-2. Temperature Effects on Relative Humidity3-4
Figure 3-3. Temperature-Dewpoint Spread Effect on Relative Humidity3-5
Figure 3-4. Latent Heat Transactions when Water Undergoes Phase Transition3-6
Figure 4-1. Earth-Atmosphere Energy Balance4-1
Figure 4-2. Greenhouse Effect on Nighttime Radiational Cooling4-2
Figure 4-3. Development of a Thermal4-3
Figure 4-4. Example of Convection in the Atmosphere4-4
Figure 4-5. Solar Zenith Angle Variations with Latitude4-5
Figure 4-6. Solar Zenith Angle Variations with Northern Hemisphere Seasons4-6
Figure 4-7. Average Seasonal Temperature Variation in the Northern Hemisphere4-6
Figure 4-8. Clear Sky Diurnal Temperature and Radiation Variations Over Land4-7
Figure 5-1. Air Has Weight5-1
Figure 5-2. Aneroid Barometer5-2
Figure 5-3. Station Pressure5-3
Figure 5-4. Air Pressure in the Standard Atmosphere5-4
Figure 5-5. Temperature Effect on Pressure5-5
Figure 5-6. Reduction of Station Pressure to Sea Level5-5
Figure 5-7. Surface Chart Pressure Patterns5-7
Figure 5-8. Weather Balloon and Radiosonde5-8
xiii
Figure 5-9. 500 Millibar Constant Pressure Chart5-9
Figure 5-10. Density is Mass (Weight) per Volume5-10
Figure 5-11. Volume Effects on Density5-10
Figure 5-12. Pressure Effects on Density in the Atmosphere5-11
Figure 5-13. Temperature Effects on Density5-12
Figure 5-14. Water Vapor Effects on Density5-12
Figure 5-15. True Versus Indicated Altitude5-13
Figure 5-16. Pressure Change Effects on Altimeter Readings5-14
Figure 5-17. Temperature Change Effects on Altimeter Readings5-15
Figure 5-18. High Density Altitude Effects on Flight5-17
Figure 6-1. Weather Observation Sources6-1
Figure 6-2. Analysis Procedure Step 1: Determine the Optimal Contour Interval
and Values to be Analyzed6-3
Figure 6-3. Analysis Procedure Step 2: Draw the Isopleths and Extrema6-4
Figure 6-4. Analysis Procedure Step 3: Interpret Significant Weather Features6-6
Figure 6-5. Example of a Surface Chart6-7
Figure 6-6. Example of a 500 Millibar Constant Pressure Chart6-8
Figure 7-1. Direction of Pressure Gradient Force7-1
Figure 7-2. Magnitude of Pressure Gradient Force7-2
Figure 7-3. Illustration of Coriolis Force7-2
Figure 7-4. Coriolis Force Variations Across the Earth7-3
Figure 7-5. Coriolis Force Magnitude Variations with Wind Speed7-4
Figure 7-6. Friction Force Magnitude Variations with Terrain Roughness7-4
Figure 7-7. Friction Force Magnitude Variations with Wind Speed7-4
Figure 7-8. Geostrophic Wind7-5
Figure 7-9. Upper Air Wind Flow7-5
Figure 7-10. Surface Wind Forces7-6
Figure 7-11. Surface Wind Flow7-6
Figure 8-1. Non-Rotating, Non-Tilted, Waterless, Earth Circulation System8-1
Figure 8-2. Earth Circulation System8-2
Figure 8-3. Speed Relative to the Earth’s Axis Versus Latitude8-3
Figure 8-4. Three Cell Circulations and Jet Stream Location8-4
Figure 8-5. Polar and Subtropical Jet Streams8-4