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AC 00-6A
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AVIATION"
WEATH ER
Editoria 1 Note:
Figures 137 through 139 and 142 on pages 160,
161, and 165 have been rearranged to align with
their proper legends. Corresponding corrections
have been made in the Contents, page XII.
Editorial Note: (9/82)
Figures 137 through 139 on pages 160 and 161
have been rearranged in order to flow in
proper sequence with the text. Corresponding
corrections have been made in the Contents,
page XII.
For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402
Preface
AVIATION WEATHER is published jointly by the FAA Flight Standards Service and 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. But as aircraft flew farther, faster, and higher and as meteorological knowledge grew, the bulletin became obsolete. It was revised in 1954 as "Pilots' Weather Handbook" and again in 1965 under its present title. All these former editions suffered from one common problem. They dealt in part with weather services which change continually in keeping with current tech- niques and service demands. Therefore, each edition became somewhat outdated almost as soon as published; and its obsolescence grew throughout the period it remained in print. To alleviate this problem, the new authors have completely rewritten this edition streamlining it into a clear, concise, and· readable book and omitting all reference to specific weather services. Thus, the text will remain valid and adequate for many years. A companion manual, AVIATION WEATHER SERVICES, Advisory Circular 00-45, supplements AVIATION WEATHER. This supplement (AC 00-45) periodically is updated to reflect changes brought about by latest techniques, capabilities, and service demands. It explains current weather services and the formats and uses of weather charts and printed weather messages. The two manuals are sold separately; so at a nominal cost, a pilot can purchase a copy of the supplement (AC 00-45) periodically and keep current in aviation weather services. C. Hugh Snyder, National Weather Service Coordinator and Training Con- sultant at the FAA Academy, directed the preparation of AVIATION WEATHER and AVIATION WEATHER SERVICES. He and his assistant, John W. Zimmerman, Jr., did much of the writing and edited the final manuscripts. Recognition is given to these meteorologists on the NWS Coordinator's staff who helped write the original manu- script, organize the contents, and plan illustrations: Milton Lee Harrison, Edward A. Jessup, Joe L. Kendall, and Richard A. Mitchem. Beatrice Emery deserves special recognition for her relentless effort in typing, retyping, proofing, correcting, and assembling page after page of manuscript. Many other offices and individuals have contributed to the preparation, editing, and publication of the two volumes.
III
Contents
Page
Preface............................................................. III Introduction ......................................................... XIII
III ustrations
Figure Page
- CHAPTER 1. THE EARTH'S ATMOSPHERE. • • • • • PART I. WHAT YOU SHOULD KNOW ABOUT WEATHER - Composition. - Vertical Structure. - The Standard Atmosphere. - Density and Hypoxia.
- CHAPTER 2. TEMPERATURE........................................... - Temperature Scales. - Heat and Temperature. - Temperature Variations.... - In Closing.
- CHAPTER 3. ATMOSPHERIC PRESSURE AND ALTIMETRY. - Atmospheric Pressure. - Altimetry. - In Closing.
- CHAPTER 4. WIND.................................................. - Convection. - Pressure Gradient Force. - Coriolis Force. - The General Circulation. - Friction. - The Jet Stream. - Local and Small Scale Winds. - Wind Shear. - Wind, Pressure Systems, and Weather.
- CHAPTER 5. MOISTURE, CLOUD FORMATION, AND PRECIPITATION. - Water Vapor. - Change of State.... - Cloud Formation. - Precipitation , - Land and Water Effects " - In Closing.
- CHAPTER 6. STABLE AND UNSTABLE AIR , ..•.. ,. .•. .•.. .• .•. - Changes Within Upward and Downward Moving Air.... - Stability and Instability. - What Does It All Mean?
- CHAPTER 7. CLOUDS. Page - Identification. - Signposts in the Sky.
- CHAPTER 8. Am MASSES AND FRONTS. - Air Masses. - Fronts , , , - Fronts and Flight Planning.
- CHAPTER 9. TURBULENCE............................................ - Convective Currents. - Obstructions to Wind Flow. - Wind Shear. - Wake Turbulence , , '" " " - In Closing.
- CHAPTER 10. ICING................................................. - Structural Icing. - Induction System Icing. - Instrument Icing. - Icing and Cloud Types. - Other Factors in Icing " - Ground Icing. - Frost , '" '" - In Closing. "
- CHAPTER 11. THUNDERSTORMS........................................ - Where and When? " - They Don't Just Happen.......................................... - The Inside Story " - Rough and Rougher " - Hazards. - Thunderstorms and Radar. - Do's and Don'ts of Thunderstorm Flying "
- CHAPTER 12. COMMON IFR PRODUCERS. - Fog............................................................ - Low Stratus Clouds. - Haze and Smoke , '" '" , , '" " - Blowing Restrictions to Visibility. - Precipitation " - Obscured or Partially Obscured Sky " - In Closing. "
- CHAPTER 13. HIGH ALTITUDE WEATHER " PART II. OVER AND BEYOND - The Tropopause " - The Jet Stream. - Cirrus Clouds " - Clear Air Turbulence... - Condensation Trails " - Haze Layers. Page - Canopy Static. - Icing " - Thunderstorms.
- CHAPTER 14. ARCTIC WEATHER ••••••..•..•..•••••••..•••......•• " • •. - Climate, Air Masses, and Fronts. - Arctic Peculiarities. - Weather Hazards " - Arctic Flying Weather. - In Closing.
- CHAPTER 15. TROPICAL WEATHER..... ..• •.•• •• ..•. •.. .•.... ...•. ....• - Circulation.
- CHAPTER 16. SOARING WEATHER.... •• .• ..•. .• •• .•• .• .•• •.. .•• - Thermal Soaring. - Frontal Soaring. - Sea Breeze Soaring.
- Ridge or Hill Soaring..
- Mountain Wave Soaring.
- In Closing.
- Glossary of Weather Terms..
- Index. - 1. Composition of a dry atmosphere. Figure Page - 2. The atmosphere divided into layers based on temperature. - 3. The two temperature scales in common use. - 4. World-wide average surface temperatures in July. - 5. World-wide average surface temperatures in January. - 6. Temperature differences create air movement and, at times, cloudiness. - 7. Inverted lapse rates or "inversions" - 8. The mercurial barometer. - 9. The aneroid barometer.
- The standard atmosphere.
- with temperature. 11. Three columns of air showing how decrease of pressure with height varies
- Reduction of station pressure to sea leveL..
- Pressure systems.
- Indicated altitude depends on air temperature below the aircraft.
- altimeter, you are losing true altitude.... 15. When flying from high pressure to lower pressure without adjusting your
- Effect of temperature on altitude.
- Effect of density altitude on takeoff and climb.
- surface temperatures. 18. Convective current resulting from uneven heating of air by contrasting
- Circulation as it would be on a nonrotating globe.
- Apparent deflective force due to rotation of a horizontal platform.
- Effect of Coriolis force on wind relative to isobars.
- westerlies and polar winds to easterlies. 22. In the Northern Hemisphere, Coriolis force turns equatorial winds to
- Mean world-wide surface pressure distribution in July.
- Mean world-wide surface pressure distribution in January.
- General average circulation in the Northern Hemisphere.
- Air flow around pressure systems above the friction layer.
- deflected across the isobars toward lower pressure. 27. Surface friction slows the wind and reduces Coriolis force; winds are
- Circulation around pressure systems at the surface.
- The "Chinook" is a katabatic (downslope) wind.
- Land and sea breezes.
- Wind shear.
- Blue dots illustrate the increased water vapor capacity of warm air.
- Relative humidity depends on both temperature and water vapor.
- Virga.
- Heat transactions when water changes state.
- Growth of raindrops by collision of cloud droplets.
- Lake effects , ,
- carry showers as far eastward as the Appalachians. 38. Strong cold winds across the Great Lakes absorb water vapor and may
- Florida , 39. A view of clouds from 27,000 feet over Lake Okeechobee in southern
- Decreasing atmospheric pressure causes the balloon to expand as it rises Figure Page
- wind......................................................... 41. Adiabatic warming of downward moving air produces the warm Chinook
- Stability related to temperatures aloft and adiabatic cooling.
- development. unstable air is forced upward, cloudiness shows extensive vertical
- Cloud base determination.
- Cirrus.....
- Cirrocumulus.
- Cirrostratus.............................................
- Altocumulus.
- Altostratus....
- Altocumulus castella nus
- Standing lenticular altocumulus clouds.
- Nimbostratus.
- Stratus..
- Stratocumulus.....
- Cumulus.
- Towering cumulus..
- Cumulonimbus , '" '" "
- Horizontal uniformity of an air mass.
- Cross section of a cold front with the weather map symbol.
- Cross section of a warm front with the weather map symbol.
- Cross section of a stationary front and its weather map symbol.
- The life cycle of a frontal wave.
- Cross section of a warm-front occlusion and its weather map symbol.
- Cross section of a cold-front occlusion.
- Frontolysis of a stationary front.
- Frontogenesis of a stationary front.
- A cold front underrunning warm, moist, stable air.
- A cold front underrunning warm, moist, unstable air.
- A warm front with overrunning moist, stable air.
- A slow-moving cold front underrunning warm, moist, unstable air.
- A warm front with overrunning warm, moist, unstable air.
- A fast moving cold front underrunning warm, moist, unstable air...
- A warm front occlusion lifting warm, moist, unstable air.
- A cold front occlusion lifting warm, moist, stable air.
- An aerial view of a portion of a squall line.
- Effect of convective currents on final approach.
- Avoiding turbulence by flying above convective clouds.. - obstructions. 78. Eddy currents formed by winds blowing over uneven ground or over
- Turbulent air in the landing area.
- Wind flow in mountain areas...
- Schematic cross section of a mountain wave.
- Standing lenticular clouds associated with a mountain wave. - mountain waves , 83. Standing wave rotor clouds marking the rotary circulation beneath - manned spacecraft. 84. Mountain wave clouds over the Tibetan Plateau photographed from a - approximately the same time. 85. Satellite photograph of a mountain wave and the surface analysis for - strong wind above the inversion. 86. Wind shear in a zone between relatively calm wind below an inversion and
- Wake turbulence wing tip vortices developing as aircraft breaks ground.
- Planning landing or takeoff to avoid heavy aircraft wake turbulence.
- Effects of structural icing.
- Clear, rime, and mixed icing on airfoils..
- Clear wing icing (leading edge and underside).
- Propeller icing.
- Rime icing on the nose of a Mooney "Mark 21" aircraft.
- External icing on a pitot tube.
- Carburetor icing.
- Internal pitot tube icing.
- Clear ice on an aircraft antenna mast.
- Freezing rain with a warm front and a cold front.
- Frost on an aircraft.
- The average number of thunderstorms each year.
- The average number of days with thunderstorms during spring.
- The average number of days with thunderstorms during summer.
- The average number of days with thunderstorms during fall.
- The average number of days with thunderstorms during winter.
- The stages of a thunderstorm...
- Schematic of the mature stage of a steady state thunderstorm cell "
- A tornado.
- A waterspout.
- Funnel clouds.
- Cumulonimbus Mamma clouds....................................
- Tornado incidence by State and area.
- Squall line thunderstorms..
- Schematic cross section of a thunderstorm..
- Hail damage to an aircraft...
- Radalphotograph of a line of thunderstorms '.
- Use of airborne radar to avoid heavy precipitation and turbulence.
- Ground fog as seen from the air "
- Advection fog in California.
- Advection fog over the southeastern United States and Gulf Coast.
- Smoke trapped in stagnant air under an inversion.
- Aerial photograph of blowing dust approaching with a cold front.
- the ceiling caused by a layer aloft. 122. Difference between the ceiling caused by a surface-based obscuration and
- A cross section of the upper troposphere and lower stratosphere.
- Artist's concept of the jet stream.
- Ajet stream segment...
- Multiple jet streams.
- Mean jet positions relative to surface systems.
- 128a. Satellite photograph of an occluded system.
- 128b. Infrared photograph of the system shown in figure 128a.. - rapidly deepening surface low. 129. A frequent CAT location is along the jet stream north and northeast of a
- Contrails.......................................................
- The Arctic... Figure Page
- Sunshine in the Northern Hemisphere "
- The permanent Arctic ice pack ,
- Average number of cloudy days per month (Arctic) ,
- Visibility reduced by blowing snow.
- A typical frozen landscape of the Arctic..
- vergence Zone. 137. Vertical cross section illustrating convection in the Intertropical 'Con-
- Prevailing winds throughout the Tropics in July.
- Prevailing winds in the Tropics in January.
- the subtropics " 140. A shear line and an induced trough caused by a polar high pushing into
- A trough aloft across the Hawaiian Islands.
- A Northern Hemisphere easterly wave "
- Vertical cross section along line A-B in figure 142..
- tions of movement. 144. Principal regions where tropical cyclones form and their favored direc-
- Radar photograph of hurricane "Donna".
- Using surface dust and smoke movement as indications of a thermal.
- Horizontal cross section of a dust devil rotating clockwise '"
- Cumulus clouds grow only with active thermals..
- Photograph of a dying cumulus...
- Altocumulus castellanus clouds are middle level convective clouds.
- type , 153. Experience indicates that the "chimney" thermal is the most prevalent
- Thermals may be intermittent "bubbles" "
- It is believed that a bubble thermal sometimes develops a vortex ring.
- Wind causes thermals to lean.
- Photograph of cumulus clouds severed by wind shear.
- Conditions favorable for thermal streeting.
- resolution camera.... 159. Cumulus clouds in thermal streets photographed from a satellite high
- The Pseudo-Adiabatic Ohart.
- chart '" " 161. An early morning upper air observation plotted on the pseudo-adiabatic
- Computing the thermal index (TI)....
- Another example of computing TI's and maximum height of thermals...
- observation or APOB. l64. An upper air observation made from an aircraft called an airplane
- Schematic cross section through a sea breeze front.
- Sea breeze flow into the San Fernando Valley " ,
- Sea breeze convergence zone, Cape Cod, Massachusetts.
- Schematic cross section of airflow over a ridge.
- Strong winds flowing around an isolated peak.
- Wind flow over various types of terrain.
- Schematic cross section of a mountain wave.
- Wave length and amplitude...
o
Part ONE
WHAT YOU SHOULD KNOW
ABOUT WEATHER
Chapter 1
HE EARTH'S ATMOSPHERE
Planet Earth is unique in that its atmosphere sustains life as we know it. Weather-the state of the atmosphere-at any given time and place strongly influences our daily routine as well as our general life patterns. Virtually all of our activities are affected by weather, but of all man's endeav - ors, none is influenced more intimately by weather than aviation. Weather is comp le x and at times difficult to un - derstand. Our restless atmosphere is almost con-
stantly in motion as it strives to reach equilibrium. These never- ending air movements set up chain reactions which culminate in a continuing variety of weather. Later chapters in this book delve into the atmosphere in motion. This c hapt er looks brief- ly at our atmosphere in terms of its composition; vertical structure; the standard atmosphere; and of special concern to you, the pilot, density and hypoxia.
COMPOSITION
Air is a mixture of several gases. When com- pletely dry, it is about 78% nitrogen and 21 % oxy- gen. The remaining 1 % is other gases such as Argon, Carbon Dioxide, Neon, Helium, and others. Figure 1 graphs these proportions. However, in na- ture, air is never completely dry. It always contains some water vapor in amounts varying from almost zero to about 5% by volume. As water vapor con- tent increases, the other gases decrease propor- tionately.
OTHER GASES .1/
OXYGEN
NITROGEN
FIGURE 1. Composition of a dry atmosphere. Nitrogen comprises about 78%; oxygen, about 21 %; and other
gases, about 1 %. When water vapor is added, the per-
centages decrease proportionately. Water vapor varies from almost none to about 5% by volume.
VERTICAL STRUCTURE
We classify the atmosphere into layers, or spheres, by characteristics exhibited in these lay- ers. Figure 2 shows one division which we use in this book. Since most weather occurs in the tropo- sphere and since most flying is in the troposphere and stratosphere, we restrict our discussions mostly to these two layers. The TROPOSPHERE is the layer from the sur- face to an average altitude of about 7 miles. It is characterized by an overall decrease of temperature with increasing altitude. The height of the tropo- sphere varies with latitude and seasons. It slopes from about 20,000 feet over the poles to about
65,000 feet over the Equator; and it is higher in summer than in winter. At the top of the troposphere is the TROPO- PAUSE, a very thin layer marking the boundary between the troposphere and the layer above. The height of the tropopause and certain weather phe- nomena are related. Chapter 13 discusses in detail the significance of the tropopause to flight. Above the tropopause is the STRATOSPHERE. This layer is typified by relatively small changes in temperature with height except for ' a warming trend near the top.
THE STANDARD ATMOSPHERE
Continual fluctuations of temperature and pres- sure in our restless atmosphere create some prob- lems for engineers and meteorologists who require a fixed standard of reference. To arrive at a stan- dard, they averaged conditions throughout the at- mosphere for all latitudes, seasons, and altitudes. The result is a STANDARD ATMOSPHERE with
2
specified sea-level temperature and pressure and specific rates of change of temperature and pres- sure with height. It is the standard for calibrating the pressure altimeter and developing aircraft per- formance data. We refer to it often throughout this book.
