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

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 techniques 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 Therefore, each edition became somewhat outdated almost as soon as published; and its obsolescence grew throughout the period it remained in print.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 Consultant 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 manuscript, 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.

Table of Contents

	Page
Preface	III
Introduction	XIII

PART I. WHAT YOU SHOULD KNOW ABOUT WEATHER

CHAPTER 1. THE EARTH'S ATMOSPHERE	1
Composition	2
Vertical Structure	2
The Standard Atmosphere	2
Density and Hypoxia	3
CHAPTER 2. TEMPERATURE	5
Temperature Scales	6
Heat and Temperature	6
Temperature Variations	7
In Closing	10
CHAPTER 3. ATMOSPHERIC PRESSURE AND ALTIMETRY	11
Atmospheric Pressure	11
Altimetry	17
In Closing	21
CHAPTER 4. WIND	23
Convection	23
Pressure Gradient Force	24
Coriolis Force	25
The General Circulation	26
Friction	30
The Jet Stream	31
Local and Small Scale Winds	31
Wind Shear	34
Wind, Pressure Systems, and Weather	35
CHAPTER 5. MOISTURE, CLOUD FORMATION, AND PRECIPITATION	37
Water Vapor	37
Change of State	39
Cloud Formation	42
Precipitation	42
Land and Water Effects	43
In Closing	45
CHAPTER 6. STABLE AND UNSTABLE AIR	47
Changes Within Upward and Downward Moving Air	47
Stability and Instability	49
What Does It All Mean?	52
CHAPTER 7. CLOUDS	53
Identification	53
Signposts in the Sky	62
CHAPTER 8. AIR MASSES AND FRONTS	63
Air Masses	63
Fronts	64
Fronts and Flight Planning	78
CHAPTER 9. TURBULENCE	79
Convective Currents	80
Obstructions to Wind Flow	82
Wind Shear	86
Wake Turbulence	88
In Closing	90
CHAPTER 10. ICING	91
Structural Icing	92
Induction System Icing	97
Instrument Icing	98
Icing and Cloud Types	99
Other Factors in Icing	100
Ground Icing	102
Frost	102
In Closing	102
CHAPTER 11. THUNDERSTORMS	105
Where and When?	105
They Don't Just Happen	111
The Inside Story	111
Rough and Rougher	112
Hazards	113
Thunderstorms and Radar	120
Do's and Don'ts of Thunderstorm Flying	121
CHAPTER 12. COMMON IFR PROCEDURES	125
Fog	126
Low Stratus Clouds	128
Haze and Smoke	129
Blowing Restrictions to Visibility	129
Precipitation	130
Obscured or Partially Obscured Sky	130
In Closing	130

PART II. OVER AND BEYOND

CHAPTER 13. HIGH ALTITUDE WEATHER	135
The Tropopause	136
The Jet Stream	136
Cirrus Clouds	139
Clear Air Turbulence	142
Condensation Trails	143
Haze Layers	144
Canopy Static	145
Icing	145
Thunderstorms	145
CHAPTER 14. ARCTIC WEATHER	147
Climate, Air Masses, and Fronts	148
Arctic Peculiarities	152
Weather Hazards	153
Arctic Flying Weather	154
In Closing	155
CHAPTER 15. TROPICAL WEATHER	157
Circulation	158
Transitory Systems	162
CHAPTER 16. SOARING WEATHER	171
Thermal Soaring	172
Frontal Soaring	191
Sea Breeze Soaring	191
Ridge or Hill Soaring	195
Mountain Wave Soaring	198
In Closing	200
Glossary of Weather Terms	201
Index	215

ILLUSTRATIONS

Figure		Page
1.	Composition of a dry atmosphere	2
2.	The atmosphere divided into layers based on temperature	3
3.	The two temperature scales in common use	6
4.	World-wide average surface temperatures in July	8
5.	World-wide average surface temperatures in January	8
6.	Temperature differences create air movement and, at times, cloudiness	9
7.	Inverted lapse rates or "inversions"	10
8.	The mercurial barometer	12
9.	The aneroid barometer	13
10.	The standard atmosphere	14
11.	Three columns of air showing how decrease of pressure with height varies with temperature	15
12.	Reduction of station pressure to sea level	15
13.	Pressure systems	16
14.	Indicated altitude depends on air temperature below the aircraft	17
15.	When flying from high pressure to lower pressure without adjusting your altimeter, you are
	losing true altitude	18
16.	Effect of temperature on altitude	19
17.	Effect of density altitude on takeoff and climb	20
18.	Convective current resulting from uneven heating of air by contrasting surface temperatures	24
19.	Circulation as it would be on a nonrotating globe	25
20.	Apparent deflective force due to rotation of a horizontal platform	26
21.	Effect of Coriolis force on wind relative to isobars	27
22.	In the Northern Hemisphere, Coriolis force turns equatorial winds to westerlies and polar winds to
	easterlies	28
23.	Mean world-wide surface pressure distribution in July	28
24.	Mean world-wide surface pressure distribution in January	29
25.	General average circulation in the Northern Hemisphere	30
26.	Air flow around pressure systems above the friction layer	31
27.	Surface friction slows the wind and reduces Coriolis force; winds are deflected across the isobars
	toward lower pressure	32
28.	Circulation around pressure systems at the surface	33
29.	The "Chinook" is a katabatic (downslope) wind	33
30.	Land and sea breezes	34
31.	Wind shear	35
32.	Blue dots illustrate the increased water vapor capacity of warm air	38
33.	Relative humidity depends on both temperature and water vapor	39
34.	Virga	40
35.	Heat transactions when water changes state	41
36.	Growth of raindrops by collision of cloud droplets	42
37.	Lake effects	43
38.	Strong cold winds across the Great Lakes absorb water vapor and may carry showers
	as far eastward as the Appalachians	44
39.	A view of clouds from 27,000 feet over Lake Okeechobee in southern Florida	45
40.	Decreasing atmospheric pressure causes the balloon to expand as it rises	48
41.	Adiabatic warming of downward moving air produces the warm Chinook wind	49
42.	Stability related to temperatures aloft and adiabatic cooling	50
43.	When stable air is forced upward, cloudiness is flat and stratified. When unstable air is
	forced upward, cloudiness shows extensive vertical development	51
44.	Cloud base determination	52
45.	Cirrus	54
46.	Cirrocumulus	55
47.	Cirrostratus	55
48.	Altocumulus	56
49.	Altostratus	56
50.	Altocumulus castellanus	57
51.	Standing lenticular altocumulus clouds	58
52.	Nimbostratus	59
53.	Stratus	59
54.	Stratocumulus	60
55.	Cumulus	60
56.	Towering cumulus	61
57.	Cumulonimbus	61
58.	Horizontal uniformity of an air mass	64
59.	Cross section of a cold front with the weather map symbol	66
60.	Cross section of a warm front with the weather map symbol	67
61.	Cross section of a stationary front and its weather map symbol	68
62.	The life cycle of a frontal wave	69
63.	Cross section of a warm-front occlusion and its weather map symbol	70
64.	Cross section of a cold-front occlusion	71
65.	Frontolysis of a stationary front	71
66.	Frontogenesis of a stationary front	72
67.	A cold front underrunning warm, moist, stable air	73
68.	A cold front underrunning warm, moist, unstable air	73
69.	A warm front with overrunning moist, stable air	74
70.	A slow-moving cold front underrunning warm, moist, unstable air	74
71.	A warm front with overrunning warm, moist, unstable air	75
72.	A fast moving cold front underrunning warm, moist, unstable air	75
73.	A warm front occlusion lifting warm, moist, unstable air	76
74.	A cold front occlusion lifting warm, moist, stable air	76
75.	An aerial view of a portion of a squall line	77
76.	Effect of convective currents on final approach	80
77.	Avoiding turbulence by flying above convective clouds	81
78.	Eddy currents formed by winds blowing over uneven ground or over obstructions	82
79.	Turbulent air in the landing area	83
80.	Wind flow in mountain areas	84
81.	Schematic cross section of a mountain wave	84
82.	Standing lenticular clouds associated with a mountain wave	85
83.	Standing wave rotor clouds marking the rotary circulation beneath mountain waves	86
84.	Mountain wave clouds over the Tibetan Plateau photographed from a manned spacecraft	87
85.	Satellite photograph of a mountain wave and the surface analysis for approximately the same time	87
86.	Wind shear in a zone between relatively calm wind below an inversion and strong wind above the
	inversion	88
87.	Wake turbulence wing tip vortices developing as aircraft breaks ground	89
88.	Planning landing or takeoff to avoid heavy aircraft wake turbulence	90
89.	Effects of structural icing	92
90.	Clear, rime, and mixed icing on airfoils	93
91.	Clear wing icing (leading edge and underside)	94
92.	Propeller icing	95
93.	Rime icing on the nose of a Mooney "Mark 21" aircraft	96
94.	External icing on a pitot tube	97
95.	Carburetor icing	98
96.	Internal pitot tube icing	99
97.	Clear ice on an aircraft antenna mast	100
98.	Freezing rain with a warm front and a cold front	101
99.	Frost on an aircraft	103
100.	The average number of thunderstorms each year	106
101.	The average number of days with thunderstorms during spring	107
102.	The average number of days with thunderstorms during summer	108
103.	The average number of days with thunderstorms during fall	109
104.	The average number of days with thunderstorms during winter	110
105.	The stages of a thunderstorm	112
106.	Schematic of the mature stage of a steady state thunderstorm cell	113
107.	A tornado	114
108.	A waterspout	114
109.	Funnel clouds	115
110.	Cumulonimbus Mamma clouds	116
111.	Tornado incidence by State and area	117
112.	Squall line thunderstorms	118
113.	Schematic cross section of a thunderstorm	119
114.	Hail damage to an aircraft	120
115.	Radar photograph of a line of thunderstorms	121
116.	Use of airborne radar to avoid heavy precipitation and turbulence	122
117.	Ground fog as seen from the air	126
118.	Advection fog in California	127
119.	Advection fog over the southeastern United States and Gulf Coast	128
120.	Smoke trapped in stagnant air under an inversion	129
121.	Aerial photograph of blowing dust approaching with a cold front	130
122.	Difference between the ceiling caused by a surface-based obscuration and the ceiling
	caused by a layer aloft	131
123.	A cross section of the upper troposphere and lower stratosphere	136
124.	Artist's concept of the jet stream	137
125.	A jet stream segment	137
126.	Multiple jet streams	138
127.	Mean jet positions relative to surface systems	139
128a.	Satellite photograph of an occluded system	140
128b.	Infrared photograph of the system shown in figure 128a	141
129.	A frequent CAT location is along the jet stream north and northeast of a rapidly
	deepening surface low	142
130.	Contrails	144
131.	The Arctic	148
132.	Sunshine in the Northern Hemisphere	149
133.	The permanent Arctic ice pack	150
134.	Average number of cloudy days per month (Arctic)	151
135.	Visibility reduced by blowing snow	154
136.	A typical frozen landscape of the Arctic	154
137.	Vertical cross section illustrating convection in the Intertropical Convergence Zone	160
138.	Prevailing winds throughout the Tropics in July	161
139.	Prevailing winds in the Tropics in January	161
140.	A shear line and an induced trough caused by a polar high pushing into the subtropics	163
141.	A trough aloft across the Hawaiian Islands	164
142.	A Northern Hemisphere easterly wave	165
143.	Vertical cross section along line A-B in figure 142	165
144.	Principal regions where tropical cyclones form and their favored directions of movement	166
145.	Radar photograph of hurricane "Donna"	168
146.	A hurricane observed by satellite	169
147.	Thermals generally occur over a small portion of an area while downdrafts predominate	172
148.	Using surface dust and smoke movement as indications of a thermal	174
149.	Horizontal cross section of a dust devil rotating clockwise	174
150.	Cumulus clouds grow only with active thermals	176
151.	Photograph of a dying cumulus	177
152.	Altocumulus castellanus clouds are middle level convective clouds	178
153.	Experience indicates that the "chimney" thermal is the most prevalent type	179
154.	Thermals may be intermittent "bubbles"	179
155.	It is believed that a bubble thermal sometimes develops a vortex ring	180
156.	Wind causes thermals to lean	181
157.	Photograph of cumulus clouds severed by wind shear	181
158.	Conditions favorable for thermal streeting	182
159.	Cumulus clouds in thermal streets photographed from a satellite high resolution camera	183
160.	The Pseudo-Adiabatic Chart	184
161.	An early morning upper air observation plotted on the pseudo-adiabatic chart	185
162.	Computing the thermal index (TI)	187
163.	Another example of computing TI's and maximum height of thermals	188
164.	An upper air observation made from an aircraft called an airplane observation or APOB	189
165.	Schematic cross section through a sea breeze front	192
166.	Sea breeze flow into the San Fernando Valley	193
167.	Sea breeze convergence zone, Cape Cod, Massachusetts	194
168.	Schematic cross section of airflow over a ridge	195
169.	Strong winds flowing around an isolated peak	196
170.	Wind flow over various types of terrain	197
171.	Schematic cross section of a mountain wave	198
172.	Wave length and amplitude	199

Weather is perpetual in the state of the atmosphere. All flying takes place in the atmosphere, so flying and weather are inseparable. Therefore, we cannot treat aviation weather purely as an academic subject. Throughout the book, we discuss each aspect of weather as it relates to aircraft operation and flight safety. However, this book is in no way an aircraft operating manual. Each pilot must apply the knowledge gained here to his own aircraft and flight capabilities.

The authors have devoted much of the book to marginal, hazardous, and violent weather which becomes a vital concern. Do not let this disproportionate time devoted to hazardous weather discourage you from flying. By and large, weather is generally good and places little restriction on flying. Less frequently, it becomes a threat to the VFR pilot but is good for IFR flight. On some occasions it becomes too violent even for the IFR pilot.

It behooves every pilot to learn to appreciate good weather, to recognize and respect marginal or hazardous weather, and to avoid violent weather when the atmosphere is on its most cantankerous behavior. For your safety and the safety of those with you, learn to recognize potential trouble and make sound flight decisions before it is too late. This is the real purpose of this manual.

AVIATION WEATHER is in two parts. Part I explains weather facts every pilot should know. Part II contains topics of special interest discussing high altitude, Arctic, tropical, and soaring weather. A glossary defines terms for your reference while reading this or other weather writings. To get a complete operational study, you will need in addition to this manual a copy of AVIATION WEATHER SERVICES, AC 00-45, which is explained in the Preface.

We sincerely believe you will enjoy this book and at the same time increase your flying safety and economy and, above all, enhance the pleasure and satisfaction of using today's most modern transportation.