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ACKNOWLEDGMENTS Producing the 4th Edition of Aviation Weather has been a team effort rather than the work of an individual author. I am indebted to Kimberly Winter-Manes, Senior Project Manager at Jeppesen, who coordinated the effort, keeping our team organized, on task, and on schedule. From the beginning, Chuck Stout, with his keen eye as both editor and pilot, made important suggestions to improve the book, and patiently incorporated those changes into the 4th Edition to produce the best training product. I know that this newest edition also benefits from the talents of the art and production personnel at Jeppesen. I don’t know each of you personally, but I know you have been there making the book shine. Thanks!

During the preparation of this book, I have had many helpful discussions with pilots, aviation weather forecasters, and atmospheric scientists on both the public and private sides of meteorology. I thank them for their input and for their ongoing dedication to deliver the most useful meteorological information to the pilot on the ground and in the air.

In preparing for this new edition, I was fortunate to be able to call on three colleagues to review the text and provide important suggestions for improvements. They are Ray Sanchez-Pescador, private pilot; Steve Senderling, commercial pilot, CFI, and Instructor of Ground School and Meteorology at Lane Community College, Eugene, Oregon; and Frank W. Lester, retired Air Force pilot and former Director of Safety and Education for the Idaho Division of Aeronautics. Many improvements and updates to the book are due to their extensive and thoughtful inputs.

My family continues to keep me centered in my writing tasks. I thank them all for understanding my passion for this aviation weather stuff and encouraging me along the way. Thank you Heather, Morgan, and David Rodd; Rachael, Andrew, Elijah, Jasmine, and Leonardo Posada. My wife, Julia, has been my personal support team, listening, providing more synonyms than I ever knew, typing, being away when I needed space and quiet to think, and feeding and encouraging me. She is a sailor who now knows much more about flying weather than she ever expected.

This book is dedicated to the people who got me into this aviation meteorology writing business. Mike Cetinich led me to Jeppesen after I described to him my first ideas about writing meteorology education and training materials both for pilots and aspiring pilots. Dick Snyder, whose extensive editorial experience at Jeppesen and whose ability to manage an author’s ego, took me from my first effort, Turbulence, A New Perspective for Pilots to this book, Aviation Weather. Along the way, he taught me many practical matters about technical writing. Today he is gone but his

influence is still strongly felt as, appropriately, my current editor, Chuck Stout, was mentored by Dick.

Peter F. Lester, 2013

TABLE OF CONTENTS

Part I — Aviation Weather Basics Chapter 1 — The Atmosphere

Section A: Atmospheric Composition Section B: Atmospheric Properties Section C: Atmospheric Structure

Chapter 2 — Atmospheric Energy and Temperature Section A: Energy Transfer Section B: Temperature

Chapter 3 — Pressure, Altitude, and Density Section A: Atmospheric Pressure Section B: Charting Atmospheric Pressure Section C: The Pressure Altimeter Section D: Density

Chapter 4 — Wind Section A: Wind Terminology and Measurements Section B: Causes of Wind Section C: Pressure Gradient Force Section D: Coriolis Force Section E: Geostrophic Balance Section F: Friction Section G: Other Effects

Chapter 5 — Vertical Motion and Stability Section A: Vertical Motions Section B: Stability Section C: The Impact of Stability on Vertical Motions

Chapter 6 — Atmospheric Moisture Section A: Moisture Characteristics Section B: Clouds Section C: Precipitation

Part II — Atmospheric Circulation Systems Chapter 7 — Scales of Atmospheric Circulations

Section A: Scales of Circulations Section B: The Largest Scale Circulations Section C: The Global Circulation System Section D: Global Circulation and Climatology

Chapter 8 — Airmasses, Fronts, and Cyclones

Section A: Extratropical Cyclones Section B: Tropical Cyclones and Hurricanes

Chapter 9 — Thunderstorms Section A: Dry Convection Section B: Cloudy Convection Section C: Weather Radar Section D: Thunderstorm Structures Section E: Thunderstorm Environment

Chapter 10 — Local Winds Section A: Thermally Driven Local Winds Section B: Mountain Lee Waves and Warm Downslope Winds

Part III — Aviation Weather Hazards Chapter 11 — Wind Shear

Section A: Wind Shear Defined Section B: Causes of Wind Shear

Chapter 12 — Turbulence Section A: Turbulence Defined Section B: Turbulence Causes and Types

Chapter 13 — Icing Section A: Aircraft Icing Hazards Section B: Observing and Reporting Structural Icing Section C: Microscale Icing Processes Section D: Icing and Macroscale Weather Patterns Section E: Minimizing Icing Encounters

Chapter 14 — Instrument Meteorological Conditions Section A: Background Section B: Causes of IMC Section C: Climatology

Chapter 15 — Additional Weather Hazards Section A: Atmospheric Electricity Section B: Stratospheric Ozone Section C: Volcanic Eruptions Section D: Space Weather Hazards Section E: Runway Hazards Section F: Cold Climate Hazards

Part IV — Applying Weather Knowledge Chapter 16 — Aviation Weather Resources

Section A: The Weather Forecasting Process Section B: Aviation Weather Forecast Products Section C: Aviation Weather Information Sources

Chapter 17 — Weather Evaluation for Flight Section A: Self-Briefing Procedure

Section B: Weather Evaluation Process

Appendices Appendix A: Conversion Factors Appendix B: Standard Meteorological Codes and Graphics for Aviation Appendix C: Glossary of Weather Terms Appendix D: References Appendix E: Review Question Answers

HOW THE SYSTEM WORKS The Jeppesen Sanderson study/review concept of learning presents information in an uncomplicated way with coordinated text and illustrations. Aviation Weather is designed to facilitate self-study. To get the most out of this textbook in the shortest amount of time, the following self-study procedure is recommended.

1. Review the Chapter Outline at the beginning of each chapter, noting major topics and subtopics.

2. Review the list of Key Terms and Chapter Questions at the end of the chapter.

3. Read the chapter Introduction and Summary Sections.

4. Skim through the chapter, reading the FAA Question Material, Insight Readings, and Illustrations.

5. Read the chapter.

6. Answer the Chapter Questions and check your answers to Review Questions in Appendix E.

7. For a general review, repeat steps 1 through 4.

The key elements in Aviation Weather are presented in color to allow you to review important points and concepts. The major features of the book are presented in the following pages.

PREFACE Meteorology is the study of the atmosphere and its phenomena; in many texts, it is simply referred to as atmospheric science. In contrast, weather is technically defined as the state of the atmosphere at an instant in time. Although the study of atmospheric impacts on aviation deals both with meteorology and with weather, it is traditionally referred to as aviation weather. We will use the latter terminology, clarifying differences where necessary.

Meteorology is a relatively “young” science. The vast majority of important developments in the field have only taken place in the last hundred years. Driven by hot and cold war technological breakthroughs and, more recently, by environmental concerns, our understanding of the atmosphere and our ability to predict its behavior have improved dramatically.

In the middle of this rapid growth has been the airplane. Much of the progress in modern meteorology has also been driven by, and for, aviation. As aircraft designs improved and more and more aircraft were able to fly higher, faster, and farther, previously unobserved details of fronts, jet streams, turbulence, thunderstorms, mountain waves, hurricanes, and many other atmospheric phenomena were encountered.

The aviation industry turned to formal atmospheric research for the practical reason that aircraft are extremely vulnerable to certain atmospheric conditions. Aircraft designers needed careful measurements of those conditions; subsequent studies by specially equipped weather research aircraft produced even more details of the weather environment of flight.

In the early days of aviation, it became obvious that a regular supply of weather information was necessary to serve day-to-day operational needs. In the 1920’s, many weather stations and the first weather data communication networks were established in the U.S. to serve the growing aviation industry. These were the forerunners of the modern observation, forecast, and data communication systems that now serve a wide variety of public and private users across the entire world.

The strong interdependence of flight and meteorology will be apparent from the beginning of your aviation experience. Whether your connection to flying is as a pilot, or as a controller, dispatcher, scientist, engineer, or interested passenger, you will quickly discover that it is nearly impossible to discuss any aspect of aviation without some reference to the meteorological environment in which the aircraft operates.

The objective of this text is to help the new student of aviation understand the atmosphere for the purpose of maximizing aircraft performance while minimizing exposure to weather hazards.

The book is also meant to provide a review of meteorology basics in preparation for the FAA examinations. It brings together information from a variety of sources and should serve as an up-to-date reference text. It is written with a minimum of mathematics and a maximum of practical information.

The text is divided into four Parts:

Part I (Chapters 1-6) addresses the “basics.” This is important background in elementary meteorology that provides concepts and vocabulary necessary to understand aviation weather applications.

Part II (Chapters 7-10) deals with the wide variety of atmospheric circulation systems, their causes, behavior, and their related aviation weather.

Part III (Chapters 11-15) focuses specifically on the flight hazards produced by the circulation systems described in Part II.

Part IV (Chapters 16 and 17) considers the weather forecast process and the task of obtaining and interpreting pertinent weather information. These final chapters provide a framework for putting the information presented in previous chapters to practical use.

As you begin your study of aviation weather, a brief “pretest” is useful to emphasize the importance of the study of aviation meteorology. Given the following meteorological phenomena:

Rain, gusts, whiteout, drizzle, high density altitude, mountain wave, low ceiling, downdrafts, haze, lightning, obscuration, microburst, high winds, snow, thunderstorm

1. Can you define/describe the specific meteorological conditions that produce each of the phenomena listed above?

2. Can you explain why, when, and where the favorable meteorological conditions that produce the phenomena are likely to occur?

3. Can you describe the specific flight hazards associated with each of the phenomena listed above, and explain how to minimize effects of those hazards?

If you cannot answer these questions, consider that each of weather items listed above was cited as a cause or contributing factor in more than 400 General Aviation accidents that occurred in a single year in the U.S. These weather-related accidents accounted for 19% of all General Aviation accidents that occurred that year. Another sobering statistic is that, if only accidents involving fatalities are considered,

weather was the cause or a contributing factor in more than 23% of fatal accident cases . . . nearly one in four!

When you complete this study of aviation meteorology, you should be able to return to this page and answer the three questions above with confidence and with respect for the atmosphere and its vagaries.

PREFACE TO THE FOURTH EDITION Changes to the 4th Edition were driven by a number of factors. Primarily, they reflect an effort to make the book more understandable and relevant through clarifications and updates of descriptions and explanations of basic physical concepts and atmospheric phenomena relevant to aircraft operations.

Since the 3rd Edition was published, there have been important developments in analysis and forecasting products to help pilots make good preflight and inflight decisions. The ability to access specific and current aviation information from new sources and from almost any location—including the cockpit—is a major change that now enables pilots to be literally “on top of the situation.” Of course, having the latest devices to obtain and display relevant meteorological information still requires pilots to make sound decisions based on the proper interpretation of that information. Being able to interpret aviation weather information is at the heart of this book.

The 4th Edition includes several new “Insight Readings,” that is, “sound bites” that help readers grasp some of the more detailed topics presented in the text. Additionally, at the recommendation of reviewers, more excerpts from NTSB reports of aircraft accidents and incidents appear throughout the book to emphasize relevant flying hazards. Finally, with the encouragement and help of outside readers, a second scenario based on a real weather situation has been added to Chapter 17.

At critical points in the text, readers are reminded that good weather knowledge is only part of good flight planning and smart inflight decision-making. Equally important, if not more so, is that pilots understand and apply the concept of “personal minimums” as well as the use of tools such as the IMSAFE checklist.

Part I

Aviation Weather Basics

PART I Aviation Weather Basics Part I provides you with the fundamentals of meteorology. These “basics” are the foundation of the entire study of aviation weather. The time you spend reading and understanding the basics will pay off in later parts of the text when you turn your attention to more complex topics such as the behavior and prediction of weather systems, and weather-related flight hazards.

When you complete Part I, you will have developed a vocabulary of aviation weather terms and a knowledge of the essential properties and weather-producing processes of the atmosphere. As a pilot, you must be fully aware of weather and its influences on flight. Your task of understanding these concepts is made much easier with a solid foundation in Aviation Weather Basics.

CHAPTER 1

The Atmosphere

Introduction The formal study of any physical system, such as an engine or an airplane, usually begins with a description of that system. Information about component parts, their location and dimensions, and terminology is necessary background for later examination and understanding of the system design and operation. Our study of aviation weather begins in a similar way. The “system” in this case, is the atmosphere.

When you complete this chapter, you should be able to describe the composition, dimensions, and average vertical structure of the atmosphere using proper technical vocabulary. Furthermore, you will have been introduced to a valuable reference tool, the standard atmosphere.

SECTION A: ATMOSPHERIC COMPOSITION SECTION B: ATMOSPHERIC PROPERTIES

Temperature Density Pressure The Gas Law

SECTION C: ATMOSPHERIC STRUCTURE Dimensions Atmospheric Layers

Temperature Layers Other Layers Standard Atmosphere

Section A ATMOSPHERIC COMPOSITION Each planet in our solar system is different, a product of the planet’s original composition as well as its size and distance from the sun. Most planets, including Earth, have an atmosphere; that is, an envelope of gases surrounding the planet.

The earth’s atmosphere is a unique mixture of gases along with small amounts of water, ice, and other particulates. The gases are mainly nitrogen and oxygen with only small amounts of a variety of other gases. (Figure 1-1)

Figure 1-1. Primary permanent components of the mixture of gases in the lower atmosphere. Below an altitude of about 43 n.m. (260,000 feet), the ratios of these gases (78:21:1) remain relatively constant. Above this altitude, energy from the sun is great enough to break down molecular structures and change the ratios.

Although nitrogen (N2) takes up most of the volume of the atmosphere, it doesn’t contribute to weather-producing processes under ordinary atmospheric conditions. Exceptions occur when N2 is subjected to very high temperatures, for example, when air passes through an internal combustion engine. In that case, nitrogen

combines with oxygen to form air pollutants known as “oxides of nitrogen” (NOx).

By and large, the most important role of atmospheric oxygen (O2) is the support of life as we know it. Because oxygen concentration decreases with altitude, all pilots must be aware of the serious effects of oxygen deprivation on aircrews and passengers. Oxygen supports combustion and contributes to both the formation and the destruction of air pollutants through chemical combinations with other gases.

In Figure 1-1, N2 and O2 and many of the “other” atmospheric gases are “permanent,” which means that their proportions remain about the same, at least in the lower 260,000 feet (about 43 n.m) of the atmosphere. In contrast, water vapor (H2O) is a “variable” gas; that is, the percentage of water vapor in the atmosphere can vary greatly, depending on the location and source of the air. For example, over the tropical oceans, water vapor may account for 4% of the total volume of gases, while over deserts or at high altitudes, it may be nearly absent.

Water vapor is an important gas for weather production, even though it exists in very small amounts compared to O2 and N2. This is because it can also exist as a liquid (water) and as a solid (ice). These contribute to the formation of fog, clouds, precipitation, and icing, well-known aviation weather problems.

Water vapor also absorbs radiant energy from the earth (terrestrial radiation). This reduces cooling, causing temperatures at the surface to be warmer than would otherwise be expected. Detailed information about this and other characteristics of water vapor, water, and ice are given in later chapters.

Gases that occupy a very small part of the total volume of the atmosphere are generally referred to as “trace gases.” Two of the more important of these are also variable gases: carbon dioxide and ozone. Although their concentrations are extremely small, their impact on atmospheric processes may be very large. For example, carbon dioxide (CO2) also absorbs terrestrial radiation. The concentration of this trace gas has been increasing over the last century due to the worldwide burning of fossil fuels and rain forest depletion, contributing to the long-term warming of the atmosphere.

Ozone (O3) is a toxic, highly reactive pollutant which is produced in the lower atmosphere by the action of the sun on oxides of nitrogen and by electrical discharges, such as lightning. The greatest concentration of ozone is found between 50,000 and 100,000 feet above the earth’s surface. The upper ozone layer is beneficial for the most part because ozone absorbs harmful ultraviolet radiation from the sun. This filtering process at high levels protects plants and animals on the earth’s surface. However, direct exposure of aircrews and passengers to the toxic properties of O3 can be a problem during high-altitude flights. These will be discussed in Part III, Aviation Weather Hazards.

Although their concentrations are small, water vapor, carbon dioxide, ozone, and other trace gases have profound effects on weather and climate.

Liquid or solid particles that are small enough to remain suspended in the air are known as particulates or aerosols. Some of these are large enough to be seen, but most are not. The most obvious particulates in the atmosphere are water droplets and ice crystals associated with fog and clouds. Other sources of particulates include volcanoes, forest fires, dust storms, industrial processes, automobile and aircraft engines, and the oceans, to name a few. Particulates are important because they intercept solar and terrestrial radiation, provide surfaces for condensation of water vapor, reduce visibility, and, in the worst cases, can foul engines.

Section B ATMOSPHERIC PROPERTIES Since the atmospheric “system” is mainly a mixture of gases, its description is commonly given in terms of the state of the gases that make up that mixture. The three fundamental variables used to describe this state are temperature, density, and pressure.

TEMPERATURE Temperature is defined in a number of ways; for example, as a measure of the direction heat will flow, or as simply a measure of “hotness” or “coldness.” Another useful interpretation of temperature is as a measure of the motion of the molecules. Kinetic energy is energy that exists by virtue of motion. A molecule possesses kinetic energy proportional to the square of its speed of movement; temperature is defined as the average of the kinetic energy of the many molecules that make up a substance. The greater the average kinetic energy, the greater the temperature. (Figure 1-2)

Figure 1-2. Temperature is a measure of the average kinetic energy of the molecules of a gas. The red molecules indicate warm temperatures with relatively large speeds (greater kinetic energy). The blue molecules represent cooler temperatures with smaller molecular speeds.

A temperature of absolute zero is the point where all molecular motion ceases. The corresponding temperature scale is known as the absolute or Kelvin scale. You will be introduced to the details of the more familiar Fahrenheit and Celsius temperature scales in the next chapter. For the moment, the Kelvin scale will serve our purposes.

On the absolute or Kelvin (°K) temperature scale, the temperature where all molecular motion ceases is 0°K. The melting point of ice is 273°K (0°C) and the boiling point of water is 373°K (100°C).

When referring to temperature, the placement of the degree symbol indicates whether the number is an actual temperature (35°C) or a temperature increment (35C°).

DENSITY Density of a gas is the mass of the molecules in a given volume. If the total mass of molecules in that volume decreases, the density decreases. If the mass remains the same but the volume increases, the density also decreases. The units of density are expressed in terms of mass per unit volume. (Figure 1-3)

Figure 1-3. In this figure, the mass within each volume is represented by a number of molecules, each with the same mass. The density is equal to the sum of the masses of all of the molecules within the box divided by the total volume. The

figure shows that density is decreased when gas molecules are removed or when the volume is increased.

PRESSURE Pressure is the force exerted by the moving molecules of the gas on a given area, for example, a square inch or square meter. Pressure at a point acts equally in all directions. A typical value of atmospheric pressure at sea level is 14.7 pounds per square inch (See table).

THE GAS LAW A unique characteristic of gases is that they obey a physical principle known as the gas law, which can be written as:

In this equation P is pressure, D is density, T is the absolute temperature, and R is a constant number which is known from experiment and theory. The equation above simply states that the ratio of pressure to the product of density and temperature is always the same. For example, if the pressure changes, then either the density or the temperature, or both, must also change in order for the ratio to remain constant. Figure 1-4 illustrates the application of the gas law and three simple ways to lower the pressure in the vessels by varying the temperature or the density.

Figure 1-4. Pressure is force per unit area. Pressure is exerted by the collective force of the molecules colliding with the sides of the vessels. When the density is kept constant (A), the only way to lower the pressure is to reduce the temperature. The molecules become less energetic and exert less force on the vessel. When the temperature and volume of the vessel remain constant (B), the pressure can only be reduced by removing gas. Although the molecules remain energetic, there are fewer of them, so the force they exert on the sides of the vessel is reduced. When the temperature and mass of the molecules in the vessel remain the same (C), the pressure can only be lowered by increasing the volume of the vessel. The molecules then exert their collective force over a larger area.

The gas law makes the measurement of the gaseous state of the atmosphere much simpler. If we know any two of the three variables that describe the gas, we can always calculate the third. In practice, we usually measure pressure and temperature and deduce the density from the gas law.

Section C ATMOSPHERIC STRUCTURE The brief introduction to the atmospheric composition and the gas law has provided you with some useful vocabulary and some simple physics to examine the structure of the atmosphere.

DIMENSIONS In much of the material in this and later chapters, we will be concerned with the size of the atmosphere and its phenomena. “How big? How high? How far?” are common questions asked in regard to atmospheric description. In order to keep distances and altitudes in a meaningful context, it is helpful to have some “measuring sticks” for reference. Some of the most useful are the dimensions of the earth. (Figure 1-5)

Figure 1-5. The earth and its dimensions. The numbers in the diagram are particularly useful for the determination of the sizes of atmospheric circulation systems such as the large cyclones that move across the earth’s surface. The figure shows the most frequently referenced dimensions: the average radius of

the earth, the circumference at the equator, and the equator-to-pole distance.

The units used in this text are those commonly used in aviation meteorology in the United States. These are given below with some useful conversions. Note: stated values are rounded. An expanded table suitable for international conversions is given in Appendix A.

LENGTH 1 degree of latitude

= 60 n.m. = 69 s.m. = 111 km

1 nautical mile (n.m.) = 1/60 degree of latitude = 6,080 ft = 1.15 s.m. = 1.85 km = 1852 m

1 statute mile (s.m.) = 5,280 ft = 0.87 n.m. = 1.61 km = 1609 m

1 f