UNDERSTANDING THE SKY by DENNIS PAGEN A SPORT PILOT’S GUIDE TO FLYING CONDITIONS "A pilot needs to understand the ways of the sky to fly successfully and safely..." A GUIDE FOR: Balloonists, RC Modelers, Parachutists, Hang Glider, Paraglider and Sailplane Pilots • Learn the cause and behavior of the wind. • Gain knowledge of lift patterns and types. • Find out how to predict flying conditions. • See how thermals are formed and act in the air. • Investigate the action of circulation and general weather. • Unlock the mysteries of seabreezes and other local effects. • Explore the world of micrometeorology-small-scale effects. • Become an expert at judging thunderstorms. • Discover the secrets of turbulent air. • Many more details including: cloud streets, heat fronts, trigger temperature, cloud types, inversions, low level jets, convergence, waves, cloud heights, site reading, gust fronts, upslope breezes, etc. 2 If you read (parts of) this document more than once, please buy the book. Support Dennis Pagen so he can write more books for us. Books like this are hard to find. I imagine writing them is even harder. So, don’t spoil the fun for the author, buy the book! I bought the book, read it a lot, and it improved my understanding of weather and of flying. Happy landings ! ☺ 3 UNDERSTANDING THE SKY by DENNIS PAGEN A SPORT PILOT’S GUIDE TO FLYING CONDITIONS 4 ILLUSTRATIONS BY THE AUTHOR UNDERSTANDING THE SKY A SPORT PILOT'S GUIDE TO FLYING CONDITIONS First Edition Copyright (C) 1992 by Dennis Pagen First Printing: January, 1992 Printed in the United States of America 1234567891011121314 Published by Dennis Pagen P.O. Box 101, Mingoville, PA 16856 Books by the Author Hang Gliding Flying Skills Hang Gliding Techniques Powered Ultralight Training Course Powered Ultralight Flying Paragliding Flight Vol Libre! (French Edition) En Parapente! (French Edition) ISBN 0-936310-03 280 pages ACKNOWLEDGEMENT I wish to thank Kathy Lee of Taos, New Mexico for her generous offering of photographs. She has truly captured much of nature's beauty with her discriminating eye. I also must thank my wife Claire for her help in all phases of this book. Her editing and layout skills are second only to her piloting skills. Dennis Pagen January, 1992 Cover Photo: Claire Vassort-Pagen Photos in text: Kathy Lee, Dennis Pagen, Claire Vassort-Pagen 5 CONTENTS PREFACE ii CHAPTER I — THE AIR AROUND US 1 The Big Picture The Structure of the Air Properties of the Air Pressure in the Atmosphere Temperature in the Air Solar Heating Cooling Cycles Daily Changes Seasonal Changes Water Vapor Humidity Water's Amazing Properties Latent Heat Summary CHAPTER II — THE LIVING ATMOSPHERE 13 The Lapse Rate Stability and Instability Indications of Stability The Stability of Layers The Moist Lapse Rate The Standard Atmosphere Density Altitude The Wind We Feel Coriolis Effect Summary CHAPTER III — THE MESSAGE OF THE CLOUDS 25 Cloud Causes Lifting the Air The Dew Point and Cloud Height Cloud Formation Cloud Bases/Cloud Tops Cloud Life Old Clouds Rain Visibility Cloud Types Altitude Classification Seasonal and Location Variations Lesser Known Clouds What They Tell Us Flying in Clouds Summary 6 CHAPTER IV — THE BIG PICTURE-GENERAL METEOROLOGY 53 Movement of the Atmosphere Air Masses The Meaning of Fronts Front Characteristics — Cold Front Characteristics — Warm Frontal Action Variations in Fronts Occluded Fronts Seasonal Changes High Pressure Systems Low Pressure Systems Local Lows Highs and Lows Together Isobars Isobars and the Wind Circulation Aloft Trough Aloft The Jet Stream Steering Winds and Pressure Patterns Tropical Weather Island Weather Summary CHAPTER V — WIND PATTERNS 87 Measuring the Winds Watching the Wind Prevailing Winds Wind Flow Near the Surface Winds Around Fronts and Pressure Systems Moving Weather Areas Beyond the Fronts Backing and Veering Winds The Wind Gradient Winds Aloft Jet Streams Diurnal Variation of the Wind Layering of the Air Low-Level Jets Special Winds North American Winds Old World Winds Summary CHAPTER VI — TURBULENCE-UNSTEADY FLOW 113 The Meaning of Turbulence The Nature of Turbulence The Causes of Turbulence Vortex Turbulence 7 Rotors Detecting Turbulence Conditions and Turbulence Cycles Surface Conditions Wind Shadow Flying in Turbulence Summary CHAPTER VII — LOCAL WINDS 137 Heating and Circulation The Sea Breeze Sea Breeze and General Wind Sea Breeze Prediction Sea Breeze Strength, Reach and Seasons Sea Breeze Effects Sea Breeze Fronts Complicated Coastlines Island Effects Land Breezes Heat Fronts From Forest to Field Upslope Winds Downslope Winds Gravity Winds Magic Air and Wonder Winds Up Valley and Down Valley Winds Complicated Systems Summary CHAPTER VIII — SOARING CONDITIONS-LIFTING AIR 161 To Soar Ridge Lift Complex Shapes Canyons and Gaps Ridge Soaring Problems Variable Lift Wave Lift Wave Properties Wave Clouds Wave Dangers Flying in Waves Finding Waves Other Wave Sources Convergence Lift Flying in Convergence Frontal Lift Lift Indicators Summary 8 CHAPTER IX — INSTABILITY AND THERMALS 189 The Birth of a Thermal Thermal Triggering Lee Side Thermals Thermal Sources House Thermals Terrain Sources Wet Ground Sources Thermal Cycles Evening Thermals Thermal Rising Real Lapse Rates The Lapse Rate Aloft Thermals and Lapse Rate Variations Dust Devils Ideal Thermal Conditions Lift in a Thermal Thermal Sink Summary CHAPTER X — THERMAL LORE 213 Thermal Sizes and Strengths Thermal Heights Thermal Duration and Daily Variation Thermals in Wind Thermal Tracks and Cycles Thermal Types Thermal Spacing Thermal Patterns Thermal Streets Cloud Streets Cloud Street Behavior Thermals and Cloud Character Near Cloud Features Wind Effects on Clouds Locating Thermals Blue Holes Summary CHAPTER XI — THUNDERSTORMS 237 Where and When The Cause of Storms A Thunderstorm Life Cycle Thunderstorm Variations Thunderstorm Dangers Downdrafts and Gust Fronts Judging the Storm Escaping a Thunderstorm Tornados Summary 9 CHAPTER XII — WATCHING THE WEATHER 253 Weather Cycles Weather and the Barometer Weather Wisdom Weather Information Sources Weather Maps Soaring Forecasts Reading an Area Site Examples Summary APPENDIX I — DENSITY CHANGES IN THE ATMOSPHERE 269 APPENDIX II — CORIOLIS EFFECT 271 APPENDIX III — DRYING WINDS 272 APPENDIX IV — THERMAL BUOYANCY 273 APPENDIX V — TRIGGER TEMPERATURE 274 GLOSSARY 276 INDEX 279 10 PREFACE In the closing decades of the twentieth century a fortunate coincidence of timing and technology has allowed our species-designed for life on the surface of planet earth-to enter the atmosphere and cavort among the clouds. Flying for fun has come into its own as a reasonable and legitimate pastime. But inhabiting the realm of the sky requires a certain amount of understanding. The air is an ever-changing environment and we must know its ways and wiles in order to fly safely and become excellent pilots. This book is designed to present a clear picture of how the atmosphere works. Naturally some simplifications must be made, for the subject of weather is a complex one requiring many years of study to master. Consequently we have distilled the important lore and knowledge necessary for pilots who fly for fun. The best way to use this book is to read it through, experience flight then reread the pertinent portions to gain a deeper understanding. We have tailored the material to suit the needs of balloonists, RC modelers, paraglider, hang glider, sailplane and ultralight pilots alike. Hopefully each reader will discover new insights and ideas within these pages to enhance the enjoyment of flight. Besides embracing all air sports, the material herein is written with an international viewpoint, for many pilots today travel in pursuit of their aerial endeavors. We include the perspective of both the northern and southern hemispheres, where appropriate, and give some attention to regional and continental specifics. We also use both English and metric equivalents in the text as well as the charts and figures. While we begin the chapters on the air's properties and general weather, we would like to point out that the emphasis in this book is on smaller-scale conditions known as local effects or micrometeorology. The reason for this emphasis is that recreational flying normally takes place within a relatively small volume of airspace where local effects play a major role. Most weather books written for general aviation do not address the small-scale conditions in enough detail to satisfy recreational pilots. This book is intended to fill this void. The background material for the information in the following chapters comes from many sources. Certainly textbooks have been very useful, but most important is the experience of almost two decades of flying and the sharing of ideas with other pilots from all forms of aviation. It is my wish to pass on some of the knowledge I have gleaned from these experiences so that we can all better savor our time in the sky. 11 CHAPTER I The Air Around Us We grew up on a planet that is surrounded by a life-giving mixture of gases. We call this mixture air and we refer to the entire gas cloud around the earth as the atmosphere. Most of us take this air and atmosphere for granted as we pass it through our lungs to borrow some oxygen, or slip through it while on the move. For the most part, the air is just simply there. But give us a set of wings and a whole new world opens up. New challenges, new vistas and new experiences alter our viewpoints forever. We become pilots with the realm of the sky to explore. We quickly become aware of the constant changes that take place in the atmosphere and the need to understand what these changes mean. With understanding we become comfortable in our new environment. With understanding we leave our fears behind and free ourselves from the limitations of an earthbound existence. In this chapter we begin to study the nature of the sky so we can later predict its behavior as we enter its domain and cast our fate to the winds.