Aviation Weather for the Private Pilot: Aviation Books Series, #2

Aviation Weather

for the Private Pilot

Book 2

Aviation Books Series

DR STEPHEN WALMSLEY

Copyright © 2021 Stephen Walmsley

All rights reserved

No part of this book may be reproduced, or stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without express written permission of the publisher.

Disclaimer

Whilst every effort has been made to ensure the accuracy of the information, the author does not give any guarantee as to its accuracy or otherwise. Nothing in the contents of this book is to be interpreted as constituting instruction or advise relating to practical flying. Students preparing for their relevant exam should consult training their syllabus published by the relevant authority to ensure they are studying towards the most up to date syllabus. The author shall not be liable nor responsible to any person or entity concerning errors and omission, or loss or damage caused directly or indirectly by the use of the information contained in this book.

ISBN: 9780473571771

Aviation Books Series

Aviation Weather for the Private Pilot is part of the Aviation Books Series that provides the reader with an educational and enjoyable reading experience. A focus has been placed on practical, hands-on aviation by linking science with the real world.

Private Pilot series includes:

Book 1: Human Factors

Book 2: Aviation Weather

Book 3: Flight Radio

Book 4: Principles of Flight

Book 5: Flight Navigation

Book 6: Aircraft Technical Knowledge

Book 7: Flight Instruments

Contents

Aviation Weather

Copyright

Aviation Books Series

Introduction

Part I: Basic Components

Chapter 1: The Atmosphere

Chapter 2: Temperature

Chapter 3: Atmospheric Pressure

Chapter 4: Air Density & ISA

Chapter 5: Water Vapour - Water water everywhere

Chapter 6: Stability - Up, up and away 

Chapter 7: Cloud Machine

Chapter 8: Precipitation: Rain, rain, everywhere

Chapter 9: Wind

Chapter 10: Local Wind

Chapter 11: Inversion

Part II: Hazardous Weather

Chapter 12: Reduced Visibility

Chapter 13: Aircraft Icing

Chapter 14: Thunderstorms

Chapter 15: Mountain Weather

Chapter 16: Turbulence

Part III: Large Scale Weather Features

Chapter 17: Frontal Depressions

Chapter 18: Climatology

Part IV: Weather Information

Chapter 19: The Big Picture

Chapter 20: Weather Briefing

Conclusion

About The Author

Introduction

When a pilot is preparing for a flight, one of the fundamental decisions they must make is to determine if the weather conditions are safe to fly in. Aircraft of all sizes are at the mercy of the elements, but private pilots operating smaller aircraft are most vulnerable. Unlike commercial airliners that primarily fly above the weather, the private pilot operates at lower altitudes where the weather is more volatile. Unfortunately, many weather conditions can be unforgiving, and if a pilot flies into hazardous weather the consequences can be fatal. Hazardous weather conditions come in many forms; from a towering thunderstorm that can toss an aircraft around like it is in a washing machine, to thick fog in which pilots will struggle to see just a few feet in front of the aircraft. However, some of the most dangerous weather conditions are those that are difficult to see, such as windshear and turbulence which can occur even on the sunniest of days.

This book explores a wide range of weather conditions that are essential for pilots to master in order to operate an aircraft safely. We will begin with the basic components of the weather and combine them to explore a range of hazardous weather conditions. We will then take a step back, and look at large-scale weather systems before touching on climatology. In the final chapters we will focus on weather reports and forecasts; how to read and assess a range of weather information to allow pilots to make sound decisions when conducting a flight.

The weather has always been a major threat to pilots. In the early days of aviation, the weather was one of the leading causes of aircraft accidents. With limited weather reports and forecasts available, pilots were frequently caught out by unfavourable weather conditions. From the 1950s, flying became safer, in part due to improvements in weather forecasting and advances in technology such as the weather radar. But despite these improvements, the weather remains a major factor in many aircraft accidents, especially those involving smaller aircraft. Considering the severe consequences that can arise when a pilot flies into unsuitable weather conditions, it is essential to understand the wide range of weather conditions that can influence a pilot’s ability to fly an aircraft safely.

Part I: Basic Components

Chapter 1: The Atmosphere

The atmosphere is the layer of gases that surround the earth. Although the atmosphere stretches up to the boundary of space, almost all flights operate in the tiny portion of the atmosphere near the surface. This tiny portion of the atmosphere also happens to be where most of the weather is found. In this chapter, we will see that as pilots stray higher in the atmosphere, various properties begin to change, such as temperature.

Structure of the Atmosphere

The atmosphere is made up of a series of different layers, with the bottom two (the troposphere and stratosphere) the most important for pilots. We can see in Figure 1.1, that the first layer is the troposphere, which begins at the surface. On average, the surface temperature at sea level is 15ºC (59ºF), and as we leave the earth’s surface, the air temperature normally reduces at a rate of about 2ºC per 1000 ft (3.6ºF per 1000 ft).

Figure 1.1:The atmosphere layers and temperature trends in the lower atmosphere

The surface temperature is not always 15ºC and we will see shortly how variations of surface temperature influence the vertical extent of the troposphere. The limit of most small aircraft (unpressurized) is about 10,000 ft. If you popped outside at this point you would find the air temperature had reduced to -5ºC (23ºF). A typical cruising level for a commercial airliner is about 30,000 ft, where the outside air temperature would be around -45ºC (-49ºF). At about 36,000 ft, where the air temperature is a cool -56.5ºC (-70ºF), the air temperature suddenly stops reducing and remains constant with any further increase in altitude. This brings us to the upper limit of the troposphere which is known as the tropopause. If we continue to climb past the tropopause, we enter the second layer of the atmosphere, the stratosphere. Initially the air temperature will remain constant with increasing altitude, before starting to increase. The stratosphere stretches well beyond the reach of all commercial airliners, even the Concorde, which cruised at around 60,000 ft. There are still several more layers of the atmosphere (e.g., Mesosphere, Thermosphere), but these are beyond the scope of aviation weather.

Troposphere and Tropopause

The troposphere is the most important layer that we must consider. Most of the earth’s weather occurs in the troposphere and it makes up 75% of the atmosphere’s mass. Large parts of the world experience surface temperatures much colder or warmer than 15ºC (59ºF), which influences the vertical extent of the troposphere. In cooler areas, such as near the poles, the surface temperature is much cooler, therefore we can say the air is denser. We will cover density in more detail in a later chapter, but this simply means the air molecules are closer together or more compact. If we climbed higher in this cooler, denser atmosphere, we will arrive at the tropopause a lot sooner, typically around 20-30,000 ft, as shown in Figure 1.2. If we now move towards warmer areas, such as near the equator, the opposite occurs. With warmer surface temperatures, the air is less dense, meaning the air molecules are spaced further apart. As a result, it takes longer to reach the tropopause, typically around 50-60,000 ft.

Figure 1.2:Variation in tropopause height from the poles to the equator

As shown in Figure 1.2, the average tropopause height of 36,000 ft (-56.5ºC) occurs around the mid-latitudes (halfway between the poles and the equator). Near the poles, the tropopause is lower and the temperature at the polar tropopause is relatively warm. Although this may sound odd, this is because the air has a shorter vertical distance to cool down at 2ºC per 1000 ft. The opposite occurs near the equator, with a high tropopause, the air has a long way to cool down, resulting in a cooler tropopause temperature near the equator.

Gases in the Atmosphere

If we take a closer look at what is contained within the air in the atmosphere, we find the vast majority of the gases have very little consequence to aviation weather. At sea level, a volume of dry air contains a mixture of gases; nitrogen contributes the largest proportion at 78%, oxygen contributes 21%, with the final 1% made up from a mixture of gases. Gases within this final 1% can have a huge impact on the weather conditions and climate in general. Here we find a range of gases, including carbon dioxide and various aerosols. These have two important consequences; some of these gases trap temperature in the atmosphere that would otherwise escape, contributing to the general warming trend in the atmosphere. Others can accelerate condensation (e.g., pollutants) which can result in smoggy conditions often experienced in some industrial areas around the world.

The final gas that we need to consider is by far the most important for our day-to-day weather; water vapour. The amount of water vapour in the atmosphere can help determine if it is going to be a nice sunny day, or muggy with a threat of thunderstorms. The amount of water vapour in the air varies widely and air’s ability to hold water vapour is directly related to air temperature. Cooler air can hold very little water vapour, resulting in water vapour only contributing 1-2% of a volume of air. Warmer air can hold up to 6-8% of water vapour. This has important consequences on a number of hazardous weather conditions; from the severity of aircraft icing to the amount of rain that can fall from a cloud.

For the private pilot, who normally flies below 10,000 ft, these tropopause heights and the gas makeup may seem pointless. However, they can significantly influence the weather conditions at the surface.  The tropopause acts like a lid to the weather in the troposphere. For example, a powerful thunderstorm that towers upwards in the atmosphere will come to a halt around the tropopause. Areas around the