Pilot's Weather E-Book

Brtian Cosgrove

• Herausgeber: Crowood
• Kategorie: Fachliteratur
• Sprache: Englisch

Veröffentlichungsjahr: 2014

Beschreibung

This book breaks new ground in the presentation of what is and should be presented as a fascinating and vitally important part of a pilot's skill. Gone are the dreary old monotone drawings of isobars and fronts, endless graphs and reams of figures and in bounce full colour photos of what you actually see - clouds and cloudscapes that tell you instantly what's happening to the air around you. For those who fly aircraft and micros, gliders or kites, this book makes the weather make sense. "The content of the book deals comprehensively with all the topics likely to come up in the PPL exams, and more importantly tries, and succeeds, to weld them together into a coherent and useful whole. Meteorology can be a dry and technical subject but this book does better than most at holding the reader's interest, helped a great deal by the excellent photos. The photos illustrate all types of cloud, frontal weather, and most other meteorological phenomena. This is a welcome change from the usual line drawings which bear little relation to reality found in the majority of aviation weather books. " - MICROLIGHT FLYING "Everything the pilot needs to know about the atmosphere, the weather and meteorology. The colour photographs are superb: these alone make the book worth having on one's shelf." - AOPA LIGHT AVIATION ". . . a few hours regularly spent within the pages of Brian Cosgrove's book would seem to be time well spent." - GUILD NEWS (GAPAN).

Leseprobe

This edition published in 1999 by Airlife Publishing, an imprint of The Crowood Press Ltd Ramsbury, Marlborough Wiltshire SW8 2HR

www.crowood.com

This e-book first published in 2015

This impression 2013

Copyright © 1999 Brian Cosgrove

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library

ISBN 978 1 84037 027 0

All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical including photocopying, recording or by any information storage and retrieval system, without permission from the Publisher in writing.

The information in this book is true and complete to the best of our knowledge. All recommendations are made without any guarantee on the part of the Publisher, who also disclaims any liability incurred in connection with the use of this data or specific details.

Photo Credits

All photos by the author except the following:

Tom Bradbury pages 37, 39, 70, 78 top/bottom, 88 bottom, 103, 140

Julian Doswell page 102

Terry Skuce page 67

Jim Ellis page 106

Dedication

To John Bannister, Julian Doswell and Peter Coles – a truly patient trio.

CONTENTS

CHAPTER 1 The Atmosphere

CHAPTER 2 Density

CHAPTER 3 Pressure

CHAPTER 4 The Altimeter

CHAPTER 5 Wind

CHAPTER 6 Effects of Wind on Flight

CHAPTER 7 Temperature

CHAPTER 8 Humidity

CHAPTER 9 Effect on Flight of the Basic Factors in the Atmosphere

CHAPTER 10 Lapse Rates

CHAPTER 11 Origin of Clouds

CHAPTER 12 Clouds

CHAPTER 13 Precipitation

CHAPTER 14 Icing

CHAPTER 15 The Cumulonimbus

CHAPTER 16 The Airmass

CHAPTER 17 Depressions

CHAPTER 18 Fronts

CHAPTER 19 Thunderstorms

CHAPTER 20 Haze, Mist and Fog

CHAPTER 21 The Anticyclone

CHAPTER 22 Visibility

CHAPTER 23 Reports/Forecasts

CHAPTER 24 Recap

CHAPTER 25 General Data

CHAPTER 26 Finale

INDEX

INTRODUCTION

Pilot’s Weather aims to provide the meteorological ‘know-how’ needed by you as a student pilot – and just maybe some existing pilots! It will be relevant whether your aircraft is a light aeroplane, sailplane, hang/para glider, ultralight, microlight, powered parachute or whatever.

Simplicity is the intention as far as possible; achieved through adopting a logical progression of the events to be absorbed and using numerous pictures of the clouds and weather patterns you will come across.

Past experience has shown that even at basic level there are a few whose curiosity likes to go that little bit deeper at the outset. Where such occasions arise, the text will be found in italics placed in a pink tinted box and can be ignored if your only wish is to learn the basics.

Should the day come when you want to extend your pilot status up to commercial or airline standard, or you wish to become a highly skilled sailplane pilot achieving a Gold C with diamonds, there are plenty of other books available to take you that stage further. In the meantime Pilot’s Weather will ensure you at least have a basic rock upon which to build the advanced knowledge that any future aspirations may demand.

Meteorological terminology and units of measurement are steadily becoming uniform internationally, but currently in the USA there are differences. For students resident or visiting the United States to learn to fly, every effort has been made to identify the major differences in terminology and procedures. Also, since the 1st January 1996 changes have been taking place – particularly in the reporting of weather – to become more in line with the international approach. To cover this transient period, terms will be duplicated in this book where necessary.

Apart from the basic principles of meteorology, the book highlights the weather problems with which you can be faced during your flying life – it is not simply confined to your passing an examination.

You may become very competent in other aspects of flight, BUT – can any of them be more crucial than a sound familiarity with the environment through which you are prepared to fly yourself and your passengers? When putting your knowledge of met into practice always err on the safe side and use the information available to you in the form of current reports and forecasts by phone, fax, radio or computer.

It is better to be on the ground wishing you were in the air than in the air wishing you were on the ground!

Brian Cosgrove

CHAPTER ONE

THE ATMOSPHERE

Quite simply the atmosphere is the air surrounding our planet Earth – mercifully kept in place by the force of gravity.

It is primarily composed of 78% nitrogen, 1% of other gases and, fortunately, 21% oxygen for which we should be truly grateful. Apart from life as we know it being unable to survive without oxygen, combustion would not be possible for engines to function.

As the force of gravity is greater nearest to its source, so the atmosphere is more concentrated near our planet’s surface.

There are four basic factors that affect the weather and flight.

They are:

DENSITY

– the weight of a given volume of air.

PRESSURE

– the weight of a given column of air in the atmosphere.

TEMPERATURE

– the warmth of the atmosphere.

HUMIDITY

– the moisture content of the atmosphere.

For occasions when uniformity or a working standard is required, the International Civil Aviation Organisation (ICAO) has agreed on an International Standard Atmosphere (ISA). This lays down standards for density, pressure and temperature based on a defined sea level — more on this later.

You must remember that these standards are but a ‘yardstick’ and they are purely theoretical. Rarely would the conditions in the atmosphere ever coincide with the standards all at the same time.

How these ISA standards play a part in aviation will be made clear as we progress.

CHAPTER TWO

DENSITY

Air has weight, and density is simply the weight of the number of molecules of air present in a given ‘parcel’ or volume of air at a given time. (The word parcel is frequently used in meteorology as no one has yet thought of a better alternative!)

Density is greatest at the surface; it decreases with height as the air thins out until in outer space it is no longer relevant.

Apart from a decrease with height, density is also affected by warmth. When a parcel of air of a given volume is heated it becomes thinner as the molecules spread out and their number becomes less than in the original parcel.

Conversely, when a volume of air is cooled it becomes denser.

Density also decreases as the moisture content in the air increases, but the effect is relatively minimal.

The ISA standard density at sea level (excepting the Dead Sea!) is 1.225 kilograms per cubic metre (or 1225 grams) or 0.764 lb per cubic foot, where density is defined as 100%. In other words, this would be the defined average weight of all the air molecules in a volume of one cubic metre of air at sea level.

Unlike the other basic factors we shall cover, it cannot be readily measured by instrumentation – its value at a given time requires calculation.

Density plays an important part in an aircraft’s performance; in fact an aircraft’s design limitations are based on the ISA standards. However, as already mentioned, reality rarely matches laid down standards; they are but yardsticks.

The effect of density below the established 100% at 1.225 kg per cubic metre has a definite bearing on an aircraft’s stated ‘book’ performance. This area we shall discuss after you have digested the aspects of pressure, temperature and humidity.

Warmed air expands – notice how the number of molecules in the original volume has decreased

CHAPTER THREE

PRESSURE

WHAT IT IS

Pressure is the weight of a given column of air in the atmosphere measured at the earth’s surface. It decreases with height until becoming nil when air ceases to exist. You may think that the reference to pressure being weight means it is the same as density — this is not so.

Imagine warming a given parcel of air in a tube. This air would expand and spread itself farther up the tube so that the column in the tube would become greater in length. But take the volume of the original parcel of air in the longer column and you will find it is lighter (less dense) than it was before the warming took place. The same amount of air remains in the column overall so the pressure at the bottom stays the same.

The surface pressure need not decrease with a decrease in density

MEASURING PRESSURE

The unit by which atmospheric pressure is measured in the UK has for many years been the millibar (mb). The ICAO has now agreed to a change of unit to that of the hectoPascal (hPa). The change is rather academic in that 1 mb equals 1 hPa so it is a case of “a rose by any other name”!

However, the millibar will continue to be used in the UK for operational purposes for the foreseeable future; the hectoPascal will be confined only to those occasions when reference is being made to ICAO International Standard Atmosphere as a definition in itself.

In the light of this present situation we will naturally refer to millibars (mbs) in this book with possibly the occasional insertion of hPa as a reminder for the future.

In the United States of America, measurement of pressure is currently in inches (in.Hg) – based on the length of a column of mercury in circumstances about to be described.

The mercury barometer consists of a long thin tube extending upwards from a reservoir of mercury. A tiny hole in the reservoir allows air pressure to enter and exert its influence on the mercury. The above is an example from where the inch is the unit of measurement

Readily recognisable with its clock face and scale marked with a range of choices from ‘Set Fair’ to ‘Stormy’, or simply a plain dial showing measurement only. A perfectly reasonable instrument can be purchased for a very modest outlay. There is usually an adjustable hand which can be set over a reading at a given time so that future changes are readily observed.

Below is a basic marine barometer usually seen on ships also with an adjustment hand.

The instrument used to measure pressure is the barometer, of which there are two main types.

Mercury barometer

This consists of a transparent (usually glass) tube in which a column of mercury extends up from a reservoir at the base of the tube. A tiny inlet hole allows air pressure to enter the reservoir, and the column rises or falls according to the increase or decrease in outside air pressure. The tube is graduated with a scale that is sometimes fitted with a vernier to permit the maximum accuracy in reading off either millibars or inches.

Aneroid barometer

This type consists of a sealed concertina-like drum wherein a fixed pressure exists. As outside pressure increases or decreases the drum contracts or expands, and its movement is transmitted to a dial or a digital read-out. Tradition in the UK still sees the inch measurement on many household barometers, sometimes accompanied by millibars on later models.

The reading can also be transmitted to a revolving drum where a continuous picture of pressure changes can be seen. This type of aneroid barometer is known as a barograph.

Here you can see the sealed, concertina-like drum and the linkage associated with an aneroid barometer. In this case the linkage transmits a continuous reading to a chart

The barograph can be very valuable for amateur weather forecasting

Conversion from millibars to inches and vice-versa is as follows:

1 millibar

0.02953 in.Hg

and 1 in.Hg

33.8653 mb

For example

1000 mb

÷

33.8653

29.53 in.Hg

or 29.53 in.Hg

÷

0.02953

1000 mb.

Those who enjoy calculations should find that 27 ft per 0,02953 in.Hg equates to 30 ft being the equivalent of 0.032811 in.Hg – hence the 0.3 in.Hg per 30 ft referred to above makes it all the more simple.

At around 20,000 ft in the more rarefied air the change becomes approximately 50 ft per unit of 1 millibar or 0.02953 in.Hg, and increases to approximately 75 ft per unit around 30,000 ft due to the markedly reduced density at that height.

PRESSURE CHANGES WITH HEIGHT

The ISA standard for a change in height is 27 ft per hectoPascal (mb) – the equivalent of 0.02953 in.Hg. However, the generally accepted change is the round figure of 30 ft per millibar which equates to 0.03 in.Hg and this combination we will use when required.

UNIFORMITY IN MEASUREMENT

Taking the accepted decrease in pressure with height at 30 feet per millibar (0.03 in.Hg), given a surface pressure of 1000 mb (29.53 in.Hg), a reading taken at the top of a 300 ft (100 metres) building would be 990 mb (29.23 in.Hg) for the same location.

You will realise that if pressure readings were taken at stations located at varying heights it would be impossible to obtain a meaningful pressure pattern at any given time.

The difference between top and bottom would be 10 mb

Uniformity is obtained by all pressure readings being converted to sea level, with even tidal movement being taken into account, thus ending up with the term Mean Sea Level (MSL).

The ISA MSL standard is 1013.25 mb (hPa) or 29.92 in.Hg.

Pressure readings, together with other weather data, are noted every day at regular intervals throughout the world and transmitted to main centres. Here the pressures are reproduced on charts with lines joining places of equal MSL pressure. These lines are known as isobars and appear similar to contours on a map.

Isobars joining places of equal MSL pressure

PRESSURE SYSTEMS

Isobars produce a pattern which depicts various pressure systems.

HIGH

– A centre of high pressure known as an anticyclone.

RIDGE

– A wedge or tongue of high pressure extending out from the centre of an anticyclone.

LOW

– A centre of low pressure known as a depression.

TROUGH

– A wedge or tongue of low pressure extending out from the centre of a depression.

COL

– The neutral area between two highs and two lows.

The main pressure systems

For operational reasons the pressure pattern in the upper air is depicted by contours. These lines join places where a given pressure is at the same height – this time the principle is the same as that of contours on a map, with the selected pressure level, say the 500 mb (13,65 in.Hg) level, shown in metres and/or feet.

Contours joining places of equal height above MSL for a given pressure level

Contours tell the same story as isobars but the location of the upper air pressure systems they depict in terms of centres of high and low pressure can differ in location from those depicted by isobars at sea level.

WORLD PRESSURE DISTRIBUTION

There are appropriate general areas of high and low pressure in belts surrounding the Earth.

The world pressure distribution

The rotation of the earth causes the flow to deflect and not move directly from high to low as you might expect

CHAPTER FOUR

THE ALTIMETER

Excluding the radio version, the altimeter is included in this book at this stage because of its direct relationship with meteorology in relying upon atmospheric pressure for its operation. It is the instrument which indicates the height of an aircraft above a pre-selected surface level and is nothing more than a sensitive aneroid barometer. But in this case it is calibrated to read feet, instead of millibars or inches, on the basis of the 1 mb (0.03 in.Hg) change in pressure equating albeit to a change of 30 ft in terms of height.

Atmospheric pressure effect on the aneroid, known as static pressure, is fed to the instrument from an aperture set into the fuselage side at right angles to any airflow.

Air pressure enters static head and on to the aneroid drum, causing it to compress and expand accordingly

The altimeter can have a three-handed dial. The first and largest hand will indicate feet in hundreds; the second, smaller hand will show thousands and the third, with possibly a ring or an inverted triangle on it, depicts tens of thousands of feet. This three-hand format can lead to inadvertent mis-reading and has done so on a number of occasions in the past, so be particularly careful in your interpretation. Some altimeters also give a digital read-out.

Below the face of the instrument is a knob which enables a selected pressure setting to be entered at any given time on the ground or in flight. Calibration of the setting scale can be in millibars (mb) or inches (in.Hg) or both. Unlike the barometer, stationary at a reporting station, the altimeter is very much a mobile entity which is subject to many variables.

Take one such situation – atmospheric pressure is constantly changing, so the altimeter has to be adjusted in order to read correctly at the time.

Eg:

The altimeter is reading 0 ft at, say, a surface pressure of 1000 mb (29.53 in.Hg). The pressure drops to 990 mb. (29.23 in.Hg) which, at the rate of 1 mb (0.03 in.Hg) per 30 ft, is the equivalent of 300 ft.

The altimeter will now read 300 ft although the aircraft is still on the ground. Re-setting to 990 mb (29.23 in.Hg) will see the altimeter once again reading 0 ft.

A basic altimeter with a millibar setting scale

This altimeter has two scales – the left one is in millibars – the one on the right is in inches

On a long cross-country flight, en route pressure changes can take place and the pressure can be totally different on arrival at the destination. If on radio, this can be checked and adjustments made to the altimeter setting; without radio the only alternative is to seek information on possible changes prior to take-off.

Before going into detail on how defined laid-down pressure settings help to control this situation, first take a look at how being unaware of such a change of pressure can affect a planned flight path to a destination.

For simplicity’s sake, the following example, albeit exaggerated in terms of pressure change, presupposes both take-off point and destination to be at sea level.

Eg: •

Airfield pressure setting at take-off shows 1020 mb. (30.12 in.Hg).

•

Altimeter reads 0 ft.

•

Aircraft climbs out and sets course at 2000 ft.

•

During the flight the pilot notices height ‘increasing’ and descends to maintain 2000 ft on the altimeter.

•

At destination on landing, being the same height as at take-off, the altimeter reads 990 ft.

•

Pressure has decreased by 33 mb (0.97 in.Hg) to 987 mb (29.15 in.Hg) – equivalent to a false increase in height of 990 feet.

•

Actual height on arrival overhead had been 1010 ft, not 2000 ft.

•

There had been a sloping downward flight path due to the decrease in pressure from take-off point to destination.

It needs little imagination to realise that, unless there is a safeguard in place, any flight path that slopes downward can pose serious problems to a pilot flying in poor visibility or ‘blind’ on instruments – particularly if high ground is on the route and a safety height has not been built in to the planned flight path.

When flying towards a centre of low pressure the altimeter can give a false higher altitude reading

Again, the likelihood of the height above mean sea level (AMSL) of the take-off and destination points being identical is rare indeed. In the above example, if the destination had been 300 ft above the take-off point, the height on arrival overhead would have been 710 ft, not 1010 ft, or the 2000 feet that the pilot expected.

To overcome such situations there is a range of defined altimeter settings of which there are four in the UK. In the USA there are only two settings; we will take the more detailed UK procedure first.

ALTIMETER SETTINGS – UNITED KINGDOM

In the UK each of the four settings forms a datum to which you must adhere in given situations. They ensure that aircraft in the airfield circuit or in en route flight are flying to a common datum.

QFE

•

Pressure setting at airfield level.

•

Set in proximity to home or destination airfield when due to land or during circuit flying. Also may need to be used in proximity to another airfield or MATZ.

•

The reading from this setting is officially referred to as height and registers 0 ft on landing.

•

QFE is currently becoming less used by some large transport aircraft.

QNH

•

Pressure setting at MSL.

•

Set just prior to take-off when going cross-country.

•

Used mainly in flight across country.

•

The reading from this setting is officially referred to as altitude and registers the elevation of the airfield above sea level on landing.

REGIONAL QNH

•

Pressure setting at a given QNH for a specified region.

•

It is based on the lowest forecast QNH for the region to ensure safer terrain clearance; it is amended at regular intervals.

•

This reading is also known as altitude.

•

Used en route when away from the airfield vicinity as an alternative to QNH.

•

The various altimeter setting regions are depicted on aeronautical charts.

•

It also assists with en route aircraft separation when positions are reported to a controller.

STANDARD PRESSURE SETTING (SPS).

•

This pressure setting is the defined ISA Standard Pressure Setting (SPS) expressed in millibars as 1013.25 at mean sea level (MSL). In practice the nearest you can probably get on the altimeter scale will be 1013 mb or the mark which appears on some scales for this setting.

•

The SPS is set on passing a stated transition altitude after take-off (more later).

•

The altitude reading at the SPS is known as the pressure altitude but it is expressed as a Flight Level (FL) where the hundreds (00s) are deleted from the pressure altitude reading. For example, 15,500 ft would be FL 155.

•

Fight levels are established at 500 ft intervals and always relate to the SPS of 1013 mb. In this way adequate aircraft separation is ensured – essential when flight is under IFR (Instrument Flight Rules).

What may give rise to concern is grasping the mechanics of the differences the two settings (SPS and QNH) can portray in the messages they give out. On this aspect you are far from alone – such concern has been experienced by many before you!

First a brief repetition:

•

SPS is the defined fixed value of 1013 mb by which altimeters are set to ensure uniformity of operation when required. It is a datum in the form of the ISA-defined standard ‘mean sea level’ MSL pressure.

•

QNH is the actual pressure at MSL at any given time and is a variable – it is constantly changing.

A QNH value below 1013 mb (29.92 in.Hg) will produce a QNH altitude reading below the SPS pressure altitude