WEATHER STUDIES L. P. SMITH, B.A. British Meteorological Office and President of the Commission for Agricultural Meteorology of the World Meteorological Organization PERGAMON PRESS OXFORD · LONDON · EDINBURGH NEW YORK TORONTO · PARIS · FRANKFURT Pergamon Press Ltd., Headington Hill Hall, Oxford 4 & 5 Fitzroy Square, London, W.l Pergamon Press (Scotland) Ltd., 2 & 3 Teviot Place, Edinburgh 1 Pergamon Press Inc., 44-01 21st Street, Long Island City, New York 11101 Pergamon of Canada Ltd., 6 Adelaide Street East, Toronto, Ontario Pergamon Press S.A.R.L., 24 rue des Écoles, Paris 5e Pergamon Press GmbH, Kaiserstrasse 75, Frankfurt-am-Main Copyright © 1966 Pergamon Press Ltd. First edition 1966 Library of Congress Catalog Card No. 63-12688 Set in io on i2pt\Baskerville and printed in Great Britain by Cheltenham Press Ltd., Cheltenham This book is sold subject to the condition that it shall not, by way of trade, be lent, re-sold, hired out, or otherwise disposed of without the publisher's consent, in any form of binding or cover other than that in which it is published. (1480/66) 1 · How to measure Temperature SUPPOSE that you had a number of bottles of different sizes and that they were filled with water. Each bottle would contain a different amount of water, but all would be full. It is much the same with heat and temperature. Different substances may contain different amounts of heat but all be at the same tempera- ture. In the first case everything depends on the size of the bottle, in the second it all depends on the capacity of the substance to take in heat. Some substances, like air, need a small amount of heat to raise their temperature, just as small bottles need only a little water before they change from empty to full. Others, such as lead, need much more heat to change their temperature, like the larger bottles. Most substances expand when they are heated and their temperature rises. This increase in size can therefore be used to measure temperature, which is done by an instrument called a thermometer. Most thermometers measure the change of length of a fine thread of mercury, which does not need much heat to change its temperature. In other words, a mercury thermometer is sensitive. If a thermometer were made of a material which was less sensitive, such as one of the solid metals, it would show the same temperature in the end but take a longer time to do so. Thermometers used to measure air temperature are placed in special screens or other forms of cover to cut out the errors caused by sunshine. Air temperature can be measured on either of two scales, Fahrenheit or Centigrade, and thermometers may be marked in either scale. The freezing point of water is 0° on the Centigrade scale and 3 4 WEATHER STUDIES 32° on the Fahrenheit scale. The boiling point of water is 100° on the Centigrade scale and 212° on the Fahrenheit. The Centigrade degree is thus almost twice the size of the Fahrenheit degree (see Fig. 1). Boiling point of water TEMPERATURE (°F) FIG. 1 To convert Centigrade to Fahrenheit the exact rule is: (a) Multiply by 9/5 Ί A simpler but less accurate rule is (*) Add 32. /double and add 30. To convert Fahrenheit to Centigrade the rule is: (a) Subtract 32 Ί A simpler but less accurate rule is (*) Multiply by 5/9 /subtract 30 and divide by 2. ASSIGNMENT 1 1 (a) On a cold winter night in Siberia the air temperature is -40°F. (b) On a cold winter night in Britain the air temperature is 34°F. (c) On a spring day in Britain the air temperature is 50°F. (d) On a warm summer day in Britain the air temperature is 84°F. (e) The temperature of a healthy body is 98-6°F. What are these temperatures on the Centigrade scale? 2 (a) On a cold winter night in Siberia the air temperature is -40°G. (b) On an autumn day in France the air temperature is 15°C. (c) On a summer day in Egypt the air temperature is 39°C. What are these temperatures on the Fahrenheit scale? 3 In the open, place a thermometer in a standard screen (or, if this is not available, shield it from the sky by some form of cover). Place another alongside it at the same height with its bulb fully exposed to the sky. Read both thermometers when the sun is shining and when the sky is overcast. Do the same thing after dark. Do the temperatures differ? Can you explain why? 5 2 · How to measure Maximum Temperature AIR temperatures are continually changing. They usually increase soon after dawn, and reach their highest values a little after mid-day. They then begin to decrease and continue to do so until dawn the next day. This is known as the diurnal change in temperature. If anyone could watch a thermometer throughout the day, taking readings every five minutes or so, it would be possible to know the highest temperature (i.e. the maximum) and also when it occurred. This is obviously not practical even on one day, let alone every day of the year, at school or anywhere else. Therefore thermometers are designed which record the maximum tempera- ture to which they are subjected. The mercury in these thermo- meters will expand up the thin glass tube but will not contract back again into the bulb because of a thin "bottle-neck" or constriction. The doctor uses such a thermometer to measure your body temperature, so that when he takes it out of your mouth it will not start cooling but will still record the temperature it reached while it was in the mouth. To enable such thermo- meters to be used more than once, the mercury can be made to return past the bottle-neck in the tube by shaking, although this has to be done with a mixture of confidence and care, otherwise the thermometer will be broken. This is known as re-setting. In practice maximum thermometers are re-set each morning at 9 a.m. (Greenwich Mean Time), so that you read the maximum temperature of each day at 9 o'clock G.M.T. the next morning. 6 ASSIGNMENT 2 1 Prepare two containers, one filled with water at room tempera- ture, the other with hot water. Practise re-setting maximum thermometers by placing them in the warm water, taking the reading and then re-setting them. After re-setting place them in the water at room temperature. Do this several times. Do you always get the same readings for each kind of water? If not, why not? 2 Repeat the comparison of screened temperature and unscreened temperatures (done in the previous study), using maximum thermometers. Which type of day causes the biggest differences? 7 3 · How to measure Minimum Temperature As we have seen, the lowest temperature in 24 hours is generally reached during the night, often around dawn: again it is necessary to have an instrument which will do this for you, and so a minimum thermometer was designed. This thermometer does not use a liquid like mercury, which you cannot see through, but a form of alcohol, which is trans- parent. Because you can see through alcohol it is possible to place in the liquid a small object looking like a dumb-bell. The thermometer is kept in a level (horizontal) position. When the alcohol expands as the temperature decreases, the alcohol retreats back towards the thermometer bulb and in so doing drags the dumb-bell with it. In this way the edge of the dumb-bell nearest the end of the alcohol column always records the lowest temperature reached. To re-set a minimum thermometer, all that is needed is to tilt the thermometer so that the bulb is uppermost and the dumb-bell slides down the tube to the end of the alcohol column. In practice this re-setting takes place at 9 a.m. G.M.T., as in the case of the maximum temperature, but the difference is that it is generally recording the minimum temperature which has occurred on the same day as it is read, not the one of the day before. 8 ASSIGNMENT 3 1 Practise re-setting in the following manner. Have two containers of water, one warm, one cold ; soak two cloths, one in each type of water. Set the minimum thermometer and place on a level surface. Place the warm cloth over the bulb. Watch the alcohol move along the tube and the dumb-bell remain stationary. Re-set the thermometer and replace on the level surface. Place a cold cloth over the bulb and watch both alcohol and dumb-bell move along the tube. Take the minimum reading, and repeat the experiment. Do your successive readings of the "cold cloth" minimum temperature agree? If not, why not? 2 Repeat the comparison of screened temperatures and unscreened temperatures, using minimum thermometers. Which type of night causes the biggest difference in screened and unscreened readings? Are the differences bigger in the maximum thermometers or the minimum? Note: Although official meteorological stations always use separate maximum and minimum thermometers, it is often con- venient for less accurate purposes to use a combined thermometer which can measure both extreme temperatures. This is known as a Six's thermometer and it contains both alcohol and mercury; the alcohol is contained in a bulb at one end of the thermometer and in a reservoir at the other end, being joined by a thin thread 9 10 WEATHER STUDIES of mercury. Two dumb-bells are in the thermometer tube at each end of the mercury thread. They are moved by the mercury but remain in the alcohol. The movement of the mercury is largely controlled by the expansion or contraction of the alcohol in the bulb. The dumb-bells register the range of this movement and hence the maximum and minimum temperatures. The dumb- bells are re-set by means of small magnets. Such thermometers can be used in either or both of the previous two studies.