THE SCIENCE OF WEATHER.*
THERE are many classes of the community to whom the weather conditions are of the utmost importance." These words, with which Dr. Geddes begins the lucid and compre- hensive work which he modestly calls " an introductory treatise on Meteorology,"1 hardly go far enough. Living as we do in an island whose geographical situation makes its weather probably less easily calculable than that of any other country in the world, we have some excuse for a preoccupation which extends from the duke to the dustman. Foreigners find it difficult to understand why, when two Englishmen meet in any part of the globe, their first remark is generally that it is a fine day—or otherwise. In people who live in a place like Mendoza, where six months of successive fine days bring the dust in the roads up to the axle of a car, or Iquique, where they used to date from the year when it rained," as Californians date irom the earthquake, this engaging habit provokes a mild scorn. But even a brief residence in England makes it comprehensible enough.
The part of Dr. Geddes's book which will be read with most interest by the layman is naturally, therefore, the last chapter, which gives a very intelligible account of the methods now adopted for predicting changes in the weather. From the earliest times men have tried to establish an empirical system of weather forecasting, fragments of which still survive in Elihu's assertion that fair weather cometh out of the North," or in such rhymes as :- " Evening red and morning grey Are sure signs of a fine day."
But scientific meteorology was only possible after the advent of two necessary conditions : the invention of such instruments of precision as the barometer, the thermometer, etc., and the c3tablishment of a set of observing stations at which regular and exact records of the weather changes were kept over a great part of the world. It was only within the nineteenth century that both conditions were fulfilled. The first serious attempt at weather prediction in this country originated in 1855, when the Board of Trade established a meteorological department under the control of the famous Admiral Fitzroy, in consequence of the memorable storm which wrought so much havoc amongst our shipping before Sebastopol. Later, this department was separated from the Board of Trade and became the Meteoro- logical Office, whose primary duty to the public is the prepara- tion and issue of the daily forecast of weather which is published in all our newspapers. This forecast is on the whole wonderfully trustworthy, though sometimes it still breaks down badly—a notable instance of such failure being afforded in the latter part of January last.
Dr. Geddes tells us in his concluding chapter how the work is done. The weather at any particular place and time is defined when fixed values are assigned to the nieteorological elements, i.e., when the condition of the atmosphere is known as regards temperature, pressure, wind, humidity, cloud and precipitation. The forecaster must be in a position to say what changes are likely to take place within a certain time—usually twenty-four hours—as regards these various elements. To this end he must be provided with the most complete and accurate information possible regarding (a) the normal conditions prevailing in the region for which the forecast is intended, and (b) the state of the meteorological elements at a particular hour in that region. The normal conditions are derived from a long series of observations made in the area in question, combined with a general study of its geographical position, of the atmospheric circulation, ocean currents, etc. The detailed information under (b) is obtained periodically from a large number of observ- ing stations scattered over the area in question, all in telegraphic or telephonic communication with the central office in which the forecast is prepared. In the British meteorological service these stations are divided into three groups. At stations of the first order continuous records or hourly readings are made of pressure, temperature, wind, sunshine and rainfall, together with cloud observations at fixed hours and notes of the general • (1) Meteorology. ByA. E. M. Geddes. London: Mackie and Son. [21s. net.] --(2) The Rainfall of the British Isles. By M. de Carle S. Salter. University of London Press. [8s. 6d. net.]—(3) Clouds. By Geo. Aubourne Clarke, iontion : Constable and Co. [21s. net.]
weather. At stations of the second order observations are made at least twice daily, and at third order stations only once a day. At a few stations additional observations of the upper air are made by means of pilot balloons, kites or aeroplanes. The information thus obtained is telegraphed in code to the central office in London as soon as possible after the hours of 7 a.m., 1 and 6 p.m., to which hours each set of observations is referred. As the telegrams come in they are decoded, and the values of the different meteorological elements are inserted on a chart of the area :-
" In this way the state of the weather over the whole of the British Isles at a given hour is known at the central office gener- ally within an hour of the time the observations are made. From this information a synchronous chart is prepared which enables the forecaster to see at a glance the distribution of the various meteorological elements at the hour of observation."
It is by means of these " synoptic charts " (as they are gener-
ally called) that the weather forecasts which appear in the newspapers are made :-
" To complete tho3 synoptic chart, observations are received from the countries of Western Europe, Spain, France, the Netherlands, Germany, Denmark, Scandinavia, North-Western Russia, Iceland and the Faroe Islands, and also from the Azores. By international agreement all observations aro made in this area at the same Greenwich mean time, in order that the observa- tions may be comparable. Additional information is also received by wireless from ships on the Atlantic, thereby completing the circle round the British Isles."
Dr. Geddes describes, in more detail than we have space to indicate, the nature of the information regarding existing weather'
conditions which is thus made available to the forecaster. As experience shows that the weather associated with an atmo- spheric depression moves, in a general way, with that depres- sion, though at the same time the pressure, temperature, etc., within the depression are themselves usually in a state of change, the forecaster has at least three points to consider : (1) how the pressure systems on the chart will move ; (2) what changes are likely to take place within these systems ; (3) what effect these changes will have on the weather conditions within the systems. Having made up his mind on these points, he is in a position to say what he considers the weather will be during the next twenty-four hours in any district within the area of his chart. As the weather is entirely different in the different quadrants of a depression, it has been found necessary to divide the United Kingdom into no fewer than twenty districts, for each of which a daily forecast is issued.
Dr. Geddes points out that a large number of synoptic charts display the existence of certain well-defined types of weather, seven of which include practically the whole of British weather conditions. The one of which we hear with most pleasure is the anticyclonic type, usually associated with light and variable winds, cloudless skies and prolonged fine weather ; it was very much in evidence last summer. An interesting detail is the tendency of a weather type to continue, leading to the well- established but not generally known fact that the chances are in favour of any given day being fine after a period of fine days, and wet after a period of wet days. Dr. Geddes goes on to give some valuable and clearly expressed hints for local fore- casting, by means of which any intelligent owner of a barometer can make himself a fairly accurate weather prophet. The reader who masters the whole of this well-arranged and clearly written book may boast that he has a singularly exact knowledge of the problems and methods of modern meteorology. Along with it we can heartily commend the detailed account of the rainfall tf our islands,' and the methods employed in its observation, which Mr. Salter writes with the highest authority, as the present superintendent of the British Rainfall Organization ; he gives a simple and interesting presentation of the various scientific questions involved. -Mr. Clarke's full and fascinating treatise on clouds' illustrates the international classification of cloud- forms with the aid of nearly a hundred admirable photographs and diagrammatic sketches. Sir Napier Shaw's preface vouches for its merit as a scientific study, and we can add from personal experience that the careful study of such a book will add immensely to the interest of a country walk when any typo of weather except the anticyclonic is in force.
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