OUR WEATHER

Evening Star, Issue 20134, 26 March 1929, Page 6

OUR WEATHER

FORECAST COMPILING HOW EXPERTS WORK DR KIDSON EXPLAINS. The one never-failing topic of conveisation the world over is the weather, everybody every day being to a greater or lessor extent affected by it. Probably the most widely-read paragraph in any daily newspaper is that which gives the forecast for the next twenty-four hours’ weather; wo may believe it or not, according to our temperament, but the fact remains that in the great majority of cases it is right, or approximately so when modified by local conditions. What the forecaster aims at doing and bow he sets to work is fully explained in language comprehensible by the layman in an article written for the ‘ New Zealand Education Gazette ’ by Dr Jvidson. Director of Meteorological Services.

“ Weather forecasts,”-says Dr Kidson, “ are concerned principally with the piediction of precipitation, wind, and temperature changes. Of these, the most important to the general public is undoubtedly precipitation. Winds are the next important, and to the mariner the most important. Temperature changes are of special consequence to the farmer, particularly in connection with the management of stock. Losses among newly-born sheep, for instance, may be very high if they experience a cold snap directly after being turned out. “ Tho great influence exercised by procipitalon oi( human occupations is too well known to require description. It is to the man on the land, however, that rainfall is of tho most vital importance. Then there are water storage' schemes and the effect on roads and transport to bo considered. The accurate forecasting of precipitation includes tho specification of its nature, whether it is to be rain, snow, or hail. Wo are not ofton troubled with snow in our cities, hut it is more frequent in the interior of the South Island. Shoe]), and in a few places, cattle have to be. brought down from the highest pastures before the first heavy snows, and loss can also bo avoided later in the winter by moving sheep on to warm or more protected slopes when snow is expected. Hail may do inncii damage to crops, hut, as a rule, little can bo done in Iho way of protection even if a warning is received. In certain circumstances, however, a ripe crop may be gathered. The amount and intensity of tho precipitation should also be indicated by the forecast. Warnings of hoods should be issued. “In addition to its obvious effects mi sea and shipping, wind is a vital factor in our comfort, it has important effects, also, in connection with many engineering activities. For instance, it cxerfs pressure on electric power transmission lines, and in certain strong winds it may bo necessary to have repair gangs standing by. Then, winds affect tides, and hence indirectly may effect tho severity ot floods, and so forth. “ Rrcsure changes have to be forecasted because the variation of the other elements depends largely on them. Air pressure also affects tides, a.i increase of pressure reducing the level of tho sea, and vice versa. “ Among the more local phenomena to bo forecasted arc thunderstorms, which may do damage to elcctrm power lines and in many other ways; frost, which may do enormous damage to fruit and other crops, and against which protective measures arc now being taken; and fog, which affects principally shipping.” SCOPE OF THE FORECASTS. Dr Kidson proccccls to consider what it is the forecast should attempt to accomplish. This, ho says, will depend largely on the degree of development of the country which, it serves. “This is for two reasons—firstly, because it a floe Is the amount, scipc, and accuracy of the information the meteorologist receives; and, secondly, because on it will depend the facilities for distributing the forecast and weather reports and tho means of displaying them to tho public. In a highly-developed region such as Europe there will be many more people of such training and in such positions as to render them suitable as observers than in a young and .sparsely-populated country like Now Zealand. Facilities for telegraphing will bo much greater. Therefore not only will it bo possible to get reports, of a, much fuller and more accurate description and from a closer network of stations, but they will be procurable at more frequent intervals. Similarly, there will he facilities for transmitting tho forecast and reports rapidly to all parts of the country, and the displaying of them promptly. Charts even may bo transmitted by telegraph, and so reproduced in distant cities by newspapers, etc. Jf necessary, subsidiary centres may have local forecasters who will amplify the predictions coming from tho central office.

4< From each of these points of view tho question of finance in tho dominant consideration. The collection of reports and their subsequent dissemination along with the forecast is a very costly business. In a closely-settled country tho cost per head is very small, and much.more can ho done than in one that is sparsely populated. There is thus a limit to what is practicable in each country. ,

“In England it is possible to give considerable local detail and precision as to time in the forecast —at least us regards special areas such as air routes. Generally, however, very definite limits arc imposed even in such countries by the length which it is possible to give a forecast, in New Zealand wo do not know the local conditions in great detail at any time of tho day, so that we can scarcely he expected to know what they will be in future. For instance, occasions arc not infrequent on which it will rain in the Hutt Valley, but not at Wellington. On these occasions, although we may indicate that wo do not expect general rain, we do not pretend to be able to define the precise areas which will receive precipitation. Similarly, it may rain at Akaroa and not at Christchurch, but wo should consider that a forecast of coastal rain for Canterbury vmnld cover the facts.

“ What the forecast should aim to do, in conjunction with the reports of the actual weather at various stations, is to enable the reader to grasp the general situation and its main probabilities and possibilities. The local observer with a knowledge of meteorology will ho able to amplify the forecast very considerably from both Ids general knowledge of local weather processes and his observations of the current weather. He will be greatly assisted in this'if he has a weather chart available ; for the weather chart is tho only satisfactory means so far found of condensing the available information into a form in which it can be hilly and quickly appreciated. “The ideal conditions, then, would be that people should receive sufficient training in meteorology to be able to

understand a weather chart and the general principles of forecasting. In this way only can the fullest use oi the meteorological service be made. In the attaining of this end the schools can help, and I have no doubt that they are anxious to do so.” FORECAST DATA. After dealing with general meteorological conditions prevailing in this part of the world, Dr Kidson says that in coming to the aciaul process of forecasting it is necessary first to consider the material on which a forecast is based, and how it is obtained. “To bo able to forecast future weather we require to know as much as possible of existing weather, not only in our own region, but over an area extending as far as possible around it on all sides, and not only at ground level, but at all levels in the atmosphere. The limit to the amount of information desired is set by the amount which can be assimilated in the time available. Some day it will probably bo possible for mathematicians to take the existing state of affairs and allowing for all the influences at work calculate future development with certainty. L. F. Richardson, who conducted a brilliant investigation of the problem from this point of view, calculated that the number of computers required for the -earth would be (33,000. In the only attempt he made his forecast was wrong. It has not been proved that his arithmetic was right, but his data were admittedly incomplete! “Actually, then, although we apply physical principles to an increasing degree as the science of lorecasting develops, our methods are still to a large extent empirical. “ For our New Zealand iorecasts wo now receive reports from Sydney and Hobart based on observations made at 9 a.in. These reach us before wc deduce the midday forecast. Reports from twelve other Australian stations follow later. Although too late for the morning forecast, these reports keep us in touch with the situation and help in the evening forecast. The reports cover pressure, wind, and weather. Then wo also receive messages twice daily from Norfolk Island and Chatham Island. Although the latter is to the cast, in the direction of our retreating weather, it is surprisingly useful. Norfolk Island also is a very valuable station. From it wo are often able to anticipate the development of cyclones and judge of the extent of the commoner typo of depression. “ From New Zealand stations we get over fifty reports in the morning and twenty-four in the afternoon. As well as pressure, ■wind, and weather, they include temperature and the state of the sea and tide. Unfortunately they ai'c characterised more by quantity than by quality. An increasing number of reports is being received irom skips at sea, and these are most valuabk.- ” CYCLONES AND ANTI-CYCLONES. Details are thou given as to bow the daily weather charts are filled in from the available data. Once the isobars are drawn in it will be see i that tin re is considerable system about them, the meteorologist being famiT.ir with the various types of pressure sv-tenn. The principal systems arc the “ anticyclone,” or centre of high' pressure, and the “depression,” or area in which pressure is lower than in its surrmndings, there being several 'nineties of Uio latter. If charts arc drawn on consecutive days the pressure systems will be found to move eastward. “ Important facts to notice,” soys Dr Kidson, “are: (I).That the direction of the wind bears a definite relation to the isobars, being such that if you look in the direction in which the wind is blowing pressure is lower on your right hand than on your left hand; and (2) that the closer the isobars or the steeper the ‘ pressure gradient,’ the stronger is the wind. The direction is approximately parallel to Iho isobars, crossing them at a wnall angle from high towards low pressure. The angle varies according to circumstances, but is generally smaller over the ocean, where friction is Jess, than over land, and in strong than in light winds. At a height of a few hundred feet the parallelism between wind and isobars is very close. The wind can, iu fact, he calculated to a close degree of approximation from the direction of the isobars and the steepness of the pressure gradient. It is important that this should be realised, since the natural impression would be that the wind would flow r from high to low pressure in order to equalise matters. It is the rotation of the earth which prevents this. At the surface, owing to friction between ground or sea and wind, the balance of forces obtaining at higher levels is upset, and there is a, slight drift from high to low pressure. “Now, the moving anti-cyclones follow tracks which are almost wholly within the area covered by our charts, so that the charade, usuts of their movements can be studied in detail. The same is, unfortunately not true of the depressions whoso centres arc more often than not beyond onr ken. Those wc can trace are, in many instances, not typical. Nevertheless, though wo have to derive our knowledge of the movement of the centres of pressure systems from , the anti-cy-clones, it is probable that much of it is applicable to the depressions which alternate with them in the eastward march. Now, it is found that though individuals may depart widely from the jnean tracks, there is a groat deal of regularity in the latter. For instance, there is a regular annual march of the average latitude of the anti-cyclone centres. The anti-cyclones travel in their farthest south latitudes in February. From March onwards there is a northward migration until tho most northward position is reached in September or October. Thereafter there is a return to tho southwards. This movement has an important influence on our weather. When the anti-cyclones are far north we come more completely into tho zona of weslorly winds, and the strength and persistence of.the westerlies from September to December is largely associated with this fact. In February and March, on the other hand, w© are nearer the zone of high pressure,, and winds are lighter and the weather more settled. Not only is there an annual variation in the latitude of anti-cyclones, but their speed of movement, as I have shown in the paper above mentioned, also varies. It is greatest over the Australian and New Zealand region as a whole in October, November and December, and least in May and June. This, no doubt, is another of tho factors associated with tho velocity of the westerly winds, and is in itself an important one in forecasting.

| “Thera is also a considerable uniformity in the distance between anticyclone centres, though there are many disturbing factors. The more I consider the matter the more am I con- , vineed that the anti-cyclones and the intervening depressions are due _to fairly regular pressure waves existing in the upper air, and a feature of the circulation of the atmosphere over the , Southern Hemisphere as a whole.’’ I THE FORECASTER LIMITED. | After a number of illustrative weather charts, Dr Kidson concludes : his article with a hint that the forei caster cannot hope to be infallible. “Jt | has to be remembered,” he says, “ that jno forecaster can be familiar with all ' the local peculiarities of the weather lin Now Zealand. Even should lie have an accurate mental picture of all the /

processes lie could not describe them to the public, because Lis forecast is neces. snrily limited to a few words. The case is very well nut by Dr Brunt, who says: ' The ideal conditions for forecasting the details of the weather at a Riven place, would be provided if a keen observer had at his disposal the use of a synoptic chart, and lived largely in the open air. An enthusiastic meteorologist so situated could readily achieve a degree of success in forecasting his local weather, which no official meteorologist stationed at a, central office could ever hope to achieve. The latter, even when he has very full telegraphic details of the weather for a very wide network of stations, can scarcely hop© to appreciate tha individual eccentricities of each locality, and, oven if he could do so, he could scarcely give effect to his appreciation, since he is called upon to sum up his views of tho coming weather over a wide area in. from twenty to thirty words.'"