HOW THE WEATHER IS MADE AND FORECAST.
In the minds of foreigners, it is held to be one of the many peculiarities of the people of these islands that so much of their casual conversation consists of remarks on the weather. The national temperament is often held to be responsible for this failing; but some of the blame must no doubt be laid at the door of the weather itself. Our climate presents such a record of change and uncertainty, that we need not wonder if it is always in our minds, and the first subject on our lips when we meet a friend. Other lands may have their cold and hot, dry, and rainy periods, that come round in the proper order year after year with unvarying monotony; but with us it may be said of the weather, that we rarely know what a day or an hour may bring forth. Even the seasons seem occasionally to be independent of any necessity of visiting us at the particular time of the year at which we have been taught to expect them. Spring weather in November, or a winter temperature in July, or a November fog in the merry month of May, all seem to be amongst the possibilities of our climate.
Happily, our meteorologists are at length beginning to define with growing clearness and confidence the laws which underlie and regulate the complicated and ever varying phenomena which we call the weather, and many of these laws, like most natural laws, are beautiful in their simplicity. Although ‘weather wisdom’ is as old as history itself, the science of the weather or meteorology is a growth of the last few years. The weather wisdom of our forefathers may in the light of present knowledge be divided into sense and nonsense. Under the nonsense may be included not only such proverbs as that which attributed to St Swithin’s day and certain other times and seasons, occult influences over the weather, but most of the information of the old almanacs, which used to ascribe the character of the weather to the positions and movements of the heavenly bodies and the age and changes of the moon. The prevalence of the belief that the weather was regulated by such influences, can only be accounted for by the well-known love of the human mind for the wonderful and inexplicable. Much of the old weather lore, however, had a large element of truth in it, and was the result of the collective experience of many generations, which had found that certain phenomena were generally followed by certain conditions of weather. The saying, that a rosy sky in the morning presages rainy weather, and the same appearance in the evening, fine weather, was current weather lore before the Christian era, and is recognised as being, in a certain sense, true at the present day. Amongst sailors, farmers, shepherds, and such like, weather maxims, the result of observation and experience, have always been current, and the value of many of these is now recognised and explained by science.
The first step towards acquiring an insight into the causes which control our weather is a study of the laws which regulate the flow and changes of the winds in these islands. The air is the great medium in which all the changes of weather are elaborated. We live at the bottom of a great ocean of air, which extends for many miles upwards, and which is always heaving and changing, like the other ocean which it covers. The winds, which are the ever-changing currents which flow through this invisible sea, are, roughly speaking, the principal factors in the making of the weather. Many of us know very well the general character of the weather which accompanies the wind from the principal points of the compass, that which comes from the moist warm south-west, for instance; or with the blustering, shower-bringing north-wester; or with the harsh, dry, east wind in spring; but to most of us the wind itself ‘bloweth where it listeth.’ The movements of the air and changes of the wind are, however, subject to laws, a knowledge of which is in some degree necessary before we can understand how our weather is made for us.
A simple definition of the wind which we ordinarily experience is that it is air obeying the force of gravity, in seeking to return to an equilibrium which has been disturbed. By the aid of the barometer we are able to form some idea of what is constantly taking place in the great ocean above us. The principle upon which this simple and useful instrument is constructed is easily understood. The air presses downwards upon the earth’s surface with a weight averaging nearly fifteen pounds to the square inch. If a portion of the surface of any fluid is relieved from this pressure by inverting over it a tube exhausted of air, the weight of the air upon the surface outside will force the fluid up into the tube until the weight of the column counterbalances the pressure which the air would exercise upon the amount of surface covered by the mouth of the tube. A column of mercury in such a case will rise in an air-exhausted tube to a height of about thirty inches; while water, from its lighter specific gravity, rises to a height of about thirty-four feet before it counterbalances the weight of the air above. The depth, and consequently the pressure, of the air overhead is, however, constantly varying within certain limits; and the column of mercury in the barometer enables us to keep a faithful record of the movements of the waves of air in the great ocean under which we live. At times, the depth of air above us is comparatively shallow, and the pressure beneath is lessened; the column of mercury is not raised so high, and the barometer is said to fall. At other times, the air is heaped up in particular places; the pressure beneath is increased, and the barometer is said to rise. In stormy weather, the column of water in a water-barometer where the scale is very large may be seen to pulsate with every change of pressure from the air-waves at the surface.
The winds are nothing more than the rush of air from the regions of high pressure to fill up the spaces where low pressure prevails. Thus, if the column of mercury should stand 28.6 inches high at London, with a gradual rise as we travelled northward, until the barometer-reading was 29 inches at Edinburgh at the same time, this would indicate that a region of depression existed over the former place, and we should expect a rush of air in the form of wind blowing upon London from the north.
When the barometrical readings taken simultaneously at stations distributed over a wide area are compared, the distribution of atmospheric pressure can be ascertained, and it is possible to tell from this the force and direction of the winds prevailing within this area, and generally also the weather which is likely to be experienced. The greater the inequality of pressure, the greater will be the rush of air to the centre of depression, and the stronger will be the wind. The wind, however, does not flow in a straight line from a region of high to a region of low pressure. The surrounding air from all quarters has a tendency to flow in, and, as with water, which rushes to the centre of a funnel when it is flowing out at the bottom, a gyratory movement is the result. The wind blows round a centre of depression in this way, always curving inward towards the centre; and in the northern hemisphere, this gyratory movement of the wind is always in a direction against the hands of a watch, while the contrary is the case in the southern hemisphere. These principles of the relation of the winds to atmospheric pressure hold good without exception over all the world. They were first definitively stated in America twenty-five years ago; but Professor Buys Ballot of Utrecht first drew attention to them in Europe, and the law expressing them is now generally recognised as Buys Ballot’s law.
In ordinary circumstances in our latitude, the winds are generally regulated by the differences in pressure induced by contrasts between continents and oceans. Where the air becomes heated, an area of low pressure is produced, the warm air becoming rarefied and ascending, and the heavier cold air rushing in from the sides to supply its place. In winter, the weather over these islands is controlled to a great extent by the winds which sweep round a large area of depression which exists over the Atlantic, the mean centre of which is about midway between the continents of Europe and America, in the latitude of the Orkney Islands. This depression is the result of the contrast produced between the comparatively warm air over this portion of the Atlantic and the much colder air over the northern portion of Europe and America, which is continually flowing in to supply the place of the lighter and constantly ascending warm air. The winds sweeping round this centre strike our shores from the south-west. This depression is not stationary, but is continually shifting over a large but well-defined area, and it gives rise to many subsidiary eddies, or small cyclone systems as they are called, which sometimes skirt our coasts, or travel over these islands, bringing with them the storms of wind and rain and sudden changes of the wind with which we are familiar. In spring, the prevailing winds from the east and north-east, so much dreaded by many, are the result of a large cyclonic system formed by the sudden increase of temperature over middle and southern Europe, as the sun’s rays gain strength and the days lengthen. The temperature is not yet sufficiently high to bring in the air from off the Atlantic, as happens when the season is further advanced, so that the cold air rushes in from the polar regions in a huge eddy, striking our coasts from the east and north-east, and bringing in its train all the attendant miseries which make our English spring a time to be dreaded by the weak and ailing.
A knowledge of the general principles which direct the flow of our prevailing winds is, however, only of general assistance in enabling us to forecast the weather which we experience in these islands. This is governed and produced to a great extent by the development of subsidiary centres of depression in and between the great cyclonic systems. These generally approach our shores from the west, travelling in a north-easterly direction; and they are responsible for most of the variable weather with which we are so familiar. They generally carry with them a certain well-defined course of weather. The readings of the barometer taken simultaneously at many places over a wide area on a system such as that now controlled by the Meteorological Office, enables us to determine the approach and development of these small cyclonic systems, and so to forecast with a certain degree of confidence the weather likely to be experienced in a certain district from twelve to twenty-four hours in advance. Most of the disturbing influences reach us from the west; and as the west coast of Ireland is the extreme limit to which our stations reach in that direction, we can receive only very short notice of their approach. This is one of the principal reasons why, with the means at present at our disposal, we cannot expect to make our weather science as perfect as in a country such as America, where the central office receives warnings from stations dispersed over the face of a vast continent. Nevertheless, we have made great advances since 1861, when the first weather forecasts were prepared and issued in this country by the Board of Trade, under the superintendence of the late Admiral Fitzroy. The forecasts at that time, although admitted to be of considerable utility to the country, were thought to be scarcely accurate enough to justify their continuance upon the system then in operation, and they were discontinued in 1866.
In the following year, the Meteorological Office was constituted upon its present footing, and the daily publication of forecasts has continued down to the present. Considering that—judging from the forecasts published daily in the newspapers—the chances of a successful forecast are on the average about seventy-nine per cent. for ordinary weather, while the percentage of successes is slightly higher in the case of storm warnings, it is evident that the Meteorological Office is capable of rendering important service to the community at large. Every morning, the central office in London receives telegraphic reports from fifty-three stations. It also receives thirteen reports every afternoon, and nineteen each evening. Besides the numerous well-placed observation stations in the British Islands, there are twenty-three foreign reporting stations, extending along the entire western coast of Europe, from which information is received, in accordance with arrangements made with the meteorological organisations in Norway, Sweden, Denmark, Germany, Holland, and France. The morning observations are made at all the British stations at eight A.M. Greenwich time, and are transmitted direct to the Meteorological Office, where they are received between nine and ten o’clock. Thus are given the barometrical and thermometrical readings at the various stations at eight A.M.; the direction and force of the wind, and the state of the weather, together with any changes of importance which may have been noticed in the course of the preceding day. From these reports, weather charts are made out, forecasts of the weather are prepared and issued to the evening papers in London and the provinces; and a telegraphic résumé of the weather, or, if necessary, intelligence of storms, is despatched to various points on our coasts and to foreign countries. The forecasts for the morning daily papers are issued at half-past eight P.M. on the previous evening. They are prepared from reports received from twenty-six home and six foreign stations; but although these are the most widely distributed and read of any issued from the office, they are much less complete than the eight A.M. forecasts.