Meteorology

Meteorology (Gr. metēōra, 'meteors, or atmospheric phenomena') was originally applied to the consideration of all appearances in the sky, both astronomical and atmospheric; but the term is now confined to that department of physics which treats of the phenomena of the atmosphere as regards weather and climate. Owing to the complexity of the phenomena, meteorology is the most difficult and involved of the sciences, and may seem, at first sight, almost incapable of being reduced to a science at all. On this account, the only procedure admissible in the first place is a faithful recording of facts by long and patient observation.

From the nature of the subjects which make up the science, it may be inferred that they occupied men's minds from a remote antiquity. From the time spent in the open air in the early ages, and from the imperfect protection afforded against the inclemency of the seasons, those appearances which experience proved to precede a change of weather would be eagerly recorded and handed down. In this way many valuable facts were ascertained and passed current from hand to hand; and perhaps there is no science of which more of the leading facts and inferences have been from so early a period incorporated into popular language. Aristotle was the first who collected, in his work On Meteors, the current prognostics of the weather. Some of these were derived from the Egyptians, while a considerable number were the result of his own observation. Theophrastus, one of Aristotle's pupils, classified the opinions commonly received regarding the weather under four heads—viz. the prognostics of rain, of wind, of storm, and of fine weather. The subject was discussed only in its popular and practical bearings, and no attempt was made to explain phenomena whose occurrence appeared so irregular and capricious; but still the treatise of Theophrastus contains about all that was known down to comparatively recent times. No real progress was made till instruments were invented for making observations with regard to the temperature, the pressure, the humidity, the purity, and the electricity of the air. The discovery of the weight or pressure of the atmosphere made by Torricelli in 1643 was undoubtedly the first step in the progress of meteorology to the rank of a science. As this memorable discovery discloses what passes in the more elevated regions of the atmosphere, it follows that the elevations and depressions of the barometric column largely extend our knowledge of the subject. Indeed, nearly all of the more important of the discoveries of modern meteorology have been made through the barometric observations.

The invention and gradual perfecting of the thermometer in the same century formed another capital advance; as without it nothing beyond vague impressions could be obtained regarding temperature, the most important of all the elements of climate. Fahrenheit constructed small and portable thermometers, which, being carried by medical men and travellers over every part of the world, furnished observations of the most valuable description. By such observations alone the comparative temperature of different countries became known, and the exaggerated accounts of travellers with regard to extreme heat and cold were reduced to their proper significance. Scarcely less important was the introduction of the hygrometer, first systematically used by De Saussure (died 1799), and subsequently improved by Dalton, Daniell, August, and Regnault. From the period of the invention of these instruments the number of meteorological observers greatly increased, and a large body of well-authenticated facts of real value was collected. The climates of particular parts of the earth were approximately determined, and the science made great and rapid advances by the investigations into the laws which regulate the changes of atmospheric phenomena.

The theory of the trade-winds was first propounded by George Hadley in the Philosophical Transactions for 1735; and it may be mentioned as a remarkable fact that for about half a century it remained unnoticed, and then was independently arrived at by Dalton. The publication of Dalton's Meteorological Essays, in 1793, marks an epoch in meteorology. It is the first instance of the principles of science being brought to bear on the explanation of the intricate phenomena of the atmosphere. The idea that vapour is an independent elastic fluid, and that all elastic fluids, whether alone or mixed, exist independently; the great principles of motion of the atmosphere; the theory of winds, their effect on the barometer, and their relation to temperature and rain; observations on the height of clouds, on thunder, and on meteors; and the relations of magnetism and the aurora borealis—these are some of the important questions discussed in these remarkable essays, with singular acuteness, fullness, and breadth of view.

One of the most interesting and fruitful subjects of inquiry that engaged the attention of meteorologists was dew. The observations on this subject were first collected and reduced by Dr Wells, and the theory he advanced, supplemented by the recent researches of Mr John Aitken, gives a complete explanation of the phenomenon (see DEW).

In 1823 Daniell published his Meteorological Essays and Observations, which, while adding largely to our knowledge in almost every department of the subject, are chiefly valuable as bearing on the hygrometry of the atmosphere. Though the practical advantages which he anticipated to flow from it have not been realised, yet this difficult department of meteorology still stands indebted to him perhaps more than to any other physicist. The law of the diffusion of vapour through the air, its influence on the barometric pressure, and its relations to the other constituents of the atmosphere are among the least satisfactorily determined questions in meteorology. Since this element is so important in originating changes of weather and as an indicator of storms, and since so much remains still to be achieved, it is to be hoped that it will soon be more thoroughly investigated, particularly in its relations to solar and terrestrial radiation. As the humidity to some extent obstructs solar and terrestrial radiation, it follows that if the air were quite drained of its aqueous vapour the extremes of heat and cold would be so intense and insufferable that all life would perish, as there would be no screen shielding the earth from the scorching heat of the sun by day, and from the equally scorching and blighting effects of its own radiation by night.

Electrical observations have been, of all meteorological observations, perhaps the least productive of results advancing the science, partly owing to their scantiness, and from the expense and trouble attending them.

Humboldt's treatise on Isothermal Lines (1817) constitutes a notable epoch in practical meteorology. Dové and, more recently, the present writer have continued the investigation, and given charts of the world, showing the temperature for each month and for the year. In 1868 another series of important charts were published by the writer, showing, by isobaric lines, the distribution of the mass of the earth's atmosphere, and by arrows the prevailing winds over the globe for the months and the year. These charts, since revised by him, and published in one of the Challenger reports, show the movements of the atmosphere and their immediate cause. It is thus seen that the prevailing winds are the simple result of the relative distribution of the mass of the earth's atmosphere; or that the direction and force of the prevailing winds are simply the flow of the air from a region of higher towards a region of lower pressure, or from where there is surplus to where there is a deficiency of air. On this broad and vital principle meteorology rests, and it is of universal application throughout the science in explanation not only of prevailing winds, but of all winds, and of weather and weather changes generally. Further, it supplies the key to the climatologies of the globe; for climate is determined by the temperature and moisture of the air, and these in their turn by the prevailing winds. In 1882 Loomis published a map representing in colours the mean rainfall of the globe. This map and others that have been constructed for separate countries show that the rainfall is everywhere determined by the prevailing winds, considered with respect to the regions from which they have immediately come, and the physical configuration and temperature of the part of the earth's surface over which they blow. The highest rainfalls are precipitated by winds which, having traversed a large breadth of ocean, come up against and blow over a high ridge lying across their path; and the amount is still further increased if the winds pass at the same time into regions the temperature of which constantly becomes colder. Of this the winter rains of north-western Europe and the summer rains of Japan are good examples. On the other hand, the rainfall is very small, or nil, where the prevailing winds have not previously traversed some extent of ocean, but have crossed a high ridge and now advance into lower latitudes, or into regions the temperature of which is markedly higher. Good examples of this are the summer rains of California and adjoining regions, and those of the Indus valley.

The establishment of meteorological societies during the last half of the 19th century must also be commemorated as contributing in a high degree to the solid advancement of the science which, more than any other, must depend on extensive and carefully conducted observation. A special object of meteorological societies is to ascertain the degrees of temperature and moisture in various localities, and the usual periods of their occurrence, together with their effects on the health of the people and upon the different agricultural products; so that, by a knowledge of the laws by which the growth of such products is regulated, it may be ascertained with some degree of certainty whether any given article can be profitably cultivated. But perhaps none of the arts have benefited to so large an extent by the results arrived at by meteorologists as navigation. The knowledge thus acquired of the prevailing winds over the different parts of the earth during the different seasons of the year, the regions of storms and calms, and the laws of storms has both saved innumerable lives, and, by pointing out the most expeditious routes to be followed, shortened voyages to a remarkable degree. In this department the name of Maury (q.v.) deserves special commendation.

Another fruit of the multiplication of meteorological stations is the prediction of storms and 'forecasts' of weather, first suggested in the United States about the middle of the 19th century. As regards the British Islands these 'forecasts' are based on telegrams which are received every morning from about sixty selected stations in Great Britain and Ireland, and on the Continent, which give the exact state of the barometer, thermometer, hygrometer, and rain-gauge, with the direction and force of the wind, and appearance of the sky, at each of these stations at eight in the morning. In the event of there being any storm or other atmospheric disturbance at one or more of these places, a full and accurate description of it is thus conveyed to London; and it is the duty of the officials there to consider the direction in which the storm is moving and is likely to move, so as to enable them to give warning of its approach at different ports by special signals. But in addition to warnings of storms, daily 'forecasts' of the weather likely to occur in the different districts of Great Britain for the following two days are also issued. As regards storms the problem to be practically worked out is this: Given telegrams showing the exact meteorological conditions prevailing over the area embraced by the stations, with indications of a storm approaching in a certain direction, to determine, not the probable area over which the tempest will sweep, but the precise localities which will altogether escape, the places where the storm will rage, its continuance, its violence, and the particular directions from which the wind will blow at places visited by the storm. Weather-registers extending over long periods give no countenance to the notion that there are regularly recurring cycles of weather on which prediction sufficiently precise and particular to be of service to agriculture and navigation may be based. The manner in which good and bad seasons occur in different places with respect to each other shows clearly that they have little direct immediate dependence on any of the heavenly bodies, but that they depend directly on terrestrial causes. Owing to its proximity to the Atlantic, Great Britain is not so favourably situated for the issue of warnings as the countries of Europe to the eastward. Since 1870 this branch of science has been prosecuted with remarkable energy and success by General Myer (familiarily known as 'Old Probability') and his successor in charge of the signal service of the United States War Department. American meteorologists were the first to undertake the representation of isothermals over the ocean; and to the United States science is also indebted for magnificent contributions and researches in international meteorology.

The study of meteorology has of late benefited largely by the establishment of high-level meteorological stations in the United States, France, Italy, Switzerland, Austria, India, Australia, Scotland, and many other countries. The nine arctic expeditions in 1882-83 devoted themselves largely to meteorological observations. Many first-class meteorological observatories are now established in all civilised countries at which hourly observations are made.

On the diurnal phenomena the more important principles of the science are based. Of the sun's rays which arrive at the earth's surface, those which fall on the land are wholly absorbed by the thin surface layer, the temperature of which consequently rises. A wave of heat is thence propagated downward through the soil, the intensity of which rapidly lessens with the depth at a rate depending on the conductivity of the soil, till at a depth of about 4 feet it ceases to be measurable. Part of the heat of this surface layer is conveyed upwards into the atmosphere by convection currents. But as regards the surface of the ocean the case is totally different. Here comparatively little of the heat is arrested at the surface, but it penetrates, as shown by the observations of the Challenger expedition, to a depth of about 500 feet. Hence in deep waters the temperature of the surface is but little heated by the direct rays of the sun, though in shallow waters, owing to the heating of the bottom, the water has a considerable daily range of temperature. Thus, in mid-ocean, from 30° N. lat. to 30° S. lat., the temperature of the surface of the sea does not vary during the day quite so much as one degree Fahrenheit. Off the coast of Scotland the daily variation is only 0.3°, and in higher latitudes still less. On the other hand, the daily variation of the upper layer of the surface of the land is frequently 50°, and in many cases very much greater. Hence the enormously different results which large masses of land and sea respectively exercise on climate. The temperature of the air over the ocean is about three times greater than that of the surface of the open sea over which it lies; but on nearing land it is nearly five times greater. The least daily variation on land is in insular situations, being at Rothesay about 5°; and the greatest in the Sahara regions of tropical and subtropical countries, where it is in many places 30°, rising on occasions to 40° and upwards. The daily minimum temperature occurs some time before dawn; and as regards the maximum, it occurs from 1 to 4 P.M., according to season and geographical situation, the earlier hour obtaining in arid climates and at true high-level observatories, and the later in climates characteristically humid.

The absolute humidity of the air, or, as it is usually termed, the elastic force of vapour, is seen in its simplest form on the open sea, as disclosed by the Challenger observations. The minimum occurs at 4 A.M. and the maximum at 2 P.M., thus approximating closely to the diurnal march of the temperature; on nearing land a secondary minimum prevails from 10 A.M. to 4 P.M., due doubtless to the drier descending aerial currents which take the place of the currents that ascend from the heated surfaces of the land. The relative humidity is widely different from the vapour pressure, and presents features of the simplest character. The maximum occurs from midnight to 4 A.M.; or when the temperature is lowest the air is nearest to saturation. On the other hand, the minimum is about 2 P.M.; or when temperature is highest the air is furthest from saturation. This feature of the humidity characterises all climates. When the air is by terrestrial radiation cooled below the dew-point dew is deposited, and when the temperature is below 32° hoar-frost is the result.

The diurnal oscillations of the barometer show two maxima and minima—the maxima occurring about 9 to 10 A.M. and 9 to 10 P.M., and the minima from 3 to 4 A.M. and 3 to 4 P.M. Since the temperature of the surface of the sea does not vary quite one degree during the day, and since these oscillations occur equally over the open sea as on land, it conclusively follows that they are independent of the temperature of the part of the surface of the globe on which the air rests. Generally speaking, the amount of the oscillations decreases with latitude. Taking latitude with latitude, the amounts are greatest over land surfaces which are greatly heated during the day and cooled during the night, and least over the anticyclonic regions of the great oceans lying to the westward of the continents from about 20° to 40° N. and S. lat. The characteristics of these anticyclonic regions is a vast descending current down their central spaces. This air necessarily increases in temperature with its descent, and consequently is further removed from saturation; and it is prob ably due to this circumstance that the amount of the barometric oscillation is here reduced to the minimum for the latitude over all anticyclonic regions.

It has been further shown from the Challenger observations that the force of the winds on the open sea is subject to no distinct and uniform diurnal variation, but that on nearing land the force of the wind gives a curve as pronouncedly marked as the ordinary curve of temperature; the minimum occurring from 2 to 4 A.M. and the maximum from noon to 4 P.M. Each of the five great oceans gives the same result—the differences between the hours of least and greatest force being Southern Ocean, 6½ miles; South Pacific, 4½ miles; South Atlantic, 3¼ miles; and North Atlantic and North Pacific, 3 miles. This diurnal peculiarity of the wind's force is even still more pronounced over all tolerably open and extended surfaces of the land. But at true high-level observatories, situated on peaks, such as Ben Nevis, the reverse everywhere holds, so that the daily minimum velocity occurs during the warmest hours of the day, and the maximum at night during the coldest hours.

Thunderstorms have well-marked periods of diurnal variation over land and over the open sea respectively. In climates where rain falls equally at all seasons they are of most frequent occurrence during the hottest portions of the day and of the year, so far as concerns the land surfaces of the globe. Taking Ekaterinburg in the Urals as representing inland climates, observations show that there, during the twelve hours from 9 A.M. to 9 P.M. when temperature is above the daily mean, 717 occurred, but only 139 during the other twelve hours when temperature is under the mean. Thus the great majority occur during the time of the day when the ascensional movement of the air from the heated ground takes place, and attain the absolute maximum when the temperature and this upward movement are also at the maximum. On the other hand, the Challenger observations on the open sea show that the maximum occurrence is from 10 P.M. to 8 A.M., 22 having been observed during these ten hours and only 10 during the other fourteen hours of the day. This remarkable result suggests that over the ocean terrestrial radiation is more powerful than solar radiation in causing those vertical disturbances in the equilibrium of the atmosphere which give the thunderstorm.

Atmospheric vapour and ascending currents, and the descending currents which necessarily accompany them, play an important part in the development, course, and termination of thunderstorms. Where the climate is dry and rainless, like that of Jerusalem in summer, thunder is altogether unknown; and where an anticyclone with its descending currents rests over a region, as happens over the centre of the Europe-Asiatic continent in winter, thunder is equally unknown during that season. The diurnal periods of hail, whirlwinds, waterspouts, dust-storms, and tornadoes have their origin in substantially the same atmospheric conditions as the thunderstorm, and occur approximately at the same hours of the day.

See Kaemt's Meteorology (trans. 1845); Drew's Meteorology (2d ed. 1860); Herschel's Meteorology (1861); Buchan's Handy Book of Meteorology (1868); Loomis' Treatise on Meteorology (1868); R. H. Scott's Elementary Meteorology (1883); Mohn's Elements of Meteorology; Hann's Climatological Atlas, and Climatology; Buchan's Atmospheric Circulation ('Challenger' Expedition); Blandford's Meteorology of India, &c. The leading points of this wide subject will be found under such heads as