CLIMATE (from the Greek klima, a slope or inclination, afterwards applied to a tract of country, with reference to its supposed inclination to the pole, and the effect of the obliquity of the sun's rays upon the temperature), a term now employed as including not merely the conditions of a place or country with regard to temperature, but also its meteorological conditions generally, in so far as they exercise an influence on the animal and vegetable kingdoms. The effect of the sun's rays is greatest where they fall per pendicularly on the surface of the earth, and diminishes as their obliquity increases; the surface which receives any given amount of the sun's rays increasing with their increased obliquity; whilst at the same time the oblique rays being subjected to the influence of a greater number of particles of the atmosphere, a greater amount of their heat is absorbed before they reach the surface of the earth at all. The greater or smaller extent of surface receiving a certain amount of heat, also makes important differences to arise from exposure by slope towards the equator or towards the nearest pole. 12.7era lion is a most important cause of differences of climate. As we ascend from the level of the sea to the greatest mountain altitudes, even at the equator, the temperature gradu ally diminishes, owing to the diminished density of the atmosphere, and we 'reach a region of perpetual snow, as in approaching the poles. The progressive diminution of the temperature is, however, affected by many other causes, so that the line of perpetual snow is far from being at the same elevation in all places of the same latitude. Thus, the snow-line on the southern side of the Himalayas is depressed by the moisture of the aria] currents from the Indian ocean; and that on the northern side is elevated by the radiation of heat in the vast dry table-lauds of Central Asia, and the consequent ascend ing streams of warm dry air; so that the difference between the two is not less than 4,000 ft. in favor of the northern side of the mountain-ranges; and Humboldt says, "millions of men of Thibetian origin occupy populous towns in a country where fields and towns would, during the whole year, have been buried in snow, if these table-lands had been less continuous and less extensive." As the actual temperature df the atmosphere depends not so much upon the direct rays of the sun as upon the radiation from the heated surface of the earth, the diversities in the character of that surface arc productive of great effects in modifying climate. A sandy desert, a tract of country clothed with luxuriant vegetation, and an expanse of water, absorb and radiate heat in very different degrees. A newly plowed field both absorbs and radiates heat much more rapidly than a field covered with grass. A sandy desert heats the atmosphere above it much more than either a fertile tract or a watery expanse, and a watery expanse still less than a fertile tract; but, on the other hand, the desert cools sooner by radiation; whilst the heat absorbed by the water being diffused through a larger mass—partly by reason of the motion continually taking place in the fluid substance—and affecting greater depths, the influence of the ocean, of seas, and of great lakes, is very powerful in maintaining a greater equality in the temperature of the atmosphere. Thus maritime places, and
particularly islands and peninsulas, have a more equal temperature, with less diversity of the extremes of summer and winter, than more inland or continental places otherwise similarly situated. The effect of the sea is modified by many circumstances, and par ticularly by currents, of which the Gulf stream (q.v.) affords a notable instance, the heated water conveyed by it from the equatorial to the polar regions having a great influence on the C., particularly of the n.w. of Europe. The temperature of Europe is also in part dependent on the warm s. winds, which have absorbed heat from the great sandy deserts of Africa; and over the world generally, atmospheric currents must be regarded as exercising even a greater influence on C. than oceanic currents. The quan tity of rain or snow that falls in the course of year, and the times and manner of its falling, are circumstances which have a great effect on climate. These arc circum stances much influenced by the distribution of land and water, and by the elevation and character of the surface of the land, which, doubtless, also influence electric and other meteorological conditions, less understood, but certainly not unimportant.
The relations of C. to vegetation are determined not merely by the mean annual tem perature, but in a great measure also—and, with regard to many plants, entirely—by the duration and C. of summer. Thus, maize, which may be mentioned as an impor tant example, succeeds well in climates of which the winter-cold is severe, the summer season alone being sufficient for its whole life; whilst, on the other hand, such plants as fuchsias, some kinds of laurel, and even the common hawthorn, which succeed well enough where maize would scarcely put forth an ear. would perish from the colder win ters of countries where it is profitably cultivated. The polar limit of particular species of animals, except those which hibernate, is generally determined by the degree of winter-cold which they can bear without injury.
Bogs and marshes exercise an unfavorable influence on C., cooling the air and caus ing fogs, as clay-soils also to some extent do, through their retentiveness of moisture; whilst marshes of some kinds, and in some situations, abound in exhalations very unfa vorable to health. Similar remarks apply to large tracts of forest. The clearing, drain age, and cultivation of land have generally favora.bl•effects on C.; .although plantations are often beneficial for shelter; and a too complete removal of natural forests may pre vent the deposition of moisture from the atmosphere to such a degree as to cause droughts, a result strikingly exemplified in some of the smaller West India islands, and the tendency to which is said to be manifested on the great scale in the eastern part of the continent of North America.
The important and difficult subject of C. will be found further elucidated in some of the principal geographical articles, and in the articles AGRICULTURE, ARBORICULTURE, ATMOSPHERE, METEOROLOGY, MONSOONS, RAIN, SEASONS, STORMS, TRADE-WINDS, WIND.