Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/12

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Climate in its modern acceptation signifies that peculiar state of the atmosphere in regard to heat and moisture which prevails in any given place, together with its meteorological conditions generally in so far as they exert an influence on animal and vegetable life. The infinitely diversified character which climate displays may be referred to the combined operation of different causes, which are chiefly reducible to these four distance from the equator, height above the sea, distance from the sea, and prevailing winds, which may thus be regarded as forming the great bases of the law of climate. Of these causes which determine climate incomparably the most potent is distance from the equator. The same sunbeam which, falling vertically, acts on a surface equal to its own sectional area is, when falling obliquely on the earth, spread over a surface which becomes larger in in verse proportion to the sine of the obliquity. Conse quently less and less heat continues to be received from the sun by the same extent of surface in proceeding from the equator toward the poles ; and this diminution of heat with the increase of obliquity of incidence of the solar rays is enhanced by the circumstance that the sun s heat, being partially absorbed in its passage through the atmosphere, the absorption is greatest where the obliquity is greatest, because there the mass of air to be penetrated is greatest. Hence arise the broad features of the distribu tion of temperature over the globe, from the great heat of equatorial regions, falling by easy gradations with increase of latitude, to the extreme cold of the poles. If the earth s surface were uniform, and its atmosphere motionless, these gradations would run everywhere parallel with the latitudes, and Ptolemy s classification of the climates of the earth would accord with fact. But the distribution of land and water over the earth s surface and the prevailing winds bring about the subversion of what Humboldt has termed the solar climate of the earth, and present us with one of the most difficult, as certainly it is one of the most important problems of physical science, viz., the determina tion of the real climates of its separate regions and localities, and the causes on which they depend. The decrease of temperature with height is perceptibly felt in ascending mountains, and is still more evident in the snow-clad mountains, which may be seen even in the tropics. The snow-line marks the height below which all the snow that falls annually melts during summer. The height of this line above the sea is chiefly determined by the following causes by distance from the equator ; by the exposure to the sun s rays of the slope of the mountain, and hence, in northern latitudes, it is higher on the south than on the north slopes of mountains, other things being equal ; by situation with reference to the rain-bringing winds ; by the steepness of the slope ; and by the dryness or wetness of the district. Since, then, no general rule can be laid down for the height of the snow-line, it can only be ascer tained by observation. Speaking generally it sinks little from the equator to 20 N. and S. lat. ; from 20 to 70 it continues to fall equably, but from 70 it falls rapidly to 78, where it is at sea-level. The following are a few of the more noteworthy of the exceptions. On the north side of the Himalayas it is about 4000 feet higher than on the south side, owing to the greater depth of snow falling on the south side and the greater dryness of the climate of Tibet, resulting in a more active&quot; evaporation from the snows and stronger sun-heat on the north side, to which is to be added the comparative want of vegetation on the north side, thus favouring a more rapid melting of the snows. The snow-line is higher in the interior of continents than near their coasts, the rain fall there being less and the heat of summer greater ; and similarly, owing to the greater prevalence of westerly over easterly winds in many regions of the globe, it is higher on the east than on the west sides of continents. In South America the snow-line rises very considerably from tho equator to 18 S. lat. and more so, markedly, on the west than on the east slopes of the Cordilleras, because of the smaller amount of precipitation of the west side of this mountain range. It is as high in 33 as in 18 S. lat., but south of 33 it rapidly sinks owing to the heavy rains brought by the westerly winds which begin to prevail there. In the south of Chili it is 6000 feet lower than among the Rocky Mountains at the same distance from the equator, and 3000 feet lower than in the same latitudes in Western Europe. It is impossible to overestimate the importance of the snow-line as one of the factors of climate in its relations to the distribution of animal and vegetable life. Glaisher, in his balloon ascents, made observations of temperature at different heights, the results of which may be thus summarized. Within the first 1000 feet the average space passed through for 1 was 223 feet with a cloudy sky and 162 feet with a clear sky; at 10,000 feet the space passed through for 1 was 4545 feet for the former and 417 feet for the latter; and above 20,000 feet the space with both states of the sky was 1000 feet nearly for a decline of 1. It must be noted, however, that these rates of decrease refer to the temperature of the atmosphere at different heights above the ground, which are in all probability altogether different from the rates of decrease for places on the earth s surface at these heights above the level of the sea the problem with which climatologists have to deal. Observation shows, as might have been expected, that the rate at which the temperature falls with the height is a very variable quantity, varying with latitude, situation, the state of the air as regards moisture or dryness, and calm or windy weather, and particularly with the hour of the day and the season of the year. In reducing temperature observations for height, 1 for every 300 feet is generally adopted. In the present state of our knowledge this or any other estimation is at best no more than a rough approximation, since the law of decrease through its variations requires yet to be stated, being in truth one of the most intricate and difficult problems of climatology awaiting investigation at the hands of meteorologists. Among the most important climatic results to be determined in working out this problem are the heights at which in different seasons the following critical mean temperatures, which have important relations to animal and vegetable life, are met with in ascending from low-lying plains in different regions of the world, viz., 80, 75, 70, 65, 63, 60, 58, 55, 50, 45, 39 (the maximum density of fresh water), 32 (its freezing point), and 20. These results, which only affect the mean daily tem perature in different seasons, and which ore due exclusively 