CLIMATOLOGY. This branch of physical geog raphy is concerned with the general distribution of the phenomena of the atmosphere. The height of the atmosphere is unknown, but from the rate at which its density diminishes with alti tude above the earth's surface, it is clear that in a few miles it becomes extremely rare, so rare that its effects may be neglected. The pressure of the atmosphere at sea-level has an average value over the earth of 14 pounds per square inch, equivalent to about 30 inches of the mer curial column. In equatorial regions the pressure is slightly below and in temperate regions slightly above this average. See AIR; ATMOSPHERE; BAROMETER.
Heat is produced by the absorption of the sun's rays by the earth. The more nearly overhead the sun is, the more heat is received per unit of area, and the higher is the temperature, other things be ing equal. The degree of temperature at the earth's surface is, however, affected by other conditions, chief among which is the relative moisture of the air, since a moist air absorbs much of the heat before the rays reach the earth. Hence very high temperatures are not observed in moist equatorial regions, while in desert regions extraordinarily high temperatures have been observed. On the whole, however, the equatorial regions receive the greatest amount of heat, and the polar regions the least. Hence the air over the equator rises, being forced upward by the pressure of air on the north and south. This produces a flow of air toward the equator from both sides, a flow which would be directly south and north were it not for the rotation of the earth, which deflects the cur rents to the westward, and thus produces the well. known uniform trade winds. (See WIND.) The land absorbs heat rapidly, and is as rapidly cooled ; the sea, on the contrary, absorbs heat slow ly, and gives it out slowly. Moreover, by means of its waves, tides, and currents the waters of the sea circulate freely, and thus tend to establish a uniformity of temperature in its various parts. hence it is that the sea is on an average through out the year warmer in the north and south and cooler in the tropics than is the land in the same latitude. Moreover, the sea is cooler in sum mer and warmer in winter than is the land. The difference in the attitude of land and sea toward temperature produces monsoons and land and sea breezes. The latter are diurnal and strictly local. The land being heated during the day, the air over it rises, and thus induces an inward draught of air from the sea. At night, the air over the land being cooled, a reverse current is set up. The monsoon (q.v.) is a similar land and sea wind, but on a much greater scale, and is induced by differences of temperature between land and sea in summer and winter. There is a monsoon
tendency on the margins of all continents, but in most cases it has little influence upon the more general movements of the atmosphere. The cool ing of the land surface, and consequently of the surface atmosphere, after nightfall induces a local circulation of air in the interior of con tinents. This air, being cooled and consequently heavier, flows down slopes, and collects in the valleys. Hence in mountainous regions there is a wind nt night down the canyons, and the air in the depressions is cooler than on the slopes above. Frosts occur in the valleys, while the slopes above may be exempt from them.
A fall of rain or snow requires the coexistence of two conditions, an atmosphere partly or wholly saturated with moisture, and the chilling of this atmosphere below the saturation point, which may be brought about by forcing the air-currents up to an elevation, to a higher latitude, or by mixture with colder air. The trade winds of the Atlantic bring to the Amazon basin and the eastern slope of the Andes an atmosphere loaded with moisture, which, as the land is, during most of the year, cooler than the air, is deposited freely, giving this region a profuse rainfall, while the summit and western slope of the Andes within the tropics are mainly desert. The southwest monsoons of India and Southern China bring vast stores of moisture from the Indian Ocean, which are deposited freely upon the colder land. The west coast of the United States and Canada, under the influence of the prevailing westerlies from the Pacific, receives in winter, when the land is cold, a profuse rainfall, while in summer, when the land is warmer, these moist air-currents carry much of this moisture over into the Rocky Moun tain region. Hence in Colorado, Arizona, and New Mexico the summer is the rainy season. The same westerly winds supply moisture from the Atlantic to Western Europe, and here, since there are no great mountain ranges to intercept it all at once, the rainfall is more generally distributed than in North America, being greatest on the coast, and diminishing gradually eastward, so that it is only in the far interior of Asia that desert conditions prevail. The southern part of South America lies within the region of the pre vailing westerly winds, and here the western slopes of the Andes have an ample rainfall, while over the pampas of Argentina these winds, drained of most of their moisture in the passage over the Andes, blow as dry winds. See METE OROLOGY.