CLIMATE (Gr. KAlpa, 6a slope or inclina tion"). The term was used to denote the ef fect of the oblique rays of the sun on the tem perature of the earth and its atmosphere. To day it.is applied to the sum of the atmospheric condiuons as recorded for a long period of tune; or, in other words, it is the totality of weather, while °weather)) is the physical condi tion of. the atmosphere at a given time or dur mg a limited period.
One may well spealc of the weather to-day, or of last month, or of some past year; but not of.the climate of a day, a month or a year. The climate of a place is what may be expected to occur as the result of a study of its contin uous weather records for a long period of years—the atmospheric pressure, the temper ature, the rainfall and snowfall, the time and frequency of frost, the extremes of heat and cold, the direction and velocity of the wind, the amount of air that flows from the different points of the compass, the amount and intensity of sunshine, the humidity and transparency of the atmosphere and its electrification.
The study of the causes of the weather and of die laws of storms constitutes that branch of science known as meteorology;. climatology is to be considered as a subdivision of meteor ology: Climates may be broadly divided into ma rine, continental, mountain and plain with the many variations produced as these conditions gradually or precipitately shade off the one into the other.
Basis of the axis of the earth's rotation were perpendicular to the eclip tic (the plane of its orbit) there would no sea sonal changes, for the rays of the sun would fall upon every point on a parallel of latitude with the same angle of incidence on each day of the year. There would be but one season at any place and it would never end; and. there would be little variation in the intensity .of storrns. But as the axis of the earth is in clined at an angle of 23.5° to the plane of. its orbit and as the direction toward which it points remains nearly constant, there are but two days in each year when both hemispheres (north and south) are exactly one-half in sunshine and one half in darlcness, that is, at the vernal and au tumnal equinoxes, when the sun cross.es the equator. At all other times in each hern.isphere
the angle at which the sun's rays strike the earth, the depth of the air through which they pass, the length of the day and the proportions of each hemisphere immersed in sunlight, are increasing or decreasing. As the two. latter conditions increase in the northern hemisphere after the vernal equinox the summer grows upon us, reaching its greatest degree of heat about four weeks after the summer solstice. The lag of temperature is due to the fact that the atmosphere, being heated much more by radiation from the earth than by the direct ac tion of the solar rays, does not attain its great est heat until after the land and water have reached their maximum temperature and in turn have communicated this heat to the air above. Up to 21 June, or the summer solstice, the northern hemisphere receives each day more heat than it loses, otherwise it could not gain in temperature; after the solstice the sun each day at meridian is found to have receded a little to the south. At places north of the tropic of Cancer its rays fall with increasing obliquity and pass through a greater depth of i air and impinge for a less time each day, so that within a few weeks the earth begins to ra diate more heat each day than it receives. The maximum heat of summer occurs, on the aver age, when the loss of heat from the earth is just equal to that gained during the day from the sun. This, as previously stated, occurs sev eral weeks after the sun is well on its way southward. About 21 September the autumnal equinox occurs, when the sun crosses the equa tor, and, as at 21 March, the days are of equal length at all latitudes of both hemispheres. On or about 21 December— the winter solstice— the sun is farthest south and the same con ditions prevail in the southern hemisphere that prevailed in the northern hemisphere on 21 June. North of the equator the sun is now least ef fective; its rays reach the earth at the lowest angle through the greatest depth of air and they are operative for the fewest hours during each day, of any portion of the year, but the greatest cold does not occur. This comes about four weeks later, when the increasing heat re ceived each day by the earth from the sun is just equal to that lost by radiation.