CLIMATE. 



653 



position of any portion, the elevation and locality of the mountains, have 

 also a share with the soil, winds, rains, and sea-board, in determining the 

 climate of any region. Many points have already been touched upon in 

 former chapters. Temperature, moisture, and vegetation are the chief 

 natural features which determine climate, and we must find out the position 

 of the land with reference to the sun first, to ascertain the climate. 



The more vertical the sun is the hotter the atmosphere, for the rays 

 strike directly upon the earth, which radiates the warmth received. These 

 heat rays are, as we know, invisible. The hottest portion of the earth, 

 must be at the equator for the sun is overhead, and the rays beat down 

 directly upon the earth. The sun is also nearer than when at the horizon, 

 and less rays are absorbed by the atmosphere. The longer the day the 

 greater the heat. 



Fig. 733. In the southern steppes. 



Temperature is registered by observation of the thermometer, and the 

 distribution of heat is represented upon a chart across which lines are drawn 

 at places of equal temperature. These lines arc called " isothermal." There 

 are also terms to denote equal winter temperature and the average summer 

 heat isocJiimines and isotlieres respectively. 



Temperature decreases as we ascend from, and increases as we descend 

 into, the earth. This fact proves that the air is not warmed by the sun's 

 heat, but by radiation from the ground. As we ascend we reach the line of 

 perpetual snow, which varies in different parts of the globe. In the tropics 

 it extends from 15,000 to 18,000 feet ; but it varies even in places of the 

 same latitude, according as the towns are inland or on the coast, as in the 

 Pyrenees and Caucasus, where there is a difference of three thousand feet in 

 the snow limit. 



