152 RELATIONS OF SOIL TO HEAT 



cold nights is thus produced. With the cloudy sky, and 

 moist air, which characterize the climate of an island, the 

 extremes just mentioned are never found, but the climate is 

 remarkably uniform in temperature. The same conditions 

 of sky which give us a cool summer produce also a mild 

 winter. 



Influence of Latitude and Aspect. The angle at which 

 the sun's rays strike the earth has a very important influence 

 upon their heating effect at the earth's surface. Every flat 

 portion of the earth's surface receives the same number of 



FIGURE 4. 



hours of sunshine in the course of the year, and at the 

 spring and autumnal equinox the day and night are of equal 

 length over the whole globe ; yet how extremely different 

 is the temperature in the tropics and in the arctic regions! 

 The enormous influence of latitude on climate is simply due 

 to the varying angle at which the sun's rays reach the earth 

 The simplest illustration is however furnished by an ordinary 

 summer's day. How different is the intensity of the sun's 

 rays at sunrise or at sunset, when the sun is near the horizon, 

 to what it is at noon, when the [sun has risen to a great 

 height! This difference in the intensity of the sun's heat 



