SOURCES OF SOIL HEAT 151 



to explain without assuming a difference in the amount of 

 heat received from the sun. It is now generally assumed 

 that the solar energy is greatest during the period of most 

 numerous sun spots, which returns about every eleven years. 



The heat received from the sun by any portion of the 

 earth's surface depends greatly on the transparency of the 

 atmosphere ; our cold and hot summers are chiefly determined 

 by the presence or absence of cloud. For the greatest amount 

 of heat to reach the earth the air must not only be clear, 

 but also dry. Dry air has but little power of absorbing the 

 heat rays from the sun, but water-vapour is an active ab- 

 sorbent. It is thus in elevated regions, having a clear dry 

 atmosphere, that the heating power of the sun is most 

 strongly felt. The highest temperature shown each month 

 during the winter of 1870-1 by a blackened thermometer 

 in vacuo at Greenwich, and at Davos in Switzerland, 5,400 ft. 

 above the sea, was as follows : 



Greenwich. Davos. 



November 95-2 ... 115.3 



December 78-8 ... 115.0 



January 79-9 ... 117.l 



February 101.8 ... 126.0 



The sun's rays were thus far hotter at Davos than at 

 Greenwich, although the ground at Davos was continuously 

 covered by snow. 



Although cloud, and to a less extent a moist atmosphere, 

 greatly hinder the sun's heat from reaching the earth, they 

 do in an equal degree prevent a loss of heat from the earth. 

 The condition of clear sky and dry air which admits a free 

 passage to the sun's radiation during the day, allows an 

 equally free radiation from the earth during the night; a 

 climate of great extremes of temperature, of hot days and 



