WATER CONTENT AND TEMPERATURE 461 



the above figures between the temperature of rolled 

 and unrolled land. Rolling generally favors deep 

 warming. The movement of heat in the soil is illustrated 

 by the curves of soil temperature on page 455. The 

 change in temperature in the subsoil lags considerably 

 behind that at the surface, and is also more uniform. 



329. Circulation of air. This is due, first, to direct 

 conduction between the air and the soil; and, second, 

 to the influence of wind on evaporation. Tillage of the 

 soil, particularly in the spring, increases the rate of 

 warming, because at that season the air is usually warmer 

 than the soil, and, by bringing all parts of the soil to 

 the surface successively, it is warmed by contact with 

 the air and by the direct receipt of the sun's heat. Wind 

 hastens the change in temperature of the soil in either 

 direction by increasing the volume of air with which 

 the soil comes in contact. 



330. Water-content. The water-content of the soil 

 is the largest factor, after the heat supply, in determining 

 the temperature of the soil. This is due to two things: 



(1) The high specific heat of water as compared to soil. 



(2) The heat absorbed in the evaporation of water. 

 The specific heat of water, as compared with an 



equal volume of soil, is shown by the table on page 456 

 to be nearly twice as great. Consequently, the more 

 water a soil contains, the more slowly will its tempera- 

 ture change with a given heat supply. The tempering 

 influence of large bodies of water upon adjacent land 

 areas is an example of this fact. 



In the evaporation of water, a large amount of heat is 

 absorbed. The vaporization of one pound of water at 



