306 SOILS: PROPERTIES AND MANAGEMENT 



influence on radiation. As a mulch is usually dry, its 

 radiant power is lower than that of the moist soil beneath. 

 Shelters decrease radiation by checking air movement. 

 The vegetation growing on soil also lowers radiation 

 through its covering effect, although the temperature of 

 soils covered with vegetation is usually low in summer 

 due to the obstruction of the sun's rays. Clouds, by 

 shutting in the heat, tend to check radiation and in many 

 cases prevent a frost that would otherwise occur. The 

 protecting effect of snow is well illustrated from the fol- 

 lowing data, taken from Boussingoult : 



EFFECT OF SNOW ON SOIL TEMPERATURE. 1 (TEMPERATURE IN 

 DEGREES CENTIGRADE) 



One of the important features of soil heat radiation is 

 its effect on air temperature. As the radiant energy from 

 the sun passes through the atmosphere, very little of the 

 heat is appropriated, due to the wave lengths. But, 

 as this energy is radiated from the soil, the heat waves 

 have become shortened and are readily taken up by the 

 atmosphere, particularly if the latter is moist. How- 

 ever, as the air is always in motion its heat is not con- 

 trolled by the soil radiation of any particular locality. 



1 Warington, R. Lectures on Some of the Physical Proper- 

 ties of Soils, p. 159. Oxford. 1900. 



