Physics of the Soil. 



may be the case in the spring, and if rains fall which re- 

 sult in heavy percolation, a large amount of heat is con- 

 veyed rapidly and deeply into the soil with the water and 

 the temperature of the ground, two to four feet below the 

 surface, may thus be very materially raised. 



268. Influence of Evaporation on Soil Temperature. 

 There is no factor, except the direct sunshine and the direct 

 radiation of heat away from the earth into space, which 

 exerts so strong an influence on the temperature of the soil 

 as the evaporation of moisture from its surface; and the 

 chief reason why an undrained clay soil is colder than one 

 well drained is the cooling effect associated with the larger 

 evaporation of soil moisture. 



To evaporate a pound of water from the surface of a 

 square foot of soil, by means of the heat contained in the 

 soil, makes it imperative that 966.6 heat units be expended 

 to do the work and this, if withdrawn from a cubic foot of 

 saturated clay soil, would lower its temperature some 

 10.3 F. 



The difference in temperature shov/n by the wet and dry 

 bulb thermometers measures, in one way, the cooling effect 

 of evaporation ; the wet bulb often reading as much as 15 

 or even 20 degrees lower than the dry one, under otherwise 

 identical conditions. 



Table showing the influence of rapid evaporation upon the 

 temperature of the soil. 



