SOIL HEAT 227 



transformation and, therefore, having done no work. This 

 reflection is much greater on sea than on land and greater 

 from snow than from soil surfaces. Reflection is influenced 

 to a marked degree by vegetation, stubble, for example, being 

 more effective than a green field, a forest or even bare soil. 

 Possibly one-fifth of the earth's insolation on the average is 

 absorbed by the land and water surfaces, being the source of 

 the energy which later functions both statically and dynam- 

 ically in the soil. 



The statement is often made that warm rain carries con- 

 siderable heat into the soil. Such an assertion is not only 

 misleading but in most cases entirely incorrect. Precipitation 

 in general is usually cooler than the soil in temperate regions, 

 especially in the summer. Rain is spoken of as warm, not 

 in comparison with soil but with average rain-water tempera- 

 ture. Even if rain water should be 10° F. warmer than the 

 soil, a very improbable assumption, an average rain would 

 raise the temperature of the surface six inches only slightly. 



117. Absorption of insolation. — The energy received 

 from the sun functions in a number of ways on reaching the 

 land surfaces of the earth. It may accelerate chemical re- 

 actions, it may be absorbed by plants, it may induce certain 

 changes in form and, lastly, it may be converted into heat. 

 It is in this latter state that insolation energy plays its most 

 important part in soil activities, since heat energy may act 

 in ways that radiant energy finds impossible. Since heat is 

 commonly conceived as the kinetic energy of the molecules 

 of a body, it is quite distinct and different from solar radia- 

 tion, which must encounter some favorable substance before 

 heat is produced. Temperature is the condition of a body 

 in respect to its heat energy and is the common mode of ex- 

 pressing heat intensity. 1 



1 Molecules are in constant motion, colliding with their neighbors, re- 

 bounding, and quivering. They possess energy which is called heat. 

 Temperature is determined by the velocity of the molecules and is a 

 manifestation of heat. 



