62 



WINDBREAKS. 



Obviously, the heating of the air in the protected zone will increase 

 its capacity for moisture, and this will in part counteract the reduc- 

 tion of evaporation due to the checking of wind. But the effect of 

 superheating will never equal that of the reduction of wind velocity. 



MEASUREMENTS OF SOIL TEMPERATURES. 



A study of the effect of windbreaks upon soil temperatures leads 

 to exactly the conclusions that have been drawn from the study of 



J /0 



DIAGRAM 30. Influence of cottonwood row (reinforced with ash) 011 nocturnal cooling of air in 

 cornfield and unplanted fields. Temperature lines in cornfield marked "C." 



their effects upon air temperatures. In addition, however, there is 

 the effect of shade on the ground in the immediate neighborhood of 

 the trees; whereas the effect of shade upon rapidly moving air cur- 

 rents is barely appreciable. 



Studies have been made of the temperature of the soil 'at a depth 

 of 20 inches (5 decimeters), which is probably about the mean 

 maximum depth of the roots of annual crops. Nearer to the surface 

 the conditions are probably very similar to those of the atmosphere. 



