EESULTS OF MEASUKEMENTS. 



63 



Throughout the period, June 17 till August 5, the relation of tem- 

 peratures at various points in the zone of windbreak influence 

 remained constant. In the zone of perpetual shade there was an 

 area in which soil temperature was low. This extended to J-ht., 

 north, where shadow fell only during part of the day. Immediately 

 beyond this the temperature of the soil was higher than in the open. 

 The highest soil temperature was recorded at 2-ht. north, which 

 corresponds to the point of maximum windbreak efficiency and to 



27.2 



26.2 



25.2 



, 242 



37.3 

 36.3' 

 35.3 

 34.3 

 33.3 



DIAGRAM 31. Influence of white pine belt on nocturnal cooling of air, with wind velocities of 1 to 4 



miles per hour. 



the point where the air temperatures are highest in the daytime and 

 lowest at night. 



This, it must be remembered, was the relation existing during the 

 period when the temperature was constantly increasing. 



On the other hand, when the ground is radiating, in the autumn, 

 the relation of temperatures in the sheltered and unsheltered areas 

 is reversed. Moving air, having retained its warmth longer than the 

 surface of the earth, is then constantly bringing new supplies of heat. 

 Stagnant air, however, imparts no heat to the surface over which 

 it is standing, but, on the contrary, is soon cooled to the tempera- 



