RESULTS OF MEASUREMENTS. 



59 



(4) The most efficient windbreaks from the standpoint of protec- 

 tion will have the most marked effect on the superheating and cooling 

 of the air. 



(5) As might be expected, clouds, by preventing insolation, and 

 serving as a blanket at the time of radiation, reduce the effect of the 



90 



DIAGRAM 27. Heating efficiency of white pine belt during hours of sunshine, as shown by summation 

 of temperatures above 5C. or 41F. (growing temperature). Thermometers read hourly between 

 9 a. m. and 5 p. m., late autumn. A with clear sky; B with cloudy sky. 



windbreak (diagram 30). When neither insolation nor radiation is 

 perceptible, as after several days of cloudiness, the windbreak has 

 no effect upon air temperatures. On cloudy days the thermometer 



