about 4° to 10° F. lower than those of the shaded surface. The greatest differences 

 between the minimum temperatures of the unshaded soil surface and air temperatures or 

 temperatures of the shaded soil surface were recorded in July and August. Below freez- 

 ing temperatures on the unshaded soil surfaces can occur anytime during the growing 

 season (fig. 6). For example, a minimum temperature of 20° F. was recorded in early 

 August of 1968. It is not unusual in late July and early August to have a spread of 

 over 115° F. in a 12-hour period between maximum and minimum temperatures on an unshaded 

 soil surface. 



Although temperatures on the unshaded surface of the soil were much more extreme 

 than temperatures of the air, the durations of these extremes were shorter (table 8). 

 Temperatures stayed within 5° F. of the maximum for an average of 1.7 to 3.4 hours, less 

 than half as long as those of the air. Temperatures of the unshaded surface stayed 

 within 5° F. of their minimiim for a monthly average of 4.6 to 8.8 hours/day. This was 

 about three-fourths as long as air temperatures remained within 5° F. of their minimum 

 during June through August and about equal to durations of minimum air temperatures 

 during May, September, and October. 



Unshaded surfaces on southwest exposures reached higher maximum temperatures than 

 those on northeast exposures. This difference averaged 4.1° F. (a = 5.8) at 7,100 feet 

 and 3.0° F. (a = 6.4) at the 8,200-foot elevation. Maximum temperatures of shaded soil 

 surfaces did not differ appreciably between exposures. 



MAY 



JUNE 



JULY 



AUG 



SEPT 



OCT 



150 



140 



130 













/ 



/ 

 / 



v.. 









k 



\ 

 \ 



Unshaded | 

 ^Absolute Maximum 











i 



/ 

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/ 



/ 





\ 







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1 



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1 

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Unsha 

 ^Mean 



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 Maximum \ 









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120 



Li. 



110 



LU 

 CC 





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a. 



90 





LU 

 1- 





80 



70 



60 



50 



18 20 22 24 26 28 30 32 34 36 38 40 42 44 



WEEK 



Figure 5 — Maximum temperatures of shaded and unshaded soil on a southwest 

 exposure at 7^100 feet elevation: average weekly means and absolute 

 maximums for the 5-year period. 



11 



