Solar Radiation 



Over the 5 study years, maximum intensity of solar radiation at the 7,100-foot 

 station on the southwest exposure during the months of May, June, and July averaged 

 slightly over 1.40 cal . /cm. ^/min . (table 1). As expected, this daily maximum dropped 

 gradually from the summer solstice until the end of October when it averaged 0.85 cal./ 

 cm.^/min. (fig. 1) • At no time did the yearly variation of monthly averages exceed 

 0.20 cal ./cm.^/min. 



No consistent difference in maximum radiation intensity was recorded between the 

 7,100-foot and the 8,200-foot elevations, although we had anticipated greater intensity 

 at higher elevations as atmospheric absorption of solar energy diminished. Apparently, 

 our instruments were insensitive to the difference caused by the 1,100-foot elevational 

 spread . 



The total solar energy received per day was greatest during July and gradually 

 decreased until observations ceased at the end of October (fig. 1). The average daily 

 insolation, 255 cal. /cm. ^, received in October , was less than half of that received in 

 July (table 2). The greater daily variability of total insolation in May and June than 

 in succeeding months reflects the changeability of weather during the early part of the 

 growing season. The extreme range in total radiation is of particular interest. Over 

 the 5-year period, the absolute maximum for any one day was 684 langleys recorded the 

 last week of June 1966; the absolute minimum for late June (1968) was only 191 langleys 

 (fig. 2). The lowest total radiation (49 langleys) for a 24-hour period was recorded 

 in late September 1965. Such wide differences in solar radiation should have a pro- 

 found influence on the daily cycle of plant growth on these mountainous areas. 



Table 1 . --Average^ daily maximum solar radiation intensity at the 

 station on the southwest exposure at the 7 ^lOO-foot 

 elevation 



Year * May " June ' July " Aug. ] Sept. _ Oct. 



------- - -Cal./am.'^/min.- - -- -- -- - 



1964 



1.35 



1. 



.43 



1, 



,33 



1, 



,29 



1, 



,18 



0.92 



1965 



1.42 



1, 



,40 



1. 



,45 



1, 



,37 



1. 



,04 



.97 



1966 



1.40 



1. 



.51 



1, 



,41 



1, 



,37 



1, 



,18 



.90 



1967 



1.43 



1, 



.43 



1, 



,43 



1, 



,33 



1, 



,24 



.97 



1968 



1.46 



1, 



,34 



1, 



,41 



1, 



.26 



1. 



,21 



.97 



5-year mean: 



1.42 



1, 



.42 



1, 



,41 



1, 



.32 



1, 



,17 



.95 



Standard deviation of monthly means averaged 0.20 cal . /cm. ^/min . 



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