Total radiation received at 8,200 feet did not differ appreciably from that 

 received at 7,100 feet. As mentioned previously, this may reflect the inability of the 

 instruments to measure the comparatively small difference involved. 



The charts produced by the actinographs clearly showed the passage of cloud shadows 

 over the sensing device. Consequently, a measure of relative cloudiness on individual 

 days was possible. Table 3 gives the 5-year percentage of days per month that were 

 virtually cloud-free (no more than two brief shadow occurrences). Cloud-free days 

 averaged 9 and 7 percent for May and June, respectively. The number of cloud-free days 

 began to increase in July and by September and October, about one-fourth of the days 

 were virtually cloud-free. As might be expected, the variation between years was con- 

 siderable; some years had over twice as many cloud-free days during the same month as 

 others . 



MAY 



JUNE 



JULY 



AUG 



SEPT 



OCT 



Figure 2. — Solar radiation at the station an the southwest exposure at 7,100 feet 

 elevation: absolute maximum and absolute minimum daily radiation and average 

 maximum and minimum daily radiation received over 5-year period. 



6 



