Table --Percent of virtually aloud-free days at the station on 

 the southwest exposure at the 7 , 100-foot elevation 



Year \ May \ June , July . Aug. . Sept. . Oct. 



■Veroent 



1964 



10 



3 



32 



23 



27 



48 



1965 



12 



13 



19 



13 



20 



29 



1966 



10 



3 



10 



16 



17 



23 



1967 



9 



7 



10 



32 



23 



20 



1968 



3 



7 



16 



16 



23 



25 



year mean : 



9 



7 



17 



20 



22 



29 



Air Temperature 



Averaged by weeks, daily maximum air temperatures increased gradually from some- 

 where between 41° and 56° F. in early May to somewhere between 71° and 81° F. in late 

 July and early August (fig. 3). A conspicuous dip in early June corresponds to a 

 decrease in total solar radiation for the same period. By late October, daily maximum 

 temperatures approximated those of early May. The drop in maximum air temperatures 

 lagged about 2 weeks behind the decrease in total daily solar radiation. 



The absolute maximum air temperature (86° F.) reached over the 5-year period occur- 

 red July 30, 1966. However, maximum temperatures close to this high were recorded 

 through August and into early September (fig. 4). In contrast, maximum temperatures 

 never exceeded 75° F. in May or 80° F. in June. The average maximum temperatures for 

 each month (table 4) were always about 20° F. lower than the absolute maximums for that 

 month over the 5-year period. The variability in maximum air temperatures between years 

 was greatest in late May, early June, late August, and early September (fig. 4). This 

 reflects the unstable weather that characteristically occurs during these periods in 

 southwestern Montana. 



The curve for average minimum air temperatures generally paralleled that for aver- 

 age maximum temperatures, but was from 20° to 30° F. lower (fig. 3). The greatest dif- 

 ferences between maximum and minimum temperatures were recorded in July and early 

 August. Average minimum temperatures did not rise above freezing until about mid-May 

 and by mid-October, average nighttime temperatures were again below freezing. At no 

 time did the weekly averages of minimum temperatures rise above 51° F. 



July was the only month during the 5-year period when the absolute minimum temper- 

 ature did not drop to freezing or below. Variation of minimum temperatures between 

 years was greatest in late August and September (fig. 4). 



Duration of a temperature extreme is probably just as important to the physio- 

 logical processes of plants as the extreme itself. An expression of this duration was 

 obtained by determining the amount of time the temperature remained within 5° F. of the 

 extreme. Monthly averages of the duration of daily maximum temperatures ranged between 

 5.2 and 7.6 hours (table 5). July was the only month in which durations of both maximum 

 and minimum temperatures were about equal. In the other months, minimum temperatures 

 always lasted longer than the maximum. 



7 



