34 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



SUNSHINE. 



In recording the sunshine it was deemed advisable to note both the 

 possible and the actual duration, since both are important to the 

 development of vegetation. These factors are graphically shown, 

 in figure 17 for the aspen-fir type during the seasons of growth in ' 



1915 and 1916. 



In 1915 the greatest actual sunshine occurred in July, while in 



1916 it was recorded in June. Owing partly to the advancement in 

 the season but chiefly to the topographic features adjacent to the 

 meteorological stations, there is a gradual decline in the potential 

 sunshine duration throughout the growing season. The potential 

 and actual sunshine durations, on the basis of three seasons' records, 



AS PEN -FIR TYPE 



\x\400 



</) 300 



200 



§ too 



X. 



1915 



1916 















































































JUNE JULY AUG. SEPT . JUNE JULY AUG. SEPT. 



ACTUAL I I . POTENTIAL — 



Fig. 17. — Actual and potential sunshine during growing season in the aspen-fir type, 1915. 



are found to be practically identical in the three associations studied ; 

 consequently, no attempt is made to correlate sunshine duration with 

 the plant activities. 



BAROMETRIC PRESSURE. 



So far no direct fundamental relations have been established 

 between barometric pressure and the development of the plant. 1 



The relation of high and low pressure to local rainstorms and high 

 winds was observed in the aspen-fir association throughout the grow- 

 ing season of 1916, and the results are shown in figure 18. 



Practically always when the pressure dropped appreciably below 

 normal a change followed in the weather. While the amount of 

 precipitation and the movement of the wind are not necessarily pro- 



1 Zon, Raphael, Meteorological observations in connection with botanical geography, 

 agriculture, and forestry. Monthly Weather Review, April, 42 ; 217-23, 1914. 



