FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 



115 



air movements at midsummer are influenced by the position of the 

 continental low-pressure area, the phenomenon of clouds is really a 

 local one, and those that form about Pikes Peak are the result of 

 daily convectional currents. Soon after noon precipitation is likely 

 to occur, and this so cools the ground that convection ceases; the 

 rest of the day is often but not regularly clear. This phenomenon is 

 of interest in that an easterly slope during this period may receive 

 almost as much insolation as a south slope. The autumn is char- 

 acterized by gradually decreasing cloudiness, but this is interrupted 

 by storm periods about the middle of each fall month. This apparent 

 regularity of the storm periods would probably disappear with a 

 longer record, although the mid-September storm is considered typi- 

 cal of the whole region and usually brings the first snow. 



Comparative sunshine in central and southern Colorado. — In Table 

 34 the mean sunshine percentages are shown for the Fremont and 

 Wagon Wheel Gap stations. The record for the latter is brought up 

 to October, 1920. 



Table 34. — Sunshine at Fremont and Wagon Wheel Gap stations. 







Percentages of the possible, by months. 









Ele- 







Grow- 





















^ 







• 



Station. 



va- 



• 



t? 















s 



,j 



,8 



H 



Ye*r 



ing 





tion. 



03 

 9 



C3 



a 





d 



>> 



d 



>> 



1 



s 



o 



1 



t> 



'a 



<0 





sea- 

 son. 











03 



ft 



S 













o 















[=i 



s 



< 



£ 



H-5 



H 



■"1 



CO 



O 



525 



A 









Feet. 































Fremont (F-l) 



8,836 



69 



71 



72 



60 



58 



62 



61 





57 



62 



64 



66 



63 



59 



Wagon Wheel Gap ( W-C) 



9, 237 



57 



59 



58 



52 



55 



56 



44 



48 



52 



59 



61 



57 



55 



50 



The sunshine percentage at Wagon Wheel Gap approaches that at 

 Fremont only in May, when the humidity is very low, and from 

 September to November, when clear weather is characteristic of 

 the Rocky Mountain region. The winter at Wagon Wheel Gap is 

 characterized by much less sunshine and more precipitation. The 

 depression is very great in July also, when the rainy season reaches its 

 height, which is about a month earlier than at Fremont. 



The general comparison indicates that the vicinity of Pikes Peak is 

 one of high sunshine percentages, a condition which is of especial 

 importance in the winter in facilitating evaporation. As we have 

 seen, this influence on south exposures may be just about counter- 

 balanced by the action of the sunlight in thawing the soil; but where 

 the sunlight does not reach the ground its influence must be solely to 

 increase transpiration. 



PRECIPITATION. 



Precipitation at the control station, by seasons. — The precipitation 

 record for the control station is complete for the entire period of oper- 

 ation of the station, beginning with January, 1910, and ending with 

 July 31, 1921. Although this record has been obtained as care- 

 fully as possible with the 8-inch standard gauge, supplemented by the 

 tipping-bucket gauge during the open season of each year, it is con- 

 sidered to be, like all other precipitation records, only an approximate 

 statement of conditions. Because of this general weakness, it has not 

 been thought necessary to check up minor discrepancies which may 



