CLIMATE 47 



Table 4. Data from the Weather Bureau on the mean relative 

 humidity by months at three places in Montana and one in the Yellow- 

 stone National Park. 



An- 

 Stations Hours Jan. Feb. Mar. Apr. May .Tun. Jly. Aug. Sep.Oct.Nov.Dec.nual 



Helena is A. M. 72 73 71 65 66 67 60 59 64 66 68 71 67 



8 P. M. 66 63 56 42 42 41 31 30 89 48 56 65 43 



Yellow- 8 A. il. 77 74 77 74 77 74 77 73 71 74 70 76 74 



stone Pk. 8 P. M. 71 64 65 48 51 46 36 38 40 55 58 59 53 



Havre 80 81 77 62 63 63 57 56 62 68 75 79 63 



Kalispell 6 A. JI. 86 86 81 78 82 82 78 77 82 86 86 87 83 



6 P. M. 80 73 58 42 49 47 37 36 50 62 76 83 58 



the year, even in relatively dry seasons, since the nights are 

 always cool. In the winter the rythmic quality of the curve is 

 much less marked, the maximum is comparatively low and the 

 peak flat, while the sharper angle in the day's tracing now falls 

 at the minimum. 



The situation is one of exceptional severity for vegetation. 

 Two months or more of drouth during the summer, mostly cloud- 

 less days, day temperatures of 80 to 90 degrees, low relative hu- 

 midity and a highly porous soil, all tend to xerophytie conditions 

 in the vegetation. This station may be considered the most severe, 

 all points considered, in western Montana. During the season 

 of 1917 10 weeks in June, July and August were without a trace 

 of rain, and but .62 of an inch fell in three months. At the 

 close of this time, in the vicinity of the instruments, soil at the 

 depth of a foot was within 1.1% of actual air dry condition and 

 when heated for three weeks at a temperature of 120 degrees C. 

 gave off less than 2% of its original weight. The lower alti- 

 tudes in this valley are treeless except where sheltered from sun 

 and wind, a fact which is evidently related to the dryness of the 

 situation. 



There is little but inferential evidence as to the rate of 

 evaporation at any point in Montana. One record in the Judith 

 Basin in 1909 showed evaporation amounting to 32.6 inches, 

 presumably for the year. At Missoula during the season of 1917 

 the evaporation was 14 inches in 10 weeks (July, August and 

 September) from an exposed water surface of 4 square feet 

 protected by wire netting of one-third inch mesh. Sometimes 

 the evaporation was as much as 2 inches per week. 



The Signal Service and the Geological Survey have eon- 



