46 FOREST DISTRIBUTION 



publications, which show that the mean maximum at Helena, 

 Yellowstone Park and Kalispell were between 60 and 87 pei cent 

 and that the lowest relative humidity of which there is a record 

 is 30 per cent at Helena in August. 



Table 2 gives the mean daily relative hiunidity, maximum 

 and minimum for a period of three years and table 3 the mean 

 daily variations during the same period. The record was made 

 by a standard hygrograph under standard conditions and cor- 

 rections made from time to time by aid of a sling psyehroineter. 

 The instrument shelter, on the Tniversity campus, was placed 

 in an area of natural prairie vegetation as dry as usually can 

 be found in the region. A mountain rising to the east shut off 

 the early morning sim, but the station was fully exposed through- 

 out the rest of the day. 



From the table of mean daily variations it will be seen that 

 these may range from oO'^^^ to 64^c. The curve traced by the 

 recording pen shows, especially during the summer months, a 

 rythmic daily rise and fall, the maximum abovit 6:00 a. m., the 

 minimum about 4:00 to 6:00 p. m. The pen in rising traces a 

 more or less zigzag and irregular line until the maximum is 

 reached. At this point there is usually a sharp peak, for the 

 maximum is of short duration, then a steady downward sweep 

 during the day to the minimum late in the afternoon. The max- 

 imum of each daj^'s i-ecord usually shows strong throughout 



Table 2. Jlean daily relative humidity at Missoula. Maximum aud 

 minimum. 



Jan. Feb. .Mch. Apr. Jlay June July .Vug. Sept. Oct. Nov. Dec. 



1915 54 51 54 61 67 49 53 49 28 



1916 22 2S 44 50 51 52 .59 62 64 49 34 18 



1917 25 29 3S 46 42 4S 59 54 52 55 21 20 



1918 .32 



