26 University of California PvMications in Botany [Vol. 9 



would increase hourly to a maximum about 2 p.m., a heavy shower of 

 rain or hail would sweep across the country and, thereafter, the water 

 vapor in the atmosphere rapidly diminish. It is somewhat surprising 

 to find Langley's observations, made also in the high mountain district 

 of Tulare County, at considerable variance with the later report : 

 during his twelve-day stay at Mountain Camp, near Mt. Whitney, 

 he noted mean humidities as follows : 



8 :15 A.M. 27.6 per cent 



12:36 P.M. 20.6 per cent 



8 :15 P.M. 40.9 per cent 



In this period the absolute maximum was 67.5 per cent at 8 :15 p.m. 

 and the minimum at 8 :15 a.m. was 4.4 per cent. The evening maxi- 

 mum decreased as a rule during the night, though some of the morn- 

 ings showed high humidities ; he found no evidence of a regular mid- 

 day maximum. "That no such law was observed on Mt. "Whitney is 

 again to be attributed to the extraordinary dryness of the climate. ' '^® 

 In the Sierran boreal region the sky is distinctly more cloudy than 

 at lower levels : 



Emigrant Gap, 5,230 feet, clear days 241.8; part cloudy 24.0 



Cisco 5,939 feet, clear days 273.2; part cloudy 5.0 



Summit 7,017 feet, clear days 226.2; part cloudy 11.1 



Tamarack 8,000 feet, clear days 189.2; part cloudy 78.8 



cloudy 96.5 

 cloudy 86.5 

 cloudy 122.7 

 cloudy 99.5 



This increase in cloudiness in the higher mountains is a sequel of the 

 rapid change in atmospheric humidity noted above and is character- 

 istic of alpine climates. 



Wind in the higher mountains is a major element in the climate. 

 It affects the plant population both indirectly as modifying the soil 

 and directly by its importance as an agent in the distribution of 

 propagative bodies and through its formative influence upon the grow- 

 ing plant. In the Sierra the prevailing winds blow from the west or 

 southwest, the storm winds, particularly, blowing from that direction. 

 There is some difference between the two slopes of the range with 

 regard to the constancy of wind direction; on the western slope the 

 air movement will be constant for days or even weeks at a, time (dis- 

 regarding those minor air movements determined by topography noted 

 below) ; on the eastern slope there is less uniformity of direction, 

 though the prevailing wind is still the west wind, yet occasionally 

 winds of considerable velocity blow from the desert. In the boreal 



