1921] • Smiley: Flora of the Sierra Nevada of California 25 



a week, the mixed forest retained about 3 per cent of its maximum, 

 and the denser fir forest 25 per cent. Still more indicative of the com- 

 parative aridit}^ of open slopes, this time for the alpine zone, are the 

 comparative measurements made on a talus slope and on a forested 

 slope on Mt. Rose : 



Talus slope (unforested) : 



Slightly protected slope below observatory 52.5 inches 



Wind-swept slope 8.1 



Protected slope 78.1 



Average of talus slope 40.8 



Forested slope 88.6 



( < 

 < ( 



The influence of the snow-cover on the seasonal temperature is well 

 brought out in the curves for stations on the opposite flanks of the 

 range. The eastern side, deficient in rainfall, has winter temperatures 

 below the western slope; with the return of spring there is required 

 less heat to melt the accumulated snow on the eastern side and the 

 monthly means for spring are higher than on the Pacific side. That 

 this milder spring temperature on the leeward side is due to the 

 lessened amount of heat required for melting is borne out by the fact 

 that, as soon as the snow-cover is melted from the western flank, its 

 mean temperature immediately rises above that of the desert side. 

 It will also be noted that in winter, when no melting occurs, the west- 

 ern side is the warmer (tables 2 and 3). 



Data concerning relative humidity in the higher Sierra Nevada 

 are extremely meager and somewhat contradictory. McAdie*^ observed 

 that on Mt. Whitney, ' ' During the mid-day hours the humidity would 

 rise as a rule to above 80 per cent, while between 2 p.m. and 5 p.m. 

 extremely low humidities were recorded, ranging from 3 to 11 per 

 cent. ' ' This diurnal change in the content of atmospheric vapor with 

 a maximum about noon followed by cloud formation and rain in the 

 afternoon is normal for all high mountain climates.*® McAdie also 

 noted that there were ' ' marked changes in short intervals in the amount 

 of water vapor present," a characteristic of the alpine climate.* In 

 August, 1913, the condition of atmospheric humidity in the high 

 mountains west of Lake Tahoe was comparable to that observed by 

 McAdie. At Glen Alpine Springs, Eldorado County, the humidity 



*L'humidite relative est sujette en niontagne aux variations les plus brusques 

 et, en apparence, les plus capricieuses. Ces variations augnientent avee 1 'altitude. 

 A des periodes de secheresse, ou, la perce gerce et les ongles eassent eomme dans 

 un desert, succedent, avee une bouffee de vent ascendent, des brouillardes pene- 

 trants.' '47 



