1921] Smiley: Flora of the Sierra Nevada of California 27 



region the force and direction of the wind are the controlling factors 

 in giving shape to the vegetation. The velocity of the winds increases 

 with altitude. On the summit of Mt. Rose the velocities of 40 to 50 

 miles per hour have been recorded. 33 In the spring perpendicular 

 winds (Chinook winds) may occur which rapidly melt the snow from 

 the higher altitudes. In addition to these general winds there are 

 "mountain and valley winds," day and night currents, induced by 

 diurnal and nocturnal temperature changes on the higher peaks and 

 ridges, which flow up and down the gorges, at times attaining con- 

 siderable force if the topography favors convergence of several minor 

 currents into a general movement. The valley or night wind flowing 

 down the slope undoubtedly plays a part in plant distribution; just 

 what, if any, importance in this connection is to be attached to the 

 feebler ascending day current is obscure. 



An account of the climate of a region with so diversified a topo- 

 graphy as that of the Sierra Nevada must take cognizance of the fact 

 that in only a very general sense is there a climate of the region as a 

 whole ; rather, there exists a number of local climates determined by 

 position. In the case of the Sierra, with its contrasted flanks, the 

 complexity becomes all the greater. It is apparent to even the casual 

 visitor that the vegetation is unlike in different parts of the range; 

 on either flank there is a change with altitude and a significant differ- 

 ence exists between the flanks in the aspect of the vegetation. The 

 general characters of the high mountain climate have now been given 

 but it has also seemed possible to arrive at some more definite under- 

 standing with regard to the unlikenesses in the plant life of the range 

 by making a study of the climates of certain stations known to possess 

 distinct assemblages of plants. 



The data are supplied by publications of the Weather Bureau in 

 which information is given concerning the mean monthly tempera- 

 tures, the monthly extreme temperatures, amount of precipitation, 

 amount of snow upon the ground at the end of each month, number 

 of rainy, clear, part clear, and cloudy days, and the prevailing direc- 

 tion of the wind. In order that the effect of the climatic elements, 

 as modified by position and topography, should be comparable and 

 serve as the basis for deduction about the vegetation, it is obvious that 

 the data should meet certain conditions: the stations should be as 

 close together as possible and still possess those contrasts in position 

 and topography which may be presumed to influence the local climate ; 

 the data should cover the same years. 



