170 



THE CLIMATIC FACTOK AS ILLUSTRATED IN ARID AMERICA. 



one alone, they give a truer impression than do the preceding sets. Finally, the curves of 

 the last set, E, representing 10 favorable and 13 unfavorable years, which combine special 

 conditions of both rainfall and growth in the highest degree, deserve careful consideration. 

 They present the same general appearance as the other two sets of curves which combine 

 our two factors, but in a higher or more intensified degree. During the four months 

 from July to October, inclusive, all three curves, favorable, mean, and unfavorable, practi- 

 cally coincide; then in November the favorable curve jumps to a great height, indicating 

 an abundant fall of snow at the beginning of winter, while the unfavorable curve reinains 



horizontal, indicating an open season with 



Jul.Aug.Sep.Oct.Nov.nec.Jan.Feb.Mar.Apr.Ma.vJun., ^^^j^ ^ f^^ j^^^j^^^ ^f ^^^^^ ThrOUghout 



December and January the favorable curve 

 remains far above the unfavorable, in Feb- 

 ruary the two almost coalesce, while in 

 March and April the favorable curve is 

 somewhat higher than the other, although 

 by no means so much so as during the first 

 part of the winter. Early snows aid the 

 growth of trees by keeping the ground almost 

 vmfrozen and thus allowing the melting snow 

 to sink in and thoroughly saturate the soil. 

 In addition to this the absence of frost in 

 the ground permits the trees to begin grow- 

 ing almost as soon as the snow disappears, 

 and thus the growing season is lengthened, a 

 matter which is of especial importance in a 

 region like the Sierras, where the drought is 

 extreme. The effect of late snows, or spring 

 rains, on the other hand, is more direct and 

 hence still more important. 



The climatic conditions indicated by 

 the curves of figure 49 can be interpreted in 

 terms of cyclonic storms. Part of the precipi- 

 tation of the Sierras is derived from cyclonic 

 storms. The growth of the trees appears 

 to be especially promoted in years when the 

 storms begin early and continue late. Al- 

 though the subject has not yet been well inves- 

 tigated, it appears that during such winters 

 the storms move farther south than usual. 

 Possibly an indirect indication of this is found 

 in the rapid decrease of precipitation during 

 February. In winters of the type character- 

 istic of northern regions the storms begin 

 early in the season and there is a rapid in- 

 crease in the amount of precipitation ; then, as 

 winter conditions come to prevail completely and the continent becomes thoroughly chilled, 

 a great continental area of high pressure and low temperature is developed. This prevents 

 storms in the area w^here it prevails and gives rise to calm, clear weather, bitter cold perhaps, 

 but sunny and free from wind. The storms meanwhile are pushed to the edges of the 

 area of high pressure — that is, toward the oceans and the south. Then, when spring 

 approaches and the high-pressui-e area is broken up, storms once more prevail, but not with 



1 s 



i 



Fig. 49. — Rainfall by Months in Favorable 

 and Unfavorable Years. 



Meaa of all Years = 



Unfavorable Years = 



Favorable Years = 



