MCGEK] CLIMATE OP SERILAND 25 



local storms, frequently accompanied by thunder-gusts or sudden tem- 

 pests, though cold drizzles sometimes occur, especially at the height 

 of the winter liumid season. Except where the local configuration is 

 such as to affect the atmospheric movements, the distribution of pre- 

 cipitation is erratic, in both time and space; some spots may receive 

 half a dozen rains within a year, while other spots may remain rainless 

 for several years; and the wet spot of one series of years may b(^ the 

 dry spot of the next. 



The climatal features of Seriland are somewhat aflfected by tlie 2)ro- 

 uounced top()grai)hic features of the district. Snow sometimes falls on 

 Sierra Seri, and probably on Sierra Knnkaak ; gales gather about the 

 rugged ranges at all seasons, and sometimes produce precipitation out 

 of season; the extreme heat of midday and midsummer is tempered 

 by the proximity of the tide-swept gulf; and since most of the local 

 derangements tend to augment precipitation and reduce temperature, 

 it would seem safe to estimate the mean annual rainfall of the tract at 

 4 or .5 inches, and the mean temperature at about TO'^, with a mean 

 annual range of some 30° and an extreme diurnal range of fully <S0o. 



The configuration and climate combine to give distinctive character 

 to the liydrogi'aphy of the Soiioran province. The melting snows and 

 more abundant rains of the high sierras form innumerable streams 

 flowing down the steeper slopes toward the piedmont plains, or soak 

 into the pervious rocks to reappear as springs at lower levels; some- 

 times the sti'eams unite to form considerable rivers, flowing S(;ores ot 

 miles beyond the mountain confines; but eventually all the running- 

 waters are absorbed by the dry sands of the plains or evaporated into 

 the drier air; and from the mouth of the Colorado to that of the Yaqui, 

 500 miles away, no fresh water ever flows into the sea. During the 

 winter wet season, and to a less extent during that of summer, the 

 mountain waterways are ()ccu|)ied by rushing torrents, rivaling great 

 rivers in volume, and these floods flow far over the plains; but during 

 the normal droughts the torrents shrink to streamlets purling among 

 the rocks, or give place to blistering sand-wastes furlongs or even miles 

 in width and dozens of miles in length, while beyond stretch low, 

 radially scored alluvial fans, built by the great freshets of milleuDiunis. 

 Only a trifling part of the rainfall of the plains ever gathers in the 

 waterways beading in the mountains, and only another small part 

 gathers in local channels; the lighter rains from higher clouds are so 

 far evai)orated in the lower strata of the air as to reach the earth in 

 feeble sprinkles or not at all; the product of moderate showers is 

 absorbed directly by earth and air; while the water of heavy rains 

 accumulates in mud-burdened sheets, spreading far over the jilains, 

 flowing sluggishly dowu the slopes, yet suffering absorption by earth 

 and air too rapidly to permit concentration in channels. These moving 

 mud-blankets of the plains, or sheetfloods,' are often supplemented by 



' Detined aiirt described in Sheetflood Erosion, Bull. Geol. Soc. Am., vol. vii. 1897, p. 87. 



