246 



The National Geographic Magazine 



The obvious deduction from the fore- 

 going is that the Salton Sea is not re- 

 sponsible for the phenomenal rainfall of 

 1905 in Arizona. 



THE INFLUENCE OF SMALL BODIES OF 

 WATER ON LOCAL CLIMATE 



It is generally believed that small 

 bodies of water have an appreciable in- 

 fluence on the local climate of contiguous 

 land areas, but it is exceedingly difficult 

 to distinguish between results which may 

 be due to purely local causes and those 

 which may be reasonably due to general 

 causes. 



The eft'ect of a small body of water 

 such as the Salton Sea on the climate of 

 the surrounding territory may be recog- 

 nized in two principal ways, iirst, in its 

 equalizing efl^ect on the temperature, 

 and second, in the increased amount of 

 water vapor thrown into the air by evap- 

 oration, since more water is evaporated 

 from a water surface than from forests 

 or fields. Owing to the fact that a water 

 surface warms up much more slowly than 

 a land surface and retains its heat much 

 longer, the water surface will, in general, 

 be warmer at night than the land and 

 cooler in the daytime. Thus there will 

 be a tendency toward lower maximum 

 temperatures and higher minimum tem- 

 peratures in a narrow zone immediately 

 surrounding the lake, but especially on 

 the leeward shore. 



The distinguishing characteristics of 

 the climate of the Salton Sea region are 

 those of the desert, viz, great heat and 

 dryness. The annual mean temperature 

 is about "7^° \ winter, 57°; spring, 75°; 

 summer, 97°, and autumn, 79° F. The 

 maximum temperatures of the summer 

 months i-ange from 115° to 130° F., and 

 the minimum temperatures of winter 

 from 20° to 25° F. The annual precip- 

 itation is about 2.50 inches, most of which 

 occurs in the cold months. The months 

 of April, May, and June are practically 

 rainless, but occasional showers fall in 

 July, August, and September in about 30 

 per cent of the years. December and 

 February are the months of greatest rain. 



In the winter snow may fall, but it rarely 

 lies on the ground more than twenty-four 

 hours ; the average number of days in a 

 year with o.oi inch or more of precipita- 

 tion is four. The winds of the Colorado 

 Desert are mostly northwesterly in win- 

 ter and southeasterly to easterly in sum- 

 mer. In the cold season they flow 

 through San Gorgonio Pass, in the north- 

 western part of Riverside County, eleva- 

 tion about 2,500 feet, as westerly winds, 

 but are deflected somewhat toward the 

 southeast by the San Bernardino Range, 

 which skirts the eastern and northern 

 limits of the desert. Being descending 

 winds and dry, they are not favorable to 

 precipitation. The cold winds are gen- 

 erally from north and east, while rain 

 winds are from east and south. In sum- 

 mer the winds are less stable as regards 

 direction than in winter. While they are 

 largely from the east and south, there is 

 at times a marked westerly component. 

 No record of the diurnal change in the 

 wind for the Salton Sea region is avail- 

 able. 



Since brisk winds promote evaporation 

 by constantly renewing the air over the 

 evaporating surface, it becomes important 

 to know something of the force of the 

 wind in the vicinity of Salton Sea. Un- 

 fortunately no instrumental records of 

 wind velocity are available for that re- 

 gion, but possibly we may learn some- 

 thing of interest by consulting the 

 records maintained at Yuma, Arizona. 

 At that place high winds are infre- 

 quent, yet there is considerable motion 

 in the air during the afternoon and 

 evening hours. Such motion, however, 

 is clearly discontinuous and not calcu- 

 lated to transport air bodily out of the 

 desert region, or to cause the importation 

 of air of different density and moisture 

 from adjoining regions. The particles 

 of air that are set in motion by the winds 

 of the daytime do not move continuously 

 in the original direction, but are carried 

 hither and thither by the light variable 

 airs of the night-time, and in some cases 

 even in a direction contrary to that in 

 which they traveled in the daytime. The 



