CLIMATIC ILLUSTRATIONS. 191 



we fi;o northward the direct convective feature of the rainfall becomes less pronii- 

 ueut, and the purely cyclonic rainfall predominates. Over the whole region 

 orographic rainfall is added to tlie other two classes when hills or mountains are 

 situated in tlie ])ath of moisture-laden currents. With this general statement tlie 

 further examination of the effect of surface conditions on the rainfiill in mi<ldle 

 latitudes must be applied to concrete cases. 



In the United States the Great Plains region of Kansas, Nebraska, and the Dakotas 

 is tliat for wliicli a variation in rainfall has most often been claimed or anticipated. 

 Many settlers believe that the so-called "rain belt" is moving rapidly westward 

 with the extension of cultivation and settlement. 



The single condition favorable to such an increase of rainfall consists in the 

 steadier and larger evaporation which no doubt takes place over the cultivated area, 

 but the other conditions necessary to condense and precipitate this moisture over 

 the same region are largely absent. There are no hills or mountains to produce 

 orographic rainfall, and without these barriers the high winds constantly carry 

 away the moisture evaj^orated from the surface and preciiiitate it far to the east- 

 ward. Hence, not only is the increased moisture in the air but a small fi-action of 

 the total rainfall, but also only a small portion of this is gathered into convective 

 or cyclonic currents and restored again by them to the prairie soil. Evidently a 

 small amount must be thus restored in every summer rain and the total rainfall in- 

 creased thereby, but quantitatively this must be very small. 



There is one region deticient in rainfall and water sujiply for which claims in be- 

 half of an actual or possible increase of rainfall due to human agency are less often 

 made, but which the preceding analysis leads me to believe would not be unreason- 

 able to anticipate. I refer to the San Joaquin Valley of California. This valley is 

 flanked by the Coast Range on the west and by the Sierras on the east. The mois- 

 ture evaporated trom the surface can not escape from the basin, but will be largely 

 precipitated either over the valley or on the sides of tlie adjacent mountains which 

 constitute its watershed. If, therefore, the increase in irrigation and in the extent 

 of cultivated area produces a material increase in the evaporation, it seems reason- 

 able to expect that this moisture will be restored by an increased rainfall in the 

 valley and in the adjacent mountain sides. One consideration only wotild appear 

 to retard and diminish this effect. The inclosure of the valley prevents that rapid 

 indraft of air which renders possible a rapid vertical circulation. Thus the ac- 

 tivity of the whole process is rendered sluggish and the total amount of moisture 

 passing through the cycle from evaporation to rainfall is smaller than with a more 

 rapid circulation. 



In the manner here outlined the possibility of a variation of rainfall may be 

 investigated for any region, andr with sufficient meteorological data even quanti- 

 tative values may be computed. The general result indicated by the analysis of 

 the physical processes involved as well as by the statistical data so far collected is 

 that the margin of such possible change is very small. In most cases it can not 

 exceed a small per cent, and in other cases it can not occur at all. It appears, tliere- 

 fore, that in the estimation of the importance and value of forests, a disjjropor- 

 tiouate and undue amount of emiihasis has been i)opularly given to such an inllueuce. 



