432 KANSAS CITY REVIEW OF SCIENCE. 



ON THE CAUSE OF THE ARID CLIMATE OF THE WESTERN 

 PORTION OF THE UNITED STATES. 



BY CAPT. C. E. BUTTON, U. S. A., U. S. GEOLOGICAL SURVEY. 



Read before Section B, American Association for the Advancement of Science, Cincin- 

 nati Meeting, August i8th, 1881. 



Many questions arising in the study of western geology involve the consid- 

 eration of the arid climate of the region, and I have frequently been led to in- 

 quire as to its cause. Arid climates are usually attributed to the passage of pre 

 vailing winds over high mountain chains. As they ascend the mountains upon 

 the windward sides they are cooled by the expansion due to diminished barom- 

 etric pressure, their capacity for moisture is reduced and an abundant precipita- 

 tion takes place. Descending upon the leeward sides these changes are reversed ; 

 the air is heated, its capacity for moisture is increased it becomes dry, and hav- 

 ing been depleted of moisture is supposed to be incapable of yielding a copious 

 supply to regions beyond. This explanation is no doubt good for some localities. 

 Peru is a case in point and for that country it seems quite perfect. It is believed 

 by many that it also explains the arid climate of the western half of the United 

 States, and that the Sierra Nevada is the range which robs the winds of that 

 region of the moisture which otherwise would make its vast expanse fertile. 

 Reflection upon this case has led me to a different conclusion. 



It is unquestionable that the Sierra Nevada abstracts a notable amount of 

 moisture from the winds blowing from the Pacific. Mr. B. B. Redding, the Land 

 Agent of the Central Pacific Railroad, has kept for several years excellent records 

 of the rainfall at many stations in California and Nevada, and informs me that 

 along the main road from Sacramento to the summit pass of the Sierra, the an- 

 nual rainfall increases at the rate of one inch for every one hundred feet of alti- 

 tude. At the summit the mean annual precipitation exceeds ninety inches. It 

 is not improbable that this large amount is considerably exceeded at numerous 

 points along the crest of the range. It seems clear therefore that the winds 

 which blow over the Sierra are to some notable extent depleted of moisture 

 and the effect must be to at least aggravate the aridity of the regions lying im- 

 mediately east of the range. But I think it can be made evident that this effect 

 is relatively not great, and that the elevated region of the west would be on the 

 whole very nearly as arid as it now is if the Sierra Nevada were obhterated as a 

 mountain range. Nor can the other and lower ranges lying east of the Sierra 

 affect the case materially, for surely more than ninety per cent of the rain and 

 snow which fall upon them are reevaporated in loco and the atmosphere ultimately 

 suffers no material loss of moisture. 



When the winds blow constantly from a cool to a warmer region they be- 

 come warm and therefore dry; and if they have no opportunity to take up more 



