AMERICAN JOURNAL OF SCIENCE. 
[THIRD SERIES.] 
ArT. XXX V.—On the Cause of the Arid Climate of the Western 
portion of the United States; by Captain C. E. Durrton, 
U.S. A., U. S. Geological Survey. 
Read before Section B, American Association for the Advancement of Science, 
Cincinnati Meeting, Aug. 18th, 1881. 
MANY questions arising in the study of western geology 
involve the consideration of the arid climate of the region, and 
I have frequently been led to inquire as to its cause. Arid 
climates are usually attributed to the passage of prevailing 
winds over high mountain chains. As they ascend the moun- 
tains upon the windward sides they are cooled by the expan- 
sion due to diminished barometric pressure, their capacity for 
moisture is reduced and an abundant precipitation 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 having been depleted of moisture is supposed 
to be ineapable of yielding a copious supply to regions beyond. 
This explanation is no doubt good for some localities. Peru 
Is 2 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, 
t is unquestionable that the Sierra Nevada abstracts a 
notable amount of moisture from the winds blowing from the 
Am. Jour. — Series, VoL. XXII, No. 1380.—Ocrosmr, 1881. 
