248 C. FE. Dutton—Arid Climate of the Western U. S. 
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 annual rainfall increases at the 
rate of one inch for every one hundred feet of altitude. 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 immediately 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 obliterated 
as a mountain range. or 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 reévaporated in loco and the atmosphere ulti- 
mately suffers no material loss of moisture. 
When the winds blow constantly from a cool to a warmer 
region they become warm and therefore dry ; and if they have 
no opportunity to take up more moisture on the way the pas- 
sage from a coo! to a warm region is a sufficient cause of 
aridity. This is, I conceive, the state of affairs which deter- 
mines the climate of the western mountain region. The winds 
blow constantly from the western quarters, being the “ return- 
trades.” Local winds and perhaps large cyclones opcasiopally 
e 
ur 
general drift of the great atmospheric ocean is ever from west 
e This prevailing air drift comes from the Pacific and 
reaches the coast nearly or quite saturated with moisture. he 
* This general statement requires sonie qualification when applied to a 
Arizona and southern New Mexico, though it is in the main applicable even : 
* 
