C.F. Dutton—Arid Climate of the Western U.S. 249 
effect on the more southerly shores is to make them cooler. 
Stated in another manner the relation is such that the tempera- 
tures of the land areas in the high latitudes are lower than those 
of the ocean, while in the low latitudes they are higher. In 
the high latitudes, therefore, the winds blowing from the 
Pacific are cooled by the land; in the low latitudes they 
are warmed by it. Hence the precipitation is copious in the 
ormer regions and meager in the latter. Between the two 
belts where these opposite effects are pronounced is a region 
where they shade into each other, and though this intermediate 
region cannot be marked out by distinct boundaries it may 
still be said to exist in latitudes lying within the valley of the 
Columbia River. : 
The cause of an arid climate thus indicated may be regarded 
as generally operative throughout the western mountain region ; 
and it will no doubt appear upon full consideration to be -much 
ven re) ountain ranges co is, however, 
greatly modified by the intervention of local causes, which 
occasionally mask or obscure it. The precipitation in different 
zero. Moreover, the westerly winds saturated with moisture 
The most frequent variants of climate are the great differ- 
— ences of altitude in different portions of the west. The moun- 
tain tops and summits of the plateaus are always well watered, 
and in any given latitude the rainfall increases or diminishes at 
a fairly definite rate with the altitude. But the variation of 
rainfall with altitude is by no means a simple ratio. Between 
4500 and 6000. feet the difference in rainfall is not great; 
between 6000 and 7500 feet it is very considerable; between 
7500 and 9000 it is still greater. 
Moreover the rainfall is greater ceteris paribus in high latitudes 
