CAUSE OF THE ARID CLIMATE OF THE WEST. 433 



moisture on the way the passage from a cool to a warm region is a sufificient 

 cause of aridity. This is, I conceive, the state of affairs which determines 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 occasionally turn the weathercock toward an easterly quarter, but the 

 general drift of the great atmospheric ocean is ever from west to east.* This 

 prevailing air drift comes from the Pacific and reaches the coast nearly or quite 

 saturated with moisture. The quantity of moisture required for saturation is de- 

 pendent chiefly upon temperature ; and the temperature of the air as it reaches 

 the coast is determined by oceanic conditions. 



From the Aleutian Islands a coastwise ocean-current moves southward, hav- 

 ing a breadth of 500 miles or more, and extending as far southward as the lati- 

 tude of Cape St. Lucas. Off British Columbia and Alaska it may be regarded as 

 a warm current relatively to the adjoining land. Off the Californias although its 

 temperature rises notably with its southward movement it may be regarded as a 

 relatively cool current. On the more northerly shores its effect is to make the 

 climate of the adjacent coast warmer than it would otherwise be; and its effect 

 on the more southerly shores is to make them cooler. Stated in another manner, 

 the relation is such that the temperatures 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 former 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 more potent and widely extended in its action 

 than any or even all of the mountain ranges could be. It is, however, greatly 

 modified by the intervention of local causes, which occasionally mask or obscure 

 it. The precipitation in different portions of the region is highly irregular and 

 several modifying causes can be indicated which, though they do not nulHfy 

 the more general one here set forth, frequently become much more conspicuous 

 in their effects. For instance, it is well known that the heaviest rainfall in the 

 United States, excepting possibly upon some mountain tops, occurs upon the 

 coast of Oregon and Washington Territory. But as aiready indicated this is the 

 locality where we find the neutral axis, so to speak, of the alleged causes favor- 

 ing respectively humidity and aridity, and where their effects are at a minimum 

 or even at zero. Moreover, the westerly winds saturated with moisture here 

 strike the coastwise mountains, and are suddenly thrown upward several thou- 



* This general statement requires some qualification when applied to southern Arizona and southern New 

 Mexico, though it is in the main applicable even there. 



