34 THE CLIMATOLOGY OF THE UNITED STATES. 



interior valleys to the eastward, which are very dry and 

 hot at this season of the year. We may include in the 

 list of localities contributing to this, the regions even to 

 the east of the Sierra Nevada range, for it seems quite 

 probable that the great enclosed basin before referred to 

 may constitute quite an important factor in the case. It is 

 true that in a large part of it the heat is not excessive, but 

 the aggregate of thermal energy throughout such a large 

 area would amount to considerable, and in the southern 

 part the heat is quite sufficient. 



The most favorable conditions, however, for producing 

 this summer wind are to be found in the great valleys of 

 the Sacramento and the San Joaquin ; these two in reality 

 forming one continuous valley, running north and south 

 between the Sierra Nevada and the coast ranges. The 

 temperature is very high here in summer and the coast 

 mountains are comparatively low and a number of open- 

 ings in the range admit a flow of air at a low level from 

 the sea to the valley. 



The principal opening through which this wind reaches 

 the valley is that through which the Sacramento flows, on 

 its way to the Bay of San Francisco ; hence this bay and 

 the adjacent localities are subject to much stronger sum- 

 mer winds than other parts of the coast north or south. 

 Now, as we have seen, the temperature of the sea along 

 the coast is quite low, and the winds passing over it are 

 cool and do not absorb much moisture ; and when they 

 strike the land which at this latitude is quite warm, what- 

 ever moisture they may contain is absorbed rather than 

 condensed ; consequently rain is impossible as long as 

 these conditions continue. 



When, however, the interior regions cool in the fall, 

 there is no longer a continued demand for this cool ocean 

 breeze, and winds, more or less variable, take its place. 



