mountain axis, and the result is that the Great Basin has a consid- 

 erably lower winter rainfall than does that portion of California 

 immediately to the west of these mountains. 



In the same fashion that these mountains act as a barrier to 

 the passage of moisture-laden winds, they also act as barriers to 

 the passage of hot or cold air masses. During the summer, the 

 Sierra-Cascade ranges protect much of California from the hot, dry 

 air masses that develop over the central United States. This, com- 

 bined with the proximity to the cool Pacific Ocean, explains why 

 California has a generally cool summer climate. During the winter, 

 these mountains serve to insulate California from the cold, dry 

 air masses that develop over the inland portion of the continent. 

 As a consequence, California has winters that are in general milder 

 than one might expect at the latitude. 



In going up the western side of the Sierra Nevada, one en- 

 counters an increase in annual (mostly winter) precipitation up 

 to a point at which it begins to drop off as one continues to 

 ascend in elevation. The zonation of vegetation in the mountains 

 reflects this variation in precipitation patterns. In the lowest por- 

 tions of the foothills the vegetation types are characteristic of 

 a relatively arid climate. In middle elevations the vegetation 

 reflects the rather favorable precipitation patterns. Above the 

 middle elevations, however, the vegetation acquires a more "arid" 

 aspect. As one goes southward in the Sierra, these climatic zones 

 move up in altitude, with the result that in the southernmost 

 Sierra Nevada the zone of highest precipitation is considerably 

 higher than it is in the northern Sierra. 



Of chief importance to the distribution and nature of the 

 plant cover of California is the amount and seasonal distribution 

 of rain or other precipitation. The wettest portions of California 

 are in the northwestern part of the state. Wet areas also occur on 

 the western slopes of the Sierra Nevada and in the Santa Lucia 

 Range of Monterey County. On the western slopes of the Coast 

 Ranges, from coastal Monterey County northward to southern 

 Oregon, the average annual rainfall exceeds 50 inches (127 cm) 

 per year. This is also true for much of the western slope of the 

 Sierra-Cascade ranges. On the other hand, the Sacramento Valley 

 averages below 20 inches (5 1 cm) of rain per year and the San 



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