134 OUTLINE OF PLANT GEOGRAPHY 



more than a hundred inches annually. On the other hand, 

 some places in the desert of southern California are almost 

 rainless. 



Topography plays an extremely important role in determining 

 the climate of different parts of California. The principal moun- 

 tains are parallel with the coast. In central California there are 

 three, the outer and inner Coast Ranges, and the Sierra Nevada 

 forming the eastern boundary of the state. At the north the 

 Siskiyous separate California from Oregon, and connect with the 

 southern Cascades. In central California the Tehachapi Moun- 

 tains connect the Coast Ranges and the southern Sierra, and in 

 southern California are more or less isolated lofty mountains, the 

 San Bernardino and San Jacinto ranges. 



These mountain ranges exercise an immense effect on the climate 

 of the adjacent regions. Thus while the western slopes of the 

 outer Coast Range receive a heavy rainfall, so that the northern 

 redwood forest is probably the heaviest stand of timber in the 

 world, the valleys to the east are too dry to support anything but 

 a scattered growth of oaks, and are often quite treeless. 



In the southern part of the state, where the precipitation at 

 best is scanty, the moisture-laden winds from the ocean are quite 

 unable to reach the interior valleys and plateaus, and these are 

 more or less complete deserts, like the Mojave desert, Death 

 Valley, and the Colorado desert. 



In central California, the great central valley, made up of the 

 Sacramento and San Joaquin, has a very light rainfall except in 

 the upper Sacramento Valley. For the most part the rainfall is 

 insufficient for ordinary farming, but under irrigation these val- 

 leys are extremely productive, and a great quantity of grain, 

 including rice, and fruit of all kinds, make this great valley the 

 most important agricultural region of California. 



There may be great differences in temperature and precipita- 

 tion due to local conditions. For example, San Francisco, situated 

 on a peninsula exposed to the full effects of the ocean wind and fog, 

 has a remarkably cold summer climate, the mid-summer maximum 

 usually being below 70°F. In the central valley, at this season, 

 intense heat prevails, while in the deserts of southern California 

 are the hottest regions in the United States. Even in the imme- 

 diate vicinity of San Francisco places lying inside the Coast Range, 



