126 OUTLINE OF PLANT GEOGRAPHY 



The effect of the ocean temperature is especially marked on the 

 immediate coast. San Francisco, lat. 38°, about the same as 

 Washington, has a mean annual temperature of 55°F, with only 

 ten degrees difference between the warmest and coldest months, 

 viz., 60°-50°. In Washington, the difference is more than four 

 times as great. The moderating effect of the ocean is particularly 

 noticeable at the north. Sitka, seventeen degrees north of New 

 York, has almost exactly the same temperature (+1°C) for its 

 coldest month. 



Away from the immediate coast the temperature is greatly 

 influenced by topography. Where the cool ocean winds in summer 

 are intercepted by mountain ranges, as in central and southern 

 California, arid regions like parts of the San Joaquin Valley and the 

 Mojave desert show the highest summer temperatures of any part 

 of the United States. For example at Needles, on the California 

 bank of the Colorado River in the Mojave desert, there are few 

 days in summer when the thermometer does not reach 100°F, 

 and 120° and even more are sometimes registered. This same 

 region, in winter, is subject to sharp frost. 



The distribution of rain on the Pacific Coast is also very different 

 from that of eastern North America. The rainiest regions are at 

 the north, where some of the stations have annual means of over 

 100 inches, and exceed any points in the eastern L T nited States. 

 Southward the rainfall diminishes rapidly until at the Mexican 

 boundary it is ten inches or less. 



Like the temperature, the rainfall is also strongly controlled 

 by topography, especially by the trend of the mountains, and there 

 may be a great difference in precipitation within a short distance, 

 due to topography. At Stanford University which lies to the 

 east of the outer coast range, the annual rainfall averages less 

 than 20 inches, but at some stations, e. g., Boulder Creek, about 

 twenty-five miles away in an air-line, it is more than three times 

 as much. 



Of course these great differences in temperature and moisture 

 exercise a great influence upon the vegetation. For example, the 

 Santa Clara Valley in the vicinity of Stanford University, is an 

 open grassy savanna, with scattered oak trees, and only on the 

 northern slopes of the hills do the trees form an approach to a 

 forest. In the mountains, however, in the regions of heaviest 



