CLIMATIC PECULIARITIES 



11 



This characteristic of our local climates is due in the main to 

 two great agencies, one active, bringing heat, the other passive, 

 shielding us from arctic influences. 



First : Our proximity to the Pacific Ocean. Professor McAdie, 

 who has charge of the San Francisco office of the United States 

 Weather Bureau, says : "The prevailing drift of the surface air 

 in temperate latitude is from west to east. Therefore the proximity 

 of the Pacific, with its mean annual temperature of 55 degrees 

 Fahr., serves to prevent large temperature changes, because of the 

 water vapor and also because the air comes landward. Whenever 

 the circulation is reversed, temperatures vary." 



Second : Another agency contributing to the mild climate of 

 the Pacific Coast consists in the mountain barriers upon our 

 northern and eastern boundaries. It was Guyot who first called 

 attention to the fact that the Sierra Nevada and the Cascade 

 Mountains reach the coast of Alaska and bend like a great arm 

 around its western and southern shore, thus shutting off or deflect- 

 ing the polar winds that otherwise would flow down over the 

 Pacific Coast States, while California has her own additional pro- 

 tection from the north in the mountain arch which has its keystone 

 in Mount Shasta. 



CHIEF TOPOGRAPHICAL AND CLIMATIC DIVISIONS 

 OF CALIFORNIA 



California is usually divided into three main areas and climates, 

 each distinct in typical conditions and yet separated by regions, 

 more or less wide, in which these conditions merge and influence 

 each other. Dr. Robertson says* : 



Isothermal lines which normally run east and west are, as they near 

 the Pacific, deflected north and south, and define three distinct climatic 

 belts. These may be named coast, valley and mountain; and while they 

 resemble each other in having only two seasons, they are dissimilar in 

 other respects. These differences depend upon the topography of the 

 country, and are of degree rather than of kind; altitude, distance from 

 the ocean, and situation with reference to mountain chains, giving to each 

 region its characteristic climate. 



How similar are the conditions which prevail in these belts 

 may be learned from the data shown in the following table, which 

 includes points separated by nearly the whole length of the State, 

 the difference in latitude of the extreme north and south points 

 being seven or eight degrees. Thus, through a north or south 

 distance great as that which separates the States of Georgia and 

 New York, similar climatic conditions prevail in California. In the 

 following table the averages are deduced from observations by the 

 United States Weather Bureau observers for a long series of years : 



* Report of State Agricultural Society, 1886, p. 322. 



