-I \ li: AGRICULTURAL SOCIETY. II.". 



Type."' already described. Another feature of the dry season is the develop- 

 ment of considerable intensity of the high in Oregon, the pressure being 

 at the same time very low in Southern California, creating the northerly 

 winds in the Sacramento and San Joaquin Valleys. 



The boundary between the wet and dry season would, I believe, be as 

 definite as the sun's march north or south if not for these disturbing weather 

 types, which bring into effect conditions overriding the gradual change of 

 temperature. It is well established that the temperature of the Pacific 

 Ocean differs very little anywhere on the coast, and the monthly variation 

 is so slight that it may be disregarded. We can in a general discussion 

 say that the temperature of the ocean washing our shores is about con- 

 stant. It is wholly different over the land, and the difference increases in 

 proportion to the distance from the sea. In the Winter, the prevalent type 

 is such as to drive the ocean winds over a country where the temperature 

 is cooler than themselves, and where the condensing conditions are strong 

 enough to well deplete them of moisture; hence rain results upon the 

 western slopes and little remains for the Rocky Mountain country. During 

 the Summer, on the contrary, the winds from the Pacific Ocean passing at 

 once over the drying country, do not precipitate their moisture at all till 

 the Rocky Mountain summits condense them. Thus the rainy season is 

 transferred from this coast to these higher regions. The change of one 

 season to another is best illustrated by projecting the curve of surface tem- 

 perature of the Pacific Ocean with the mean daily temperature of a place 

 in proximity to it, for instance, San Francisco. As soon as the air temper- 

 ature curve permanently crosses the former, the change of season takes 

 place. A specific case of this principle is discussed and well illustrated in 

 the last Bulletin of this Academy, by Prof. Davidson, in his paper on the 

 air and water temperature at the Golden Gate. 



