266 TRANSACTIONS OF THE 



the great central valley of California. 'He said: "These winds are most 

 virulent and desiccating in the extreme north and the extreme south ends 

 of the valley; the atmosphere from the Golden Gate and the bays seem to 

 modify the wind ordinarily in the center of the valley." 



The tables of temperature above confirm his inference. Tulare is 4° 

 cooler for the year than the next station south, and 2° cooler than the next 

 station north, which may be referred to the influence of the cold air from 

 the high mountains at whose base it is situated, and to the evaporation 

 from Tulare Lake. Another effect of these deserts is to create a daily sea 

 breeze from the southwest return trade winds that prevail on the coast as 

 surface winds during the Summer months. Each day, after the sun rises 

 over these great deserts, they become heated and increase the temperature 

 of the air over their surface; this air rises, and as the whole current of cool 

 air is from the ocean on the west, it rushes in to fill the vacancy. A gentle 

 southwest wind may be blowing on the coast at night or in the morning ; 

 by eleven or twelve o'clock the full force of the sun's rays is felt in the 

 Nevada Desert — the gentle breeze has increased to a brisk wind, and con- 

 tinues until evening, after the setting sun has withdrawn his rays and the 

 desert has radiated its heat into space. The gentle southwest wind resumes 

 its sway until the next day, when, from the same cause, the high wind is 

 again repeated. Dr. Gibbons, in an article on the climate of San Fran- 

 cisco, says: "Whatever may be the direction of the wind in the forenoon, 

 in the Spring, Summer, and Autumn months it almost invariably works 

 round towards the west in the afternoon. So constant is this phenomenon 

 that in the seven months from April to October, inclusive, there were but 

 three days in which it missed, and those three days were all rainy, with 

 the wind from the south or southwest." He adds: " I cannot discover that 

 in any other spot on the globe the wind blows from one octant one hundred 

 and eighty-six days and from the opposite octant only six days in the year." 



The mean Summer temperature at Wadsworth and Brown's Station, on 

 the Nevada Desert, on the line of the Central Pacific Railroad, is 80.3°, and 

 for Brown's, 78.1°. The Summer temperature at Fort Mohave is 92.6°, and 

 at Fort Yuma, 92.7°. The mean temperature of the Mohave Desert for 

 July is 93.1°. Fort Yuma is about five hundred miles south-southeast 

 from Wadsworth. The country intervening is entirely desert. 



The indraught of westerly winds from the Pacific in Summer does not 

 appear to be alone sufficient to satisfy the demands of the heat of these 

 great deserts. Lieutenant Wheeler states that on the Mohave Desert 

 " southeast winds are by far the most prevalent in the Summer time." He 

 adds: "It is also easily observed that the clouds and Summer rains come 

 from that direction." From this it would appear that the deserts create 

 an indraught from the Gulf of California, as well as from the Pacific Ocean. 

 I have shown that we are in the latitude of the southwest return trade 

 winds, and that their force is augmented by the effects of the radiation of 

 heat from the deserts on our eastern border. The configuration of the 

 immediate coast near San Francisco, from Point San Pedro to Point Reyes, 

 and the open Golden Gate, cause an increased quantity of this daily sea 

 breeze to pass by and over this city. This increased wind and accom- 

 panying fog, coming directly from over the cool Japan Gulf Stream, so lowers 

 the Summer temperature of this city that, as has been shown by Dr. Gib- 

 bons and the records of the Smithsonian Institute, there is no other place 

 in the whole territory of the United States, of the same elevation, that has 

 so low a mean temperature; the mean Summer temperature at the Golden 

 Gate being 56°. Another cause affecting the climate of California is in the 



