278 TRANSACTIONS OF THE 



general rule, they are more virulent in the extreme north and the extreme 

 south end of the Sacramento (and San Joaquin) Valleys. 



The atmosphere from the Golden Gate, and the three bays, seems to 

 modify the wind, ordinarily, in the center of the valley. The general course 

 of these currents of heated air is, of course, determined by the trend of the 

 valley and its walls. 



Without attempting an accurate statement of the periodic recurrence of 

 these winds, we may say that the ordinary current returns about once in 

 two weeks. Hot blasts occur about ten times during the year, while the 

 extremely vicious blasts occur only once in six or eight years. We are, 

 however, without data for the settlement of these points, unless we can find 

 the needed information in the meteorological records of the Central Pacific 

 Railroad Company. Their continuance is from one to thirteen days, 

 and the average about three days. Ordinarily, the wind ceases at night, 

 and is followed by a slight breeze from the south, though frequently the 

 movement continues all night with considerable force. Counter currents 

 are not generally noticeable during the prevalence of the north wind. 



THE ORIGIN AND CAUSE OF THE NORTHERLY WINDS. 



In discussing this point, we must distinguish between the cause of these 

 winds and the cause of their disagreeable characteristics. If we are to 

 regard the northern winds as special or local winds, we must first seek for 

 local causes. Wind is, of course, a certain quantity of air set in motion by a 

 change of equilibrium, and this loss of equilibrium is produced by the rari- 

 fication of the atmosphere. The sun's rays penetrate the atmosphere at the 

 south end of the valley, and being incapable of returning to celestial space 

 through the same medium, they leave their heat in the sand and decom- 

 posed granite, where it is entrapped and stored. This accumulated heat 

 rarities the air, which ascends and creates a practical vacuum at the south 

 end of the valley. The contiguous air to the northward then rushes in to 

 restore the equilibrium, while the heat in the soil creeps northward until 

 the whole surface of the valley becomes heated, when we have a practical 

 vacuum four hundred and fifty miles long, with an average width of forty- 

 five miles. The air north of the Sacramento end of the valley then rushes 

 into this long vacuum, which is more perfect at the south end, and the 

 result is our northerly winds. Considering the northers as local winds, this 

 is the only explanation that science has to offer as to their origin and cause. 

 But, as the science of meteorology advances, we are led to look for more 

 general causes — causes connected with the general circulation of wind 

 currents. 



For a long time the cold southwest winds of France were attributed to 

 local causes, but more thorough investigation of barometric pressures in 

 Spain, France, and Italy, during the prevalence of that wind, established 

 the fact that it belonged to a broader system of circulation. And it is 

 probable that more extended observations of barometric pressures will con- 

 nect our northers with a wider system of atmospheric circulation. As 

 having a possible bearing upon this point, we note the fact that the north- 

 westerly winds of this valley are sometimes almost simultaneous with the 

 northwesterly winds of the regions about Santa Barbara. Hittell, in his 

 "Resources of California," gives an account of two such nearly coincident 

 currents occurring at Stockton and Santa Barbara in June, 1859. It is 

 also to be noted that, in the Winter season, the southeast wind, which 

 brings on rain, is preceded by the northerly wind, carrying southward the 

 rain clouds at a very high altitude. Further discussion on this point must 



