94 CARNEGIE INSTITUTION 



The sea breeze in the San Gabriel valley is usually' strong and is 

 felt almost every day during summer. But, quite unexpectedly, on 

 Mount Ivowe and Mount Wilson, overlooking this valle3^ it is always 

 very light. On almost all of the days I spent on these mountains 

 there were sea breezes, very gentle, fresh, but not cold. At such 

 an elevation, with the ocean only a few miles away to the south, 

 and nothing but low hills between, I should have expected much 

 stronger winds from this quarter. The only suggestion I have to 

 offer as a possible explanation is that the mountains to the west and 

 northwest may be a sufficiently diverting barrier to give these winds 

 a course that passes clear of Mount Wilson, and that the higher 

 ridges to the northeast of Mount Wilson form a barrier to currents 

 that otherwise would pass directly to the desert from the sea. It is 

 80 miles from Mount Wilson westward to the ocean. In this direc- 

 tion protection is afforded by the Verdugo and Santa Monica moun- 

 tains, which, though lower than the San Gabriel range, are never- 

 theless sufficiently high to have considerable influence upon the 

 winds. Moreover, it is probable that they help to divert west winds 

 toward the south, causing them to pass to one side of Mount Wilson. 



To the west-northwest of Mount Wilson there are many moun- 

 tains, reaching all the way to Point Conception, 150 miles away ; and 

 to the northwest, further inland, are still more extensive ranges. It 

 cannot be doubted that these ranges have an influence on the winds 

 that come from their direction, tempering, deflecting, or even turning 

 them completel}' aside. In any event, it is pretty certain that they 

 would find an easier passage from the sea to the desert than by pass- 

 ing over the San Gabriel range. 



In this connection we may consider the dust in the air over the Los 

 Angeles plains, for this is very intimately connected with the winds 

 that blow from the Mohave desert to the sea. When the desert is 

 dry — as it is during much of the year, and particularly in summer — and 

 strong winds blow over it, clouds of dust are raised high into the air. 

 When this dust laden air passes through Cajon canyon and out to the 

 sea, the air over the L,os Angeles plains becomes charged wdth dust. 

 In general, the dust filled region of the air has a clearly defined 

 upper limit, here called the dust line, having an elevation of from 

 3,000 to 5,000 feet, sometimes falling lower, at others rising higher, 

 according to the force and height of the wdnds. The air above the 

 dust line is not entirely free from dust. Here, as in most other 

 places, there is some dust even at very great elevations. Perhaps 

 no place of moderate height in the temperate zones can be found 

 that is wholly free from it. 



