WEATHER CONDITIONS ON THE PACIFIC COAST 

 attaining a slightly higher velocity. After a few 

 hours the velocity decreases and, with the change 

 to northwest, the weather clears. At such times 

 the air is remarkably pure and the distant moun- 

 tains can be seen distinctly. 



The average hourly wind velocity in San Fran- 

 cisco is nearly 4.5 meters per second (ten miles 

 per hour), and the prevailing direction west. The 

 wind attains its greatest velocity about 4:30 P.M. 

 and its least about 6 A.M. The average movement 

 during the twelve hours from 7 A.M. to 6 P.M. is 

 approximately 4.9 meters per second (eleven miles 

 per hour), and from 7 P.M. to 6 A.M., 4 meters per 

 second (nine miles per hour). 



Fog. One of the most marked climatic features 

 of San Francisco is the prevalence of fog. In sum- 

 mer afternoons sea fog moves through the Gate, 

 appearing about 1 P.M. and covering the whole 

 sky by 3 P.M. The average depth of the fog layer 

 is 518 meters (1700 feet). Comparing the percent- 

 age of possible sunshine at San Francisco and 

 Mount Tamalpais, it is at once apparent that the 

 summer afternoon sea fog shuts out 50 per cent or 

 more of the possible sunshine between 3 and 7 

 P.M. during June, July, and August. There is also 

 curtailment of sunshine between 7 and 9 A.M. dur- 

 ing May, June, July, August, and September. 



In the winter, morning fogs, or, as they are 

 commonly called, "tule" fogs, frequently occur. 

 These are low-lying banks of condensed vapor 

 formed by cooling due to radiation and contact. 

 The land surfaces are much cooler than the water 

 surfaces, and hence these fogs have a decided mo- 

 tion from the land to the sea. The average number 

 of foggy days is twenty-four per year. 



In addition to the summer afternoon sea fog, 

 moving from west to east, and the land or tule fog 

 of winter mornings, there is a third kind of fog, 

 which may be called smoke fog. Under certain 

 atmospheric conditions the smoke of the city moves 

 seaward during the forenoon and returns about 

 1 P.M. as a dense black pall. This is the cause of 

 the so-called dark days. The phenomenon is of 

 brief duration, seldom exceeding two hours; but 

 while it lasts causes some apprehension. 



Humidity. Relative humidity is a term fre- 

 quently used in attempts to describe relative damp- 

 ness of a locality. The term is misleading and 

 while generally used can not give a proper basis 

 for comparison unless temperatures are also given, 

 inasmuch as relative humidity is only a ratio and 

 does not indicate a fixed amount of water vapor. 



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