WEATHER CONDITIONS ON THE PACIFIC COAST 

 and some by excessive loss of heat by radiation. 

 It is interesting to note that in summer San Fran- 

 cisco receives but 68 per cent of the possible sun- 

 shine, while at Mount Tamalpais more than 90 per 

 cent is recorded. 



The Winds. The prevailing drift of the sur- 

 face air along the California coast is from west 

 to east. Briefly, the summer winds may be grouped 

 as follows: West to northwest, 75 per cent; north 

 to northeast, 4 per cent; east to southeast, 3 per 

 cent; south to southwest, 3 per cent; and calms, 

 15 per cent. The winter winds show a greater per- 

 centage of motion from the south. Southerly gales 

 are not infrequent, and there is a common belief 

 that southeast is the prevalent direction in winter 

 months. This, however, is not true. West to north- 

 west winds have a frequency of 30 per cent; north 

 to northeast, 18 per cent; east to southeast, 17 per 

 cent; south to southwest, 22 per cent; and calms, 

 13 per cent. 



It is plain from the above figures that north- 

 west is the predominant direction along the coast 

 in the vicinity of San Francisco. Owing to the 

 topography there are certain deflections and 

 changes in the direction of the wind, especially 

 through the Golden Gate from northwest to west 

 or even west by south. These are referred to else- 

 where. 



During certain portions of the year, especially 

 May and June, the northwest wind attains a re- 

 markably high velocity. 



Another type of high wind experienced at San 

 Francisco is the north-northeast wind of November, 

 December, and occasionally January. This wind 

 differs from the northwest wind of summer and 

 is distinctly a mountain wind. The highest wind 

 velocity recorded in San Francisco, 28.6 meters 

 per second (sixty-four miles per hour), from the 

 northeast, occurred on November 30, 1906.* 



The most prevalent high wind of winter is from 

 the southwest, closely followed by the southeast. 

 The latter is the well-known wind preceding winter 

 storms in this section. At San Francisco south- 

 east winds will sometimes prevail for several days; 

 but in most storms the wind after blowing for 

 twelve or more hours increases in velocity to 18 

 meters per second (forty-one miles an hour), more 

 or less, and then shifts suddenly to the southwest, 



* The velocities used here are Weather Bureau instru- 

 mental records. These are only approximately correct. For 

 conversion into meters per second it may be convenient to 

 remember that one meter per second equals 2.2 miles per 

 hour. 



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