THE SMITHSONIAN INSTITUTION. 231 



CORRESPONDENCE. 



THE CLIMATE OF SAN FRANCISCO. 



BY DR. H. GIBBONS. 



No. 1.— GENERAL REMARKS. 



Since the 1st of December, 1850, I have kept a record of ob.serva- 

 tions on'the weather in this city, of which I propose to give you a summary, 

 for the especial benefit of distant enquirers. It may be well to observe, 

 that whife the climate of the western coast of North America possesses 

 some pecuHar features, that of San Francisco and the immediate vicinity 

 differs from every other place on the coast, and is, in some respects, 

 the most extraordinary climate in the world. This is owing to the 

 pecuhar position of the city, having the ocean on one side, and on the 

 other a vast bay, extending north and south near a hundred miles, and 

 separated from the ocean by a mountain wall, except at the break 

 where the city is located, and where the bay communicates with the 

 sea by a narrow strait. On the coast, a trade-wind from the northwest 

 blows almost constantly, in the summer season especially, and a strong 

 ocean current flows in the same direction. 



The thermometrical observations forming the basis of the following 

 summary were made three times a day, viz : about sunrise, which is 

 the coldest period; at noon, or after, being the warmest period; and 

 at eleven in the evening. In computing the mean temperature for 

 the month, I have used two observations only, the extremes, at sun- 

 rise and at mid-day ; experience having shown that the mean thus cal- 

 culated is very near the true temperature ibr the twenty-four hours. 



In the Atlantic States, the warmest period of the day in winter is 

 from one to two o'clock, and in summer from two to three. In San 

 Francisco the same rule holds in winter but not in summer ; for the 

 sea breeze, which springs up about noon, or soon after, instantly de- 

 presses the temperature, so that the warmest time of the day, from 

 May to August, inclusive, is an hour or two earlier than in winter. 



For the want of proper care in the location of the thermometer, many 

 of the observations which are thrown into print lose much of their value. 

 The greatest error is commonly at mid-day, when the instrument is ex- 

 posed to reflection from buildings and other objects on which the sun is 

 shining. Every such object acts as a mirror, and tends to elevate the 

 column of mercury above the proper mark for the air. The ther- 

 mometer should, therefore, be excluded, not only from the direct, but 

 also from the reflected heat of the sun, and it should at the same time 

 be exposed to a free circulation of air ; hence, to obtain a proper loca- 

 tion is otlen very difficult. The figures in my observations will be 

 found lower, in many instances, than those obtained by other ob- 

 servers, in consequence of the care exercised in this respect. In 

 making the morning observation, I use a self-registering thermometer, 

 which is certain to give the minimum temperature. 



