"The cold, dry currents of air from the north-east, after passing the Rocky Mountains and the Sierra Nevada, descend to the Pacific, and absorb the moisture of the atmosphere, to a great distance from the land. The cold air from the mountains, and that which accompanies the great ocean current from the north-west, thus become united; and vast banks of fog are generated, which, when driven by the wind has a penetrating, or cutting, effect on the human skin, much more uncomfortable than would be felt in the humid atmosphere of the Atlantic, at a much lower temperature.
"As the sun rises from day to day, week after week, and month after month, in unclouded brightness during the dry season, and pours down its unbroken rays on the dry, unprotected surface of the country, the heat becomes so much greater inland than it is on the ocean, that an under-current of cold air, bringing the fog with it, rushes over the coast range of hills, and through their numerous passes, towards the interior.
"Every day, as the heat, inland, attains a sufficient temperature, the cold, dry wind from the ocean commences to blow. This is usually from eleven to one o'clock: and, as the day advances, the wind increases and continues to blow till late at night. When the vacuum is filled, or the equilibrium of the atmosphere restored, the wind ceases; a perfect calm prevails until about the same hour the following day, when the same process commences and progresses as before; and these phenomena are of daily occurrence, with few exceptions, throughout the dry season.
"These cold winds and fogs render the climate at San Francisco, and all along the coast of California, except the extreme southern portion of it, probably more uncomfortable, to those not accustomed to it, in summer than in winter.
"A few miles inland, where the heat of the sun modifies and softens the wind from the ocean, the climate is moderate and delightful. The heat, in the middle of the day, is not so great as to retard labor or render exercise in the open air uncomfortable. The nights are cool and pleasant. This description of climate prevails in all the valleys along the coast range, and extends throughout the country, north and south, as far eastward as the valley of the Sacramento and San Joaquin. In this vast plain, the sea-breeze loses its influence, and the degree of heat in the middle of the day, during the summer months, is much greater than is known on the Atlantic coast in the same latitudes. It is dry, however, and probably not more oppressive. On the foot-hills of the Sierra Nevada, and especially in the deep ravines of the streams, the thermometer frequently ranges from 110° to 115° in the shade, during three or four hours of the day, say from eleven until three o'clock. In the evening, as the sun declines, the radiation of heat ceases. The cool, dry atmosphere from the mountains spreads over the whole country, and renders the nights cool and invigorating.
"I have been kindly furnished, by Surgeon-General Lawson, U.S. Army, with thermometrical observations, taken at the following places in California, viz: At San Francisco, by Assistant-Surgeon W. C. Parker, for six months, embracing the last quarter of 1847 and the first quarter of 1848. The monthly mean temperature was as follows: October, 57°; November, 49°; December, 50°; January, 49°; February, 50°; March, 51°.
"At Monterey, in latitude 36° 38' north and longitude 121° west, on the coast, about one degree and a half south of San Francisco, by Assistant-Surgeon W. S. King, for seven months, from May to November inclusive. The monthly mean temperature was: May, 56°; June, 59°; July, 62°; August, 59°; September, 58°; October, 60°; November, 56°.
"At Los Angeles, latitude 34° 7', longitude west 118° 7', by Assistant-Surgeon John S. Griffin, for ten months, from June, 1847, to March, 1848, inclusive. The monthly mean temperature was: June, 73°; July, 74°; August, 75°; September, 75°; October, 69°; November, 59°; December, 60°; January, 58°; February, 55°; March, 58°. This place is about forty miles from the coast.
"At San Diego, latitude 32° 45', longitude west 117° 11', by Assistant-Surgeon J. D. Summers, for the following three months of 1849, viz: July, monthly mean temperature, 73°; August 75°; September, 70°.
"At Suttersville, on the Sacramento River, latitude 38° 32' north, longitude west 121° 34', by Assistant-Surgeon R. Murray, for the following months of 1849: July, monthly mean temperature, 73°; August, 70°; September, 65°; October, 65°.