KILLING FROSTS. The following table gives the dates on which occurred killing frosts during the year 1899the last in spring and first in autumn-at stations recording and reporting frosts. The blank spaces in the table indicate that there were no killing frosts at those stations for the period shown, or, in some few cases, that the record is incomplete. NOTE. Killing frosts occurred in every month of the year at Bodie, Mono County. The observer at Los Angeles reports that there were no killing frosts at that station during the year; the first heavy frost occurred December 11, damaging vegetables in exposed places. At San Diego the most severe frost for five years occurred February 6, but did not damage orchards or fruit in citrus region. Apr. 10 Apr. 28 Feb. 6 Dec. 18 Sargent Shasta. San Luis Obispo... San Mateo San Miguel.. Feb. 4 Dec. 27 Feb. 9 Dec. 18 Mar. 17 Dec. 19 Mar. 13 Dec. 14 Feb. 73 Soledad Feb. 6 Feb. 8 Oct. 24 22 Dec. 19 FOG. One of the most marked atmospheric conditions with which the forecaster has to deal on the coast of California is fog. Particularly in the vicinity of San Francisco are the effects of the fog noticeable. Owing to the peculiar topography of this section, there are certain well-marked stream lines in the general movement of the air from west to east; and in these streams great masses of the condensed-water vapor lying beyond the heads and along the coast are carried in through the Golden Gate. There are times when, judging from such reports as are available, fog prevails along the entire Pacific coast. On the daily weather map which is issued at San Francisco attention has been called several times within the last few years to the fact that nearly every coast station reported fog, thus indicating the presence of a bank of fog at sea from 1,400 to 1,600 miles in length. The distance seaward to which this fog extended is not known, but from the records contained in logs of vessels sailing from San Francisco it is thought that an average value would be about 50 miles. Now, fog, whether it appears for a few hours at certain seasons, as in the harbors of the Atlantic coast, or regularly on summer afternoons and winter mornings along the Pacific coast, and whether it lies in sharply defined streams and strata, as at San Francisco, or in ill-defined general banks, as off Newfoundland, indicates certain sharp contrasts of temperature and air motion. On the Pacific coast there are several well-marked types of fog-the summer afternoon fog, moving from the sea to the land; the morning winter land or tule fogs, which move seaward, and occasional nondescript smoke fogs near the larger cities. At Mount Tamalpais we are able to look down upon the fog streaming through the Golden Gate. On one side the ocean maintains a temperature of about 55° F., while inland the temperature is much higher. Some illustrations of the fog effects as photographed at the Weather Bureau Observatory were published in the Monthly Weather Review for July and November, 1900, and January, February, and March, 1901, and are here reproduced. The differences in temperature, humidity, and air motion are so marked within comparatively small distances, both horizontally and vertically, in the bay district that it seemed advisable to tabulate in comparative form the meteorological elements for a year at the higher station (elevation approximately half a mile) and the station at sea level. The present paper aims to present, with some photographic evidence of fog forms and drifts, a rough study of the air drainage of the locality in which fog streams and counter streams are of such frequent occurrence that they serve excellently as exponents of air motion. The topography of the section is remarkable, because of the close juxtaposition of ocean, bay, mountain, and foothill. A valley, level as a table, 450 miles long and 50 miles wide, having afternoon temperatures of 100° or over, is connected by a narrow water passage with the Pacific Ocean, the mean temperature of the water in this locality being 55°. Thus within a distance of 50 miles in a horizontal direction there is frequently a difference of 45° in temperature, while in a vertical direction there is often a difference of 30° in an elevation of half a mile. High bluffs, ridges, and headlands are at such an angle to the prevailing strong westerly surface air currents that an air stream is forced with increased velocity through the Golden Gate, and there must of necessity be considerable piling up of both air and water vapor at this point. The locality may indeed be considered as a natural laboratory in which experiments connected with cloudy condensation of water vapor are daily wrought, and it is therefore of more than passing interest to the meteorologist. |