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. Much faithful work has been done in physical laboratories on the behavior of water vapor at varying volumes, pressures, and temperatures. Regnault, Thomson, Broch, Aitken, Kiessling, R. von Helmholtz, Hertz, Rayleigh, Von Bezold, Barus, Marvin, and others have worked upon the change of state from vapor to liquid and from liquid to solid; and while many irregularities are noted in the behavior of water vapor, the general problems of decreasing volumes and increasing pressures until condensation points are reached have been solved, and it is well understood that the vapor-liquid and liquid-solid condensations are in themselves but two phases in a chain of condensation phenomena. The problem of fog is therefore a limited one. It may be considered as a special case of cloud development, occurring in the first and second stages of Hertz, viz, the unsaturated and saturated stages. Condensation in the free air, as in these fog formations, takes place under conditions different from those obtaining in the laboratory. There are no fixed restraining walls, though the strongly stratified outlines suggest sharply limited air streams. Again, saturation as it occurs in free, constantly changing air and true adiabatic saturation are not identical. Saturation in the free air must be studied under disadvantageous circumstances, for the work must be done at a distance, with instruments neither sufficiently delicate nor accurate, and there is no control of conditions possible. In passing it may be noted that, except for traces of salt, the air of the section under consideration is partially filtered, as it presumably comes from off the broad ocean and is as free from land dust and smoke as normal air can be. Offshore winds are infrequent and light. An attempt has been made at the Mount Tamalpais station to correlate the surface pressure conditions with fog. A typical pressure distribution accompanying sea fogs has been recognized. In general, a movement southward along the coast of an area of high pressure in summer means fresh northerly winds and high temperature in the interior of the State, with brisk westerly winds, laden with fog, on the coast. An excellent illustration of a successful forecast of fog may be found in connection with the daily weather map of June 30, 1899, and also July 1, 1899. It should, however, be stated that fog does not always occur with these typical fog maps. For example, on June 28, 1899, the pressure distribution was such as to lead to the expectation of fog, but no fog was reported on this date. COMPARATIVE DATA FOR SAN FRANCISCO AND MOUNT TAMALPAIS. In Bulletin No. 28 issued by the Weather Bureau, entitled The Climate of San Francisco," meteorological data pertaining to the city of San Francisco are given up to the beginning of data for 1899, being available, It is not necessary to repeat here these records; but the possess special interest: These temperatures, compared with those of 1898, show that the annual mean temperature of both stations for the two years is practically 55°, which is also the mean annual temperature, so far as we can judge from somewhat scattered data, of the ocean in the vicinity of San Francisco. The temperature of the lower station naturally approximates sea conditions throughout the year, while the departures at the more elevated station are marked in both winter and summer. The highest temperature recorded on the mountain during the year was 960, on July 18; the maximum temperature on the same date at San Francisco being 66°, and at Point Reyes 52°. It is worthy of note than within so short a distance as 25 miles, between Mount Tamalpais and Point Reyes, there should be a difference of 44°. The highest temperature recorded at San Francisco during 1899 was 94° on October 8, while on this date the maximum temperature on Mount Tamalpais was 88°, and at Point Reyes 74°. The lowest temperature recorded during the year on the mountain was 23°, on February 4, and on the same date 34° at San Francisco and at Point Reyes. The minimum temperature was, therefore, 11° lower at the higher station. As has been elsewhere stated, during summer months there is very frequently, owing to the fog, a cooling of 11° at the lower station. In all of these instances the retarding influence of the water is apparent, in summer the temperature near the sea remaining cool, and comparatively warm in winter. Including every day in the month of June there was found to be a mean daily difference of 11.4°, or in other words from the data obtained by means of this mountain station, checked by data from Point Reyes and Mount Hamilton, the temperature rose at the rate of 1° for every 203 feet of elevation. This increase held at least up to 2,380 feet. If we consider only those days in the month (twenty-four) when there was a rise with elevation, we have for the mean daily difference in temperature between Mount Tamalpais and San Francisco 15.3°, i. e., the temperature rises 1° for every 155 feet elevation. This may be adopted as a working normal summer-day gradient. For days when there was a decrease in temperature with elevation, six in all, we find a difference of 5.9°, or there was 1° fall for every 402 feet, which, it may be noted, is not quite as steep a gradient as values generally given in text-books, 1° for every 300 feet. We notice first that periods of rainy or cloudy and cold weather occur when the surface temperatures are higher than those of the upper level. It would seem as if at these times the different air strata from sea level to 2,500 feet had been intermingled to some degree and the cold layer usually existing close to the surface had been temporarily displaced. Fogs seem to occur at times of steep inverted gradients; in other words, when the temperature of the 2,500-foot level is considerably higher than at sea level. The conclusion to be drawn from what precedes is that the summer fogs of San Francisco result from the chilling of the upper warm air over the ocean by the water, particularly the cold current close to the shore. The strong indraft through the Golden Gate on summer afternoons (see charts of hourly wind velocity, 3 to 7 p. m., fig. 6) carries with it the fog. The movement of the lower fog-laden air eastward and into the valley is compensated by a westward air movement at higher levels. The great difference of temperature between the valley and the ocean, often 50° within as many miles, is probably the prime factor in controlling the circulation. The mountain, as might be supposed, is the drier station, the mean relative humidity being 59 per cent, while it is 83 per cent at San Francisco. Especially during the summer months is the difference noticeable, and, doubtless, it is this dryness which causes such an agreeable "change of climate" to visitors at this season. The difference may perhaps be stated more clearly in this way: The weight of water vapor per cubic foot varies from 1.9 grains to 3.5 grains on the mountain during the year, while at San Francisco it varies from 3.3 grains to 4.4 grains. The average hourly wind velocity seems to increase with elevation, the values for the mountain station far exceeding those of the lower station. The maximum velocities recorded are, respectively, 91 and 47 miles per hour. The total wind movement was 177,017 miles at Mount Tamalpais and 96,602 miles at San Francisco. 1176-Bull. L-03-16 Mean actual pressure. Lowest temperature Dew-point Relative humidity. Total rainfall Greatest 24-hour rainfall.. Mean cloudiness.... Average hourly wind velocity. Prevailing wind direction Maximum wind velocity Partly cloudy days. Cloudy days Days with 0.01 inch rainfall Mean actual pressure.. Mean maximum temperature Lowest temperature Dew-point.. Relative humidity Total rainfall Greatest 24-hour rainfall.. Mean cloudiness... Average hourly wind velocity. Prevailing wind direction Maximum wind velocity Clear days...... Partly cloudy days. Cloudy days... Days with 0.01 rainfall. Mean daily range of temperature.. Jan. 40 Feb. 46 80 3.67 0.98 6.7 7.9 COMPARATIVE DATA. MOUNT TAMALPAIS, 1899. 27.62 27.65 27.53 27.54 27.53 27.51 27.50 27.49 27.56 66.8 71.0 73.3 56.3 69, 71 69 82 86 96 79 92 32 23 32 32 33 41 46 45 47 34 29 36 35 35 33 39 32 70 56 58 62 30 50 29 1.70 0.29 77 4.1 1.8 22.9 19.8 NW. NW. N. 84 NW. 76 NW. 7 78 NW. W. NW. 86 NW. 24 8 14 13 17 9 13 7 8 9 5 14 1 17 9 5 1 14 4 17 7 3 3 14 174.9 57 8.0 2.8 2 263.8 87 11.4 3.8 16 162.6 44 9.9 4.1 6 300.7 76 13.9 6.3 31 1 344.6 404.5 78 91 13.8 14.3 4.7 1 6.1 17, 074 14, 257 17, 821 15, 608 16,986 14,234 1 Mar. Apr. May. June. July. Aug. Sept. SAN FRANCISCO, 1899. 7 822188 39 43 3 13 365.1 382.4 0.00 0.00 1.2 17.6 86 13.0 3.1 1,019 NW. 61 N. 29 000 445.4 99 14.5 5.9 13, 108 294.1 65 11.3 2.1 11,356 29.98 30.00 29.89 29.87 29.87 29.78 29.78 29.78 29.83 29.83 29.88 29.98 78 80 74 80 80 75 73 94 65 34 42 43 43 47 48 46 48 37 45 45 45 49 50 52 50 52 44 84 78 86 T. 0.00 3.92 3.79 2.65 0.00 48 87 1.51 79 3.3 3.0 1.94 5.8 8.7 14.2 15.3 14.4 6.6 SE. W. W. SW. 12.6 SW. 40 W. SW. SE. 39 36 37 44 41 41 47 SW. 5 W. W. W. W. W. W. W. W. W. 30 11: 18 21 23 16 18 20 21 11 10 9 10 5 11 11 8 5 15 7 16 * 2 2 4 2 2 5 11 11 2 15 5 1 0 0 0 9 12 0 11 129.1 9 83 0 308.4 73 10.4 2.0 0 292.5 78 13.8 6 272.5 78 13.6 3.4 6,298 42 8.4 1.9 4,757 2.2 10,722 9,066 0 10 0 11 0 373.4 0 Oct. Nov. 37 63 0.00 4.26 7.48 4.65 36.86 0.00 1.32 2.51 0.83 8.0 4.6 2.51 2.0 4.6 4.1 17.1 18.2 16.7 23.9 20.2 NW. NW. NE. NW. NW. NW. 16 5 89 Dec. 41 80 56 W. 2 10 18 11 19 13 15 105.9 35 12 177.7 60 7.0 9.3 2.1 3.1 11, 996 17,782 76 N. 16 4 Annual. 10 190.5 64 10.4 2.3 6, 430 27.55 61.5 49.8 55.7 96 23 36 59 91 NW. 196 80 89 92 77 3, 342 73 11.7 4.3 177, 017 29.87 60.7 49.1 54.9 94 34 48 83 23.23 2.15 4.0 11.0 W. 47 SW. 185 104 76 67 52 312.3 69 11.6 2.6 96, 602 |
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