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The following notes on "Fog at Mount Tamalpais " are reprinted from the Monthly Weather Review, November, 1900, and January, February, and March, 1901:

In fig. 21, Plate I, is shown perhaps the most common type of fog. It may be of interest to compute roughly the weight of water vapor existing under such conditions. From a number of records, a fair average dew-point temperature is 51° F. (10.6° C.). It is estimated that an area 10 miles east and west and an equal distance north and south is covered with fog The upper level of the fog may be taken as half a mile. If the fog were solidly packed, we could not be much in error if we estimated its bulk at 50 cubic miles.

There are, therefore, 52803 X 50 cubic feet of water vapor at a mean temperature of 51° F. A cubic foot of vapor at this temperature weighs 4.222 grains, and we therefore have as a gross weight 2,219,535 tons of 2,000 pounds each. But most generally the fog disappears between sea level and 1,200 to 1,500 feet altitude, and there are also wide swaths or channels fog free. The amount given above, therefore, would need to be cut in two, and a liberal estimate of the weight of the water vapor in a fog outside the Heads is 1,000,000 tons. This is carried through the Golden Gate by westerly winds, blowing 22 miles per hour, from 1 to 5 p. m.

For each square mile of surface there would be about 10,000 tons of water vapor and for each acre about 15.63 tons. This is equivalent to a rainfall of 0.14 inch.

In Waldo's Modern Meteorology a an example in the use of Hertz's graphical tables for following the changes in a given quantity of water vapor under varying conditions is given. With little change, the problem will apply in this case.

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CHART

DAILY FOG BELT

1ON1

SAN

BAY

SAN

FRANCISCO

Golden Gate

OCEAN

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PRESIDIO

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BAY

FRANCISCO

FRANCISCO
AND VICINITY

Forecast: Cloudy with

Fog to night, fair
Wednesday.

FIG. 20.-Fog service at San Francisco. Corner of large map standing in main corridor of Ferry Building. By means of frequent reports from Point Reyes and Mount Tamalpais the extent and character of fog over Drakes Bay, the roadstead, and the Gate itself are known in the city.

At San Francisco the mean actual pressure is 29.87 inches (758.7 mm.) and at Tamalpais 27.55 inches (699.8 mm.); the elevation of the latter station is 724 meters, and the former is practically at sea level.

With a pressure of 750 mm. and a temperature of 27° C. (80° F.), a given mass of air, half saturated, lifted upward under adiabatic conditions, will not change its initial 11 grams of water contents per kilogram until at an elevation of 640 meters, when condensation would begin. At an elevation of 700 meters, the pressure being 687 mm., the temperature would be 19.3° C. (67° F.).

At 640 meters the dew-point would be 13.3° C. (56° F.) or 2.5° C. lower than the initial dew-point 15.8° C. (60° F.), the difference being due to the increased volume. At 1,000 meters the temperature would be 8.2° C. (49° F.), or at a rate of 0.51° C. decrease per 100 meters elevation.

It is pointed out, however, that in all theoretical values the assumption is made that the kilogram of mixed air and water vapor retains its mass unchanged, but this can not be the case with a mixture in free air performing a journey of any extent. It is also to be remembered that in the actual case before us the horizontal movements of the given mass would be of far more significance than the vertical movements.

a Page 236. The paper in full is translated in Professor Abbe's Mechanics of the Earth's Atmosphere, No. XIV, pp. 198-211. [Improved methods are given by Professor Bigelow in his Report on the International Cloud Observations. Washington. 1900.]

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FIG. 22.-LIFTED FOG. HEIGHT ABOVE GROUND ABOUT 500 METERS.

PLATE I.

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FIG. 23.-SEA FOG POURING OVER SAUSALITO HILLS AND THROUGH GOLDEN GATE.

FIG. 24.-FOG WAVES.

PLATE II.

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