Page:Popular Science Monthly Volume 59.djvu/549

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539
FOG STUDIES ON MOUNT TAMALPAIS.

vin meteorograph from the summit to the valley and back, the descent and ascent requiring about 100 minutes, the distance being about 16 miles. The instrument was hung at the top of an open canopied car in such a way as to secure a good air circulation. The average dew-point was 50° F. (10° C.) and the maximum weight of a cubic foot of water vapor at this temperature and saturation 100 per cent, is a little over 4 grains. Estimating an average fog bank as covering an area of fifty square miles and extending from the 500-foot level to the 1,500-foot level, the maximum weight of the water vapor would be about 400,000 tons; and if this condensed vapor could be suddenly precipitated it would be the equivalent of a rainfall of about one-tenth of an inch. But condensation and precipitation are not identical. The processes

PSM V59 D549 Temperature records.png

Temperature Records.

which cause the collapse, if it may be so called, of a cloud of fog are obscure. Elaborate experimentation is needed at this point before the problems of fog dissipation or rain-making can be solved. With the fogs of the San Francisco Bay district and indeed with every dense cumulo-nimbus or cumulus cloud, the condensation is considerable and it would seem at times as if but a very gentle initiative would lead to precipitation. Various methods of removing dust particles from the atmosphere have been suggested within the past few years, and possibly in thus removing the dust the essential nuclei of condensation may be removed. Conversely, in order to bring about precipitation, it may be found necessary to supply at the proper time the proper nuclei. At any rate, it is known that by various methods, of which may be mentioned filtering, clarifying, recondensing, calcining and electrifying, smoke, fog, dust and condensed vapor may be removed from limited spaces. The