Popular Science Monthly/Volume 59/October 1901/Fog Studies on Mount Tamalpais
|FOG STUDIES ON MOUNT TAMALPAIS.|
By ALEXANDER McADIE.
LATE on a February afternoon the passengers on a large Pacific Mail steamship sighted the Farallones and doubtless thought as the pilot came aboard that the long run across the broad ocean not always true to its name was safely over and danger past. The 'Rio de Janeiro' came to anchor a little before six o'clock on Thursday night, February 21, 1901, and the weather being foggy, the captain wisely remained at anchor until about 4 a. m. when the fog lifted. The lights of the Cliff House two or three miles away could be seen and the vessel started on a northeast course with Lime Point dead ahead. There is some difference of testimony as to whether the Captain or Pilot gave the order to go ahead. The fog closed down again and the Pilot steered by the whistle hoping to get the echo from Point Diablo. No echo was heard. The vessel was not moving at full speed, the First Officer was standing on the starboard side listening for the Fort Point bell and the Captain and Pilot were on the bridge. No soundings however were taken. At about 5:30 a. m. the vessel struck the Fort Point Reef, backed off and within twenty minutes had gone from sight with 130 of the 210 persons aboard.
The two diagrams herewith show the general approach to the Bay of San Francisco and in more detail the probable path steered by the 'Rio' with zones of inaudibility of the fog signals.When all is said and done it appears that the fog was the prime cause of this appalling accident. Now, while an accident of such magnitude gives startling emphasis to the need of studying fog, a summation of the minor accidents for a single year due to fog in any large seaport would be equally impressive. One cannot cross the sea, run down the coast or even go over a bay upon a ferry-boat without experiencing at times this troublesome condition. Nor is it only when on the water that we are at the mercy of the fog. Study the statistics of railway accidents and you will be surprised how often, in the column giving the cause of collision or other accident, the word fog appears. Can we help ourselves? Yes; and the first step is to study patiently and systematically the various types of fog formation. Already the ability to communicate by means of wireless telegraphy between vessels at sea and the land removes the greatest element of danger to vessels caught in fog. The 'Rio de Janeiro' was lost at the entrance to
The region about San Francisco and Probable Conditions at Time of Wreck.
Probable Course of 'Rio de Janeiro.'
what might have been thought to be a well-protected harbor. That is, there were light-houses and fog whistles along the shore, but the vessel was helpless, nevertheless, when the fog closed down, for all guiding points were lost and, owing to the peculiar reflections and refractions of sound waves in the air, the whistles and bells, as the accident too sadly proved, were inaudible.
In the vicinity of San Francisco the processes of cloudy condensation in the free air are very active. It is no uncommon occurrence on summer afternoons, when the wind is blowing at the rate of twenty-two miles an hour, to see sharply marked fog drifts hang like white blankets over the city hills or stream through the Golden Gate like a spectral army. From the U. S. Weather Bureau Observatory on Mount Tamalpais, elevation about 2,400 feet, one looks down upon such remarkable fog formations as are shown in the accompanying illustrations.Now fog, like frost, may be considered to be largely a problem in air drainage. The condensed vapor, like the frozen vapor, indicates air motion with certain accompanying changes in temperature. Therefore the first line of study in connection with fog formation is concerned with temperature gradients; and chiefly the vertical gradient. Instead of the usual fall in temperature of 1° for each 183 feet elevation, we find in these San Francisco fogs an increase of temperature from sea-level upwards. In a given summer month the mean daily temperature at the upper station was eleven degrees or more warmer than at the lower station. If the rate of increase were uniform throughout the 2,500 feet, this would mean a rise of one degree for two hundred feet elevation. The rate is not uniform, and between the 1,500 feet and 2,000 feet levels is probably often as much as one degree for fifty feet. Days without fog are as a rule days without this steep inverted gradient, and it would seem as if the temperature throughout the entire mass of air was more uniform. Some approximate vertical sections of the temperature in a fog bank were obtained by carrying a
Fog Studies from Mount Tamalpais.
Marvin 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
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 removal of the fog for even a small distance in the neighborhood of a fog-bound vessel might be of advantage. The chief difficulty at present would seem to be the quick influx of the circumjacent fog. The supply of fog might be so great that our dissipators would seemingly produce no effect. The dissipation of fog and smoke in enclosed areas by electrical agencies, as strikingly shown in Dr. Lodge's experiments, leads to the wish to reproduce these experiments in the free air and upon a large scale. Moreover within the past two years there has been growing up a theory due chiefly to Zeleny, Elster, Geitel and T. C. K. Wilson concerning the part played by ions in causing rain. It is known that the negative ions move more rapidly than the positive ions and that water vapor will condense more readily on the negative ions. It may
be that under certain unstable conditions some of the more energetic ions, by relieving the electric tension, inaugurate the formation of the rain-drop. In studying the electrical potential of the atmosphere, it has been shown that the approach or retrocession of clouds, especially cumulus and cumulo-nimbus, could be determined by the changes in the potential values. There was also good reason for believing that the electrometer gave in certain fluctuations indications of the proximity of invisible vapor masses. Certainly the one instrument upon which we now rely in studies of fog formation and influence, the mercurial thermometer, is far from being a sufficiently sensitive instrument. Optical methods may furnish apparatus sufficiently delicate. It has been claimed by some that the polarization of blue sky light can be used in studying the vertical distribution of fog, and that changes in atmospheric conditions are shown by this means several hours in advance of other precursory appearances.
Strangely enough within the past year and from an unexpected source, suggestions have been made which should be considered with some care and then tested. In discussing the mortar batteries used at Windisch Feistritz, Dr. Pernter has given us some data concerning vortex rings. These are the rings which, according to Burgermeister Stiger and his associates, successfully protect their vineyards from hail. Whatever the real cause may be regarding hail, we are thankful for the opportunity to study such large and energetic vortices. These rings are powerful enough to tear a thick paper screen to pieces at a distance of 100 meters. On leaving the mortars in a horizontal direction the whirls have a velocity of about 170 miles per hour or eight times the velocity of the stiff surface indraft of air on summer afternoons through the Golden Gate. At a distance of 100 meters the velocity was reduced nearly 50 per cent. With the Suschnig apparatus, the charge of powder being 250 grammes, Dr. Pernter found an initial velocity of about 55 meters per second. The probable limit of upward movement was 400 meters. Dr. Hann has suggested that the results obtained by shooting these rings into winter fogs should be carefully studied. The suggestion is pertinent. At Mt. Tamalpais, as we have tried to show, unusually good opportunities exist for experimenting upon fog. Many varieties of formation occur. The tule fogs of winter, in one of which the 'Rio de Janeiro' was lost, sometimes do not exceed 100 feet in depth. The summer afternoon sea fogs are more dense and more sharply delined. Some of the fogs are due to direct cooling by contact; in some the cooling is due to radiation, and, in the great majority of cases, the cooling is due to mixture. The differences in temperature, humidity and air motion are so marked that it is likely that differences in electrical potential, dust-content and ionization also exist. There is urgent need of bettering our knowledge of these matters. Practical applications will speedily follow.
The sacrifice of life on that ill-fated steamship on the morning of February 23 will not have been altogether in vain, if it leads to a thorough study of the conditions governing fog.