Page:Popular Science Monthly Volume 59.djvu/547

Rh 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