FOG STUDIES ON MOUNT TAMALPAIS. 



537 



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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 (he accident too sadly 

 proved, w^ere 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 Avhite 



blankets over the city hills 



or stream through the 



Golden Gate like a spectral 



army. From the U. S. 



Weather Bureau Observa- 

 tory on Mount Tamalpais, 



elevation about 2,400 feet, 



one looks down upon such 



remarkable fog formations 



as are shown in the accom- 

 panying 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. There- 

 fore the first line of study in connection with fog formation is con- 

 cerned with temperature gradients; and chiefly the vertical gradient. 

 Instead of the usual fall in temperature of 1° for each 183 feet eleva- 

 tion, 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 in- 

 verted gradient, and it would seem as if the temperature throughout the 

 entire mass of air was more uniform. Some approximate vertical sec- 

 tions of the temperature in a fog bank were obtained by carrying a Mar- 



