San Francisco fog

For other uses, see San Francisco Fog.

Fog over San Francisco Bay

The fog of San Francisco Bay is a specific type of fog. It is a sea fog identified as advection fog, which is characterized by the lateral transfer of temperature by wind blowing over cooler water. In circumstances such as these, often, the water is cool enough to lower the temperature of the air to the dewpoint, causing fog generation. Coastal areas having Mediterranean climates, such as that of San Francisco, have especially frequent sea fog blowing off the ocean to just a few miles inland. Once the fog has been generated as such, heated air in California's Central Valley lowers the air pressure, causing the relatively higher pressure air out at sea to force the fog inland into the San Francisco Bay Area.

San Francisco's fog is created when warm, moist air blows from the central Pacific Ocean across the cold waters of the California Current, which flows just off the coast, creating a cool, moist wind.

San Francisco is called The Naturally Air Conditioned City because of the frequent fogs that roll in from the Pacific Ocean, especially during the summer.

San Francisco panorama from Twin Peaks

Further reading

• "Advection" and "Advection Fog" Facts on File Dictionary of the Weather and Climate. Ed. Jaqueline Smith. Facts on File, Inc: New York. 2001.
• D.J. Croft. "Fog" in Encyclopedia of Atmospheric Sciences. Eds. James R. Holton, Judith A. Curry, and John A. Pyle. Academic Press: New York. 2003.
• Ross Reynolds. Cambridge Guide to the Weather. Cambridge University Press: New York. 2000.