The fog of San Francisco Bay

San Francisco Bay

San Francisco Bay is a shallow, productive estuary through which water draining from approximately forty percent of California, flowing in the Sacramento and San Joaquin rivers from the Sierra Nevada mountains, enters the Pacific Ocean...

is a specific type of fog

Fog

Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...

. 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

Mediterranean climate

A Mediterranean climate is the climate typical of most of the lands in the Mediterranean Basin, and is a particular variety of subtropical climate...

 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

Pacific Ocean

The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

 across the cold waters of the California Current

California Current

The California Current is a Pacific Ocean current that moves south along the western coast of North America, beginning off southern British Columbia, and ending off southern Baja California. There are five major coastal currents affiliated with upwelling zones...

, 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.

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
  Judith Curry
  Judith A. Curry is an American climatologist and chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. Her research interests include hurricanes, remote sensing, atmospheric modeling, polar climates, air-sea interactions, and the use of unmanned aerial...
  
  , and John A. Pyle. Academic Press: New York. 2003.
• Ross Reynolds. Cambridge Guide to the Weather. Cambridge University Press: New York. 2000.