Why is San Francisco so foggy?

The fog in San Francisco forms when a warm airmass flows across a
cooler ocean - the cooler ocean cools the warm air and the moisture in
the warm air condenses and forms the fog.  This effect is similar to
why mirrors in the bathroom fog-up - water in the warm, moist air from
the shower, once in contact with the cooler surface of the mirror,
condenses onto the glass.
 
An explanation of this effect along with a description of types of fog
and mist:
http://www.rcn27.dial.pipex.com/cloudsrus/mistnfog.html#formation

A brief description of the situation from a weatherman in Florida:
http://www.click10.com/weather/quizdon/

A brief description from a meterology course explaining why the fog is
close to the ground, rather than higher in the sky:
http://tornado.sfsu.edu/geosciences/classes/m356/radiation_fog.html