“All that is sunny does not glitter, not all those in the fog are lost.” –Karl the Fog
Today, I went to the San Francisco Bay with a view of the Golden Gate Bridge.
Karl the fog is an advection fog. This means that it is formed by wind moving moist air over a cold surface. In this case, the wind blowing inland moves moist air from the Pacific Ocean over the cool bay. As sunlight penetrates the fog and heats up the ground, the ground gives, or conducts, some of its heat to the cool air, heating the air up. As the fog moves inland, the air warms and the fog lifts above the surface as the warmer, less dense air parcel rises above other cooler and denser masses of air. Eventually, the air becomes warmed enough to totally evaporate the fog.
This is why in the morning, the fog forms just above the surface of the cool bay waters but, as the day progresses, the fog rises up and eventually dissipates.
Bay Area Earth Science 