The ocean is dark blue, the land is green (cool) to orange (hot). Clear sky is light blue and fog is white. The yellow arrows represent rising hot air. We start the cycle at an arbitrary time.

1: Interior land has been heated by the sun, and hot air is rising there. The air replacing this hot air is cool air pulled in from the ocean.

2: Fog is beginning to be pulled over costal land by rising hot air, while the interior continues to heat up.

3: Continuation of frame 2.

4: Fog is starting to cool the costal land and eliminate the rising hot air there.

The Summer Fog Cycle

5: Continuation of frame 4.

6: Fog has cooled the land and eliminated the wind which was pulling fog from the ocean.

7: Cool new fog isn't being pulled in from the ocean; the old fog is heating and dissipating.

8: Now the interior land is warming and the rising air currents are generated again.

The animation above is of course an idealization. In many cases the cycle only continues for the first few stages. In other cases it lasts much longer than normal. Frequently the fog only blows through the Golden Gate, generating a plume that moves over the middle of the Bay toward Berkeley. A small cycle may occur on a daily basis.

However, the basic principals still apply: fog comes from the cool ocean and is drawn into the interior by rising hot air.