Mike McCarron, Director of Community Affairs for SFO, says flights are delayed anywhere from a few minutes to an hour-and-a-half because of low visiblity due to fog. Flights less than two hours in length are the most impacted.
"On a clear day we can land 60 planes an hour, on a foggy day you can only land 30," he says. "The schedule's designed to land 60. It's kind of like driving, when it's foggy you slow down. We can only use one of the two runways, because they're too close together to operate in bad weather."
McCarron says you should contact your airline before you head out for the airport.
That's something you should probably do anyway, but today, more so. Just sayin'...