The weather in the San Francisco can change very quickly; in summer, a foggy morning often turns into a completely clear afternoon. On this day however, the fog rolling into the Bay through the Golden Gate was already fairly low after sunrise, only just covering the deck of the Golden Gate bridge (just under the bottom of the frame) but not San Francisco itself.
Fun fact: I took this picture one day after this picture; I was lucky to get a morning with perfect fog right after a clear morning. This particular shot is also part of a timelapse to be published in my 'Sunrise in the Bay' video. This is currently still in progress, but a preview can be found here.