While taking a walk in the Presidio the other day, I was treated to this spectacle. Looking through the trees along the path and over Presidio Parkway, I watched fog slide in from the straits, slip under the Golden Gate Bridge, and cover the north side of the bay. Eventually it covered the roadway of the bridge, leaving the towers visible.
Meanwhile the south side of the bay, where I was walking, stayed clear. In fact I spent the rest of the afternoon in the Presidio and enjoyed warm, sunny weather, while the bridge and the bay remained shrouded. Sometimes the fog does that: goes just so far and stops.
Fog's gotta do what fog's gotta do.