Just to tell the end of the story, ACV may be the worst for fog, but nothing like pea-soup at SFO to foul up everybody for hundreds of miles around. My return from ACV to SFO was yesterday was booked on UA#5378 slated to depart ACV at 5:47pm. But since I was done early, I got on the standby list for UA#5374 slated to depart at 2:44pm. Because of morning fog in SFO (including, apparently, a full ground stop for awhile) everything was mucked up. The flight that had been due to arrive at ACV from SFO at 10:30am arrived at little past 3:30pm. I did clear the last seat on the 2:44pm flight back to SFO, but it didn't depart until 5:45pm. I do have to say that the Skywest gate agents at ACV were incredibly helpful and competent and patient. I have also never had anybody on United do anywhere near as well keeping the waiting pax updated every few minutes on the status of things.