Actually, I’ve seen low clouds at SFO at all times of the day. It could come at anytime during the day. I was there for my SEA flight at 5am in the morning and there were low clouds. I departed SFO for CLT and there were low clouds
During the winter, low clouds come even more often so SFO can be very unreliable during their busiest times. And this can come especially if the 19s are used for landings and 10s are used for takeoffs. They can’t handle it.
SJC in the winter despite bad weather, can get you to your destination with minimal delay. I’ve tracked SJC and it’s longest delays during winter are mainly because of the flight’s destination airport.
Unless we talk about destinations, SJC is more convenient than SFO especially in winter and rainy seasons