My sister used to work for United a couple of years back as ground crew at a small airport that flew directly to SFO. She always told me how notorious SFO was for weather problems, and would cause flight delays or cancellations day-to-day (multiple times a week) with only a couple of hour's warning.
SFO's issue is not "weather" so much as it's a combination of the local conditions and the airport itself.
SFO has two sets of parallel runways, and usually likes to use 28L/28R for arrivals. There are plenty of protocols for airports to do simultaneous landings on two runways parallel and next to each other, but (simplifying a bit) SFO has the slight problem of them being too close together to do instrument landings for both runways at the same time.
This leads to compromise protocols which rely on the ability of one plane to maintain visual contact with the other plane when doing simultaneous landings. But of course in foggy/cloudy/rainy weather, visibility goes away and SFO can no longer handle simultaneous landings on those runways, which basically cuts the capacity of the airport in half.
