SFO is even worse during the fog -- if there is insufficient visibility, they have to close one of the two runways because they're too close to use both using instruments.
It’s actually a bit more wild than that! SFO is always too close to land both runways at once via IFR alone. The second tail plane is required to land while maintaining separation visually, which can’t be done in bad weather. Also, since wakes are what they are, the second plane has to be the heavier one. Which is how you get extremely large planes landing at a major international airport under requirements in part for VFR.
