What’s the Deal With Cloud Delays

In what has become a fairly regular occurence while flying out of SFO, I find myself delayed by over an hour due to “cloudy conditions.” I get that it’s foggy – it was in evidence as James and I caught a cab back from the St Regis. And I get that fog isn’t great for visibility – I used to have to drive home through it all the time when I was living in Maine. But at some point I was lead to believe that the actual landing was mostly a matter of instruments, radar, and other air traffic wizardry. It’d be one thing if there was no visibility on the ground, but there’s not that much fog. So I’m not entirely clear on why we’re being delayed. Snow, ice, even high wind – those are obvious reasons to put everything on hold – but fog? Maybe someone with pilot experience can clear that up for me.