it happens alot into SFO or SAN...

I have been on several flights that have been delayed or diverted due to fog at either SFO or SAN... twice diverted (ONT and LAX) on an IAD-SAN transcon, three or four times delayed departure on SAN-SFO or SFO-SAN

I'm sure they try to avoid the possibility on long-haul int'l flights - if they can - but if the fog develops during the flight for instance I'm assuming it will divert... this has never happened to me on an int'l flight albeit... and its probably b/c they would have to hold you on the plane at the airport you diverted to (remember the Caracas-EWR flight on CO a few months ago which got stranded for 5 hours at BWI) - so I'm sure for PR UA will try to avoid this (fingers crossed)