I'm not a fan of rain or night time, but fog is actually kind of cool. It can take maps everybody knows, and throw a kink in them. One time fog was so thick that people didn't even know they were playing on the map "Field by a river" for a few minutes into the round (some didn't know for multiple rounds if they died early). It changes where people naturally flow on the maps, which is a good thing IMO.
Only problem with weather like "fog" is that it's client side only, meaning you can join before or after and not get hit with fog. It should be a server variable.