I actually had low cloud cover built into CraterMA's TT at one stage, permanent clouds starting at 1.5k above each V base. Then HTC changed the cloud system. I have since submitted a new weather system for the terrain with heavy cloud cover over the TT area but I guess currently it would be too much of an FPS hog for many older PCs. Maybe it will get introduced with the next version of the game as I've read clouds are going to be redone to make them work better.