This is a pretty tricky skull to explain but I'll try my best. Okay the fog skull is on the level [b]Floodgate[/b]. It takes away the whole radar so you don't know what's behind you. To me I think this is okay considering you can go on one path most of the game and not have anything pop up behind you. Anyways, onto finding the skull.
Take out a battle rifle/carbine (just a long range type weapon or something that isn't automatic).
Walk to where there are sand bags. They should be close to the AA gun. Look to the first building on you're right.
Stop and look along the top edge of the building.
Keep walking forward and a flood should appear on the top about to jump.
Shoot it and the skull should fall down to the ground.
Remember to not shoot it too early or else the skull will stay up on the roof! Now you have another skull and 10g. If you need any help send a friend request to J Manizzy.
Oh yeah, if you want the skull and the 10g, pick up the skull after step 5. Happy Skull Hunting.