Graff listened to Walter explain, the snow would cover much of the sound and help to dampen the vibration considerably.  Although he was talking more about the enemies Mark 1's than their own, "Thank you commander, but, uh...what about their eyes and ears?  Those turrets will chew up the lighter among us pretty quickly, especially when they get in the compound."  At the ranges some of the lighter folk need get in the fog won't really cover them from the turrets, for that matter anyone with range will be relying on shooting with sensors only, no visual confirmation that there's nothing between them and the target.  This fog will hamper both sides equally.