As for the reasoning behind the Clairvoyance thing, er... it's enhanced visual acuity up to seeing the future. Whilst that would help with identifying movement in the mist and other clues to indicate where somebody is... fundamentally, the reason you can't see through fog is because the fog itself is in the way. Seeing right through magical fog as if it's clear air is the same as using clairvoyance to see through a wall.