Ice fog is nasty...little ice crystals floating around like tiny, tiny snowflakes. You almost don't notice the stuff, but the effects are as bad as black ice, and black ice doesn't melt on the windshield.

I was driving in to work last night in sub -20F, and felt the car getting squirrely, but there was nothing visibly wrong until I flipped on the high beams for a second and saw the ice in the air.