Originally Posted by
cyccommute
The only component of a car's exhaust that can condense is the water vapor and, due to the properties of water and hydrogen bonding, form "clouds" that are visible. But, as I pointed out post 156 above, even if every last drop of water could be wrung out of a car's exhaust...which is impossible...there isn't enough water in car's exhaust (or even hundreds of car's exhausts) to cause icing.
But you made some assumptions in that post that weren't true. The black ice you get from exhaust in extreme cold isn't anywhere near 3 mm thick. It's extremely thin. It also doesn't form on horizontal surfaces, it's on the streets, typically in patches around intersections and not on the sidewalks nearby even. So the amount of water required is much much much less than what you're imagining. It doesn't happen at 20° or 30°, it happens at -20° or -30°, - temps you probably don't see often in Denver. Boiling water will freeze instantly when thrown in the air under those conditions.
It's not that hard to believe when you think about it. At those temps the exhaust from my own face will produce quite a bit of ice on the stuff around it, often including (unfortunately) the inside surface of the lenses of my goggles.