Originally Posted by
cyccommute
No, rime ice isn't different. It is, essentially, ice that forms from the gaseous form (aka water vapor) to the solid. It's the opposite of sublimation. If water vapor from car exhaust could form "black" ice, it would form it using the same mechanism passing from vapor to solid without going through the liquid phase.
I think you're confusing rime ice with hoar frost. Rime ice is formed from small liquid water droplets (like fog). Car exhaust is very visible in cold temps so a substantial portion of the water vapor is condensing before freezing. It's not going straight from gas to solid, or at least not all of it. In either case, black ice from car exhaust looks nothing like hoar frost or rime ice.
The ice that's getting formed on the street is different from fog freezing on a tree branch. A water droplet may freeze on contact with the street (like it would a tree) but it's getting hit by the warm exhaust and alternating bursts of cold air so it melts and refreezes perhaps many times while a car is sitting there. That is not the same process that produces rime ice or hoar frost. At least that's my theory. I could be wrong but as I've said before, they don't look the same.
Why are you arguing with people who experience this every winter?