Originally Posted by
tjspiel
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.
No, I'm not confusing them. They all form from the same mechanism. Hoar frost and rime ice both form from water condensing out of the atmosphere. Droplet size has little to do with the mechanism and water vapor could be consider very, very small droplets.
"Black ice" as defined by the
American Meterological Society as
black ice
1. Thin, new ice on freshwater or saltwater, appearing dark in color because of its transparency, which is a result of its columnar grain structure.
2. On lakes, black ice is commonly overlain by white ice formed from refrozen snow or slush.
3. A mariner's term for a dreaded form of icing sometimes sufficiently heavy to capsize a small ship.
4. A popular alternative for glaze. A thin sheet of ice, relatively dark in appearance, may form when light rain or drizzle falls on a road surface that is at a temperature below 0°C. It may also be formed when supercooled fog droplets are intercepted by buildings, fences, and vegetation.
Definition 4 is the one that I'm referring to and is always recognizable by a wet appearance. I've never seen "dry" water ice.
Originally Posted by
tjspiel
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.
Watch the exhaust from cars the next time you are out in the cold. Even in the coldest of conditions and even when the tailpipe exhaust is at it's thickest, the fog rises up. It doesn't go down.
Originally Posted by
tjspiel
Why are you arguing with people who experience this every winter?
Um, I
do live where we have winter. I experience rime ice (aka freezing drizzle), hoar frost (aka freezing fog), freezing melt ice, snow, snow pack, polished snow pack, ice from compacted snow pack and every form of snow and ice that you experience. The only thing I've
never seen is this "invisible" "black" ice you guys are talking about. Granted, Colorado weather can be quite goofy...it was 65°F a week ago and back in November we dropped 50°F in 2 hours and 75°F in 12 hours...but that goofy weather would cause us to have more of the "invisible" black ice from car exhaust than you do. When you drop 50°F in 2 hours all kinds of things come out of the sky.