Originally Posted by
cyccommute
No, my understanding of how "black" ice forms isn't "lacking". Ice, of any variety, can form below 0°C...i.e. the "freezing" point of water. If the temperature is 0°C and there is air is at the dew point, ice can form. The water coming out of the air can be rain or snow or fog but if the temperature is below the freezing point of water, it is reasonable to expect some kind of ice formation. You drove on snow packed and icy streets today. Was any of it "invisible"? Have you ever been JRA or JWA and slipped on ice that you couldn't see or didn't have any reasonable expectation of knowing was there. dscheidt said "there are lots of situations where the ice has formed where none of your conditions [are met]." None may mean something else to him but at least one of those conditions has to be met and that is a temperature below the freezing point of water. Having a moisture source is another. If the temperature is above the freezing point, ice can't form and if there is no moisture source, ice can't form.
Nor is -18°C a "magical point" for either de-icers. -18°C is a point where only one deicer becomes ineffective and even that depends on the concentration of the deicer. Sodium chloride is ineffective below -7°C in "normal" concentrations. Magnesium chloride is the one that becomes ineffective below -18°C. Calcium chloride is ineffective below -32°C. And change the concentration of anyone of those salts and you'll change the freezing point depression.
I'm even dubious about the water vapor from exhaust causing the problem. Assuming 2,2,4-trimethylpentane (isooctane), a gallon of gasoline has a mass of 2.7 kg. Combusted, it makes 3.8kg of water and 8 kg of carbon dioxide. Assuming 26 miles per gallon, .14 kg of water per mile of driving (sorry about mixing units but it's easier to think that way). For every foot of that mile, that car is putting out 0.027g of water. That's not a lot of water per foot. A cubic foot of air can carry 1.2g of water per cubic meter at -18°C. That's the exhaust of 44 vehicles assuming that the water stays in one place which it wouldn't. It's going to be mixed with a lot of dry air from everywhere around the car. The water may come out of the air but I doubt that the exhaust from vehicles is the major source.
Still not getting it after a lot of years...
On one of my morning routes the daily traffic volume is 40,000 cars which sit at lights, idling... bring the temperature down to that where the de-icer stops working (our locality is generally -18C) and that water vapour condenses on the roadway, does not melt, and makes it like a curling rink that has just been misted.
We know when water freezes and that snow can be compressed and polished... that compressed snowpack and ice that has formed is highly visible and those roads are also rough and rutted with no sanding so drivers expect them to be slippery and most drive accordingly.
Getting back to that 40,000 car a day route, the cumulative effect of cold temperatures and high traffic volume equals black ice (we have had no precipitation) and drivers do not realize how slippery the roads are until they touch their change speed by accelerating, braking, or change lanes.
When the sun hits and temps come up few degrees the whole situation changes save for those shaded areas where the road just looks like a clear road... carnage usually ensues.