Originally Posted by
Sixty Fiver
Your understanding of how black ice forms is lacking... -18C / 0F is that magical point where de-icers stop working and water vapour from exhaust can condense and freeze.
It might not look wet because of the lighting or lack thereof, and there may even be a little sand to make you think things are okay and then you gently touch the brakes to find that things are smooth as glass.
For the record, my Jeep is all wheel drive with ABS and excellent winter tyres and I have 30 years of accident free driving in extreme weather conditions... this morning was a highly cautious driving experience when I left the packed and icy streets where the AWD works really well to the streets that had been cleared and had built up black ice at every intersection.
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.