Originally Posted by
FBinNY
Air temp is less critical than ground or pavement temp in black ice formation. There are many ways black ice can occur, including vapor deposition (ie. dew) melt/refreeze, etc.
As far as car exhaust is concerned, the exhaust doesn't have to reach the pavement. As the exhaust hits the cold air, the vapor in it condenses (why we see the "steam") and settles on the road surface. The process is nearly identical to dew formation which causes major black ice issues in places like the Plain of Venice. It's also the same basic process that causes skiers to form ice beards.
So there's one label that can describe many similar problems. What makes it Black Ice isn't how it forms, but the fact that it's thin and just about invisible, so there's no warning before you're on it.
BTW - you get the most black ice when air temps are near o slightly above freezing, and the pavement is well below freezing.
I expect that's why we rarely if ever saw black ice in Winnipeg though it's a phenomenon in the East. Prairie air temperatures are too cold -- rarely hovering around freezing -- and the air is too dry. If water vapour precipitates (i.e. if the air can't hold the water vapour at that temperature) the precipitate freezes in the air and turns directly to ice crystals.
Your point is taken that moisture-laden air (whatever the source of the moisture and more likely natural moisture than exhaust), just-above-freezing air will deposit the moisture on colder surfaces. Hence "bridge ices" signs. Those 32F/0C temperatures lead to treacherous conditions.
And I just picked up my 45nrth Gravdal studded tires so bring on the ice.