Originally Posted by
asmac
Aren't you thinking of below zero Celsius, not Fahrenheit? And how does the hot exhaust move downward to the pavement? In my experience hot air rises and, when it's really cold, water vapour instantly crystallizes or just vanishes..
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.