TIL: Black ice is caused by tailpipe exhaust

I'd always assumed the moisture for black ice just came from the weather — it rains a little, then it freezes, and voila. But that's not the case. Black ice, in case this is a regional colloquialism that doesn't translate everywhere, is actually transparent ice. It's a thin layer of slippery stuff that forms on roads and is almost imperceptible to the eye. You look and see a normal road. You don't see the ice.

Technically, black ice can form from any source of moisture, but the big one turns out to be the droplets of water that condense out of vehicle tailpipe exhaust and dribble onto the roadway.

Image: Black Ice, a Creative Commons Attribution (2.0) image from cata13es's photostream

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  1. Hm. And I thought Black Ice came from computer viruses. You know, like in William Gibson? wink

  2. Beeki says:

    Everybody in Finland (that whatched tv in 1977) knows that black ice is caused by evil man painting the roads with the stuff.

  3. bwv812 says:

    I'm not super convinced by Maggie's interpretation of the Discover blog's explanation. If it's from the exhaust condensate that "dribbl[es]" onto the roadway (presumably from the tailpipe), then you would expect black ice to be highly localized and appear primarily below tailpipes. Maybe condensation from exhaust fumes coming into contact with the ground causes black ice, but I'm not buying that it's mainly dribblings.

    I've experienced black ice on roads with relatively little traffic, and given the speed at which cars travel, the amount of exhaust moisture emitted per car per meter, and the dissipation patterns of vehicle exhaust, I would be very surprised if the black ice I've experienced comes primarily from car exhaust as opposed to fog or other meteorological phenomenon. Maybe on heavily trafficked roads cars could be a real contributor, but I'm having problems buying this as the predominant contributor to black ice: if it was true, black ice would be very predictable based on temperature and traffic, but this isn't the case at all.

    The Discover blog's statement that "salt is also not as effective at freezing temperatures" is pretty ridiculous. Salt is really unnecessary in the absence of freezing temperatures, and is only effective after the temperature has dropped below freezing. And it's only when it's well below freezing (-21°C) that regular NaCl salt becomes ineffective.

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