Originally Posted by
pdlamb
Living in the temperate zone, I'd never heard of the special <0F definition of black ice. Black ice, as I understand it, is simply ice that's difficult to see.
I'd argue that the "refreeze" ice is pretty specific. It's been a few years, but I vaguely remember from chemistry class that salt won't melt ice below a certain temperature, which was either 7 or 9 degrees (F) below freezing. So perhaps you won't see "refreeze" ice on the road on a 25F morning, but be careful if it's down to 20F, because it's possible then. (And be careful in situations like Virginia on I-95, when it was impossible to pre-treat the road surface because it was raining while it froze!)
I've got family in Williamsburg, VA, and drive I-95 getting to/from DCA to their place ... I-95 is a nightmare on a sunny summer day, I truly feel for the people that got stuck in that mess. We have the same thing happen here every year or two, rain prior to a cold front with snow. There's not much to do but sit at home for a day or two, roads are just dangerous until the salt can cut through the mess.
Funny thing here right now, we've been so cold for so long that residential streets are just snow-packed ... you can't see the pavement. Main roads and highways are clear and dry, but the less traveled roads are gonna be snow packed for a while.