Originally Posted by
dscheidt
Black ice is nearly invisible. That's the common thread between all the various definitions of it. It can form at freezing, it can form at oh-heck-its-cold -- any time there's liquid water on a surface that's below freezing.
There were some interesting bits of it on my ride home today. High today was something like 15F, but it was sunny. Late feburary the sun angle is high enough that the sun will melt ice even in quite cold conditions. The meltwater flows down hill, and when the sun stops hitting it, it freezes. It's particularly bad today, because it's not snowed in some time, so there's very little residual salt around, to keep the ice melted. Fortunately, there's enough auto traffic on the roads I ride to keep the ice to a minimum, by spreading the puddles out.
The other place we see it in Chicago is under railroad bridges. Water drips down, forming nice puddles of ice underneath.
"Black ice" is an over used excuse. It's only "invisible" to those who don't look. If the road is wet, near the freezing point of water and there is water in some form falling out of the sky, anyone using the roadways should expect slick conditions. I see far too many people who see a wet road at anything less than the freezing point of water, assume that it's just wet and drive at stupid speeds.