Originally Posted by
FBinNY
The thing to keep in mind about black ice is that it's different than normal cold weather ice or icy roads in general.
Black ice forms only within a narrow temperature band near 32F. (lower on salted roads). There are many techniques for riding safely on icy roads, but black ice hazards relate to riding on roads that are generally clear or unfrozen. So it's the element of surprise that creates the greatest danger.
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.