What the heck is black ice?
Wisconsin winters are at once beautiful and treacherous. One threat we deal with when driving through the frozen months is “black ice.” But what is it, exactly? Glad you asked. Black ice is a deceptive hazard that forms when a new layer of clear ice forms on water. The ice can appear dark in color because it has trapped fewer air bubbles (which are what can usually make ice appear white) and as a result is almost completely transparent, showing the black color of the roadway beneath it. This can make a roadway appear merely wet rather than covered in a sheet of ice. Freezing rain causes this condition to occur because it adds new layers of water to the roadway and very quickly turns it to ice. The temperature of the ground, where ice actually forms, is always lower than what your thermostat, more than likely placed above ground, reads. Remember to take caution in the coldest days of winter when roadways or sidewalks appear wet.
Read more here about how black ice forms.