What is ‘black ice’?
The term ‘black ice’ refers to either a new layer of transparent ice on water, which allows us to see the deep water below, or a layer of clear ice on a roadway, which makes for hazardous driving. In both cases, the ice is transparent, not black, and so it shows the color of the underlying surface.
The ice is clear because no air bubbles are trapped in it. Lots of trapped air turns ice white; snow is white because of air trapped between crystals.
Driving on a road covered with black ice is particularly dangerous because the roadway can appear to be merely wet, so drivers may not recognize the slippery conditions until too late. Similarly, a sidewalk covered with clear ice will look dark gray – like a wet sidewalk. This ‘grey ice’ can be hazardous to walking!
Because bridges span the open air, they cool faster than other roadways, so black ice often occurs first on bridges. That’s why we see those warnings that “Bridge May Freeze Before Road Surface.”