Black ice, which can also be called gray ice, is hard-to-see ice on the roadway and causes slick conditions for drivers.
The road may look normal or even a little wet when there are patches of black ice on top.
It forms when temperatures fall below 32 degrees Fahrenheit and there was already rain, snow, or slush on the roadway. It’s especially prevalent in the early morning hours when snowmelt has refrozen on surfaces overnight.
Black ice can also develop when the water droplets in fog freeze on contact with a road or bridge. Such black ice, or freezing fog, tends to occur first on bridges or overpasses, because the cool air beneath allows the road surface to cool faster, than if the road was on top of ground, which retains more heat.
Drivers should be especially careful when they encounter fog and know temperatures are cooling toward 32 degrees.