Black ice is a very simple way to describe slippery and dangerous road conditions. It gets the name from a layer of ice that covers a section of road and is transparent due to freezing without very many air bubbles and appearing the color of the material beneath it, which is usually black asphalt.
At low temperatures (below 0°F/-18°), black ice can form on roadways when the moisture from automobile exhaust condenses on the road surface. Salt’s ineffectiveness at melting ice at these temperatures compounds the problem. Black ice may also form even when the ambient temperature is several degrees above the freezing point of water 0°C (32°F) if the air warms suddenly after a prolonged cold spell that leaves the surface of the roadway well below the freezing point temperature.
Always beware of the black ice! Bridges and overpasses can be especially dangerous, and accidents occur very quickly.