Black Ice - On Roads and Pavements

On Roads and Pavements

Because it represents only a thin accumulation, black ice is highly transparent and thus difficult to see as compared with snow, frozen slush, or thicker ice layers. In addition, it often is interleaved with wet road, which is nearly identical in appearance. This makes driving or walking on affected surfaces extremely dangerous. Deicing with salt (sodium chloride) is effective down to temperatures of about -18 °C (0 °F). Other compounds such as magnesium chloride or calcium chloride have been used for very cold temperatures since the freezing-point depression of their solutions is lower.

At low temperatures (below -18 °C), black ice can form on roadways when the moisture from automobile exhaust condenses on the road surface. Such conditions caused multiple accidents in Minnesota when the temperatures dipped below -18 °C for a prolonged period of time in mid-December 2008. Salt's ineffectiveness at melting ice at these temperatures compounds the problem.

Black ice may 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 has left the surface of the roadway well below the freezing point temperature.

The term black ice is sometimes used to describe any type of ice that forms on roadways, even when standing water on roads turns to ice as the temperature falls below freezing. The American Meteorological Society Glossary of Meteorology includes the definition of black ice as "a thin sheet of ice, relatively dark in appearance, may form when light rain or drizzle falls on a road surface that is at a temperature below 0 °C."