<Li> A mariner's term for a dreaded form of icing sometimes sufficiently heavy to capsize a small ship; </Li> <Li> Another term for ice on rocks in the mountains known equally as verglas (glaze ice). </Li> <P> The American Meteorological Society Glossary of Meteorology includes the definition of black ice as "a thin sheet of ice, relatively dark in appearance, (that) may form when light rain or drizzle falls on a road surface that is at a temperature below 0 ° C (32 ° F)." 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 pavement, which is nearly identical in appearance . This makes driving, cycling 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 . </P> <P> At low temperatures (below − 18 ° C (0 ° F)), 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 (0 ° F) 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 . </P>

At what temperature does water freeze on the roads