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 ." Because it represents only a thin accumulation, black ice is highly transparent and thus difficult to see compared with snow, frozen slush, or thicker ice layers. In addition, it is often interleaved with wet pavement, which is nearly identical in appearance. This condition makes driving, cycling or walking on affected surfaces extremely dangerous. Deicing with salt (sodium chloride) is effective down to temperatures of about . 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.