Black ice

Black ice is ice frozen without many air bubbles trapped inside, making it transparent. Black ice takes the color of the material it lies on top of, often wet asphalt or a darkened pond. Its difficult-to-detect nature makes it a significant hazard to drivers, pedestrians, and sailors.

On Roads

Black ice, also known as "glare ice" or "clear ice," typically refers to a thin coating of glazed ice on a surface, often a roadway. While not truly black, it is transparent, allowing the usually-black asphalt/macadam roadway to be seen through it, hence the term. It is unusually slick compared to other forms of roadway ice.

Because it contains relatively little entrapped air in the form of bubbles, black ice is transparent and thus very difficult to see (as compared to snow, frozen slush). In addition, it often is interleaved with wet road, which is identical in appearance. For this reason it is especially hazardous when driving or walking because it is both hard to see and unexpectedly slick.

Bridges and overpasses can be especially dangerous. Black ice forms first on bridges and overpasses because air can circulate both above and below the surface of the elevated roadway, causing the pavement temperature to drop more rapidly. This is often indicated with "Bridge May Be Icy" warning signs.

Black ice may form even when the ambient temperature is several degrees above the NTP freezing point of water (0°C) if the air warms suddenly after a prolonged cold spell that leaves 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. However, this use of the term black ice is not included in the American Meteorological Society Glossary of Meteorology.

A similar hazardous condition can also occur when diesel fuel spills onto a road surface because the lighter fractions evaporate quickly to leave a greasy slick which is difficult for oncoming drivers to spot in time to prevent skidding.

Ice skating

In New England, "black ice" refers to a clear type of pond ice that forms in very cold weather. Black ice has the appearance of thick, slightly cracked glass laid on the water, and its transparency reveals the darkness of the pond beneath: hence the name. Black ice is very hard and smooth, making gliding easier, but stopping slower and more difficult.

Thin, clear ice also has acoustic properties which are useful to tour skaters. Skating on clear ice radiates a tone whose frequency depends on the thickness of the ice.

Maritime black ice

Black ice is a danger for cold-weather fishing trawlers. As ice forms on its superstructure, a boat can become top heavy, and in rough weather this unbalanced extra weight may capsize it. Thick layers of black ice can form rapidly on boats where they encounter a combination of air temperatures cold enough to freeze seawater and rough seas that splash seawater over the entire boat.