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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.
Additional recommended knowledge
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.
It is usually deposited by extremely cold rain droplets, mist, or fog. The process of freezing is slowed down due to latent heat given off in sublimation, allowing the rain droplets to flow and merge together on the surface forming a film before freezing into clear ice. Nevertheless, 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, rime ice, or other typical forms of ice on roadways). In addition, it often has a matte appearance rather than the expected gloss; and often is interleaved with wet pavement, which is identical in appearance. For this reason it is especially hazardous when driving or walking because it is both hard to see and extremely slick.
Black ice may form even when the ambient temperature is several degrees above the NTP freezing point of water (0°C). This occurs typically (and treacherously) when terrain contours and/or prevailing winds cause a local steep differential of atmospheric pressure and/or temperature, or when the atmosphere has warmed up after a prolonged cold spell that leaves the temperature of the ground and roadway well below the freezing point.
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 temperature to drop more rapidly than on regular pavement. This is often indicated with "Bridge Ices" warning signs. It is often missed in the investigation of motor accidents due to the fact that it often melts quickly, before investigators can detect it. Black ice is one of the possible causes of a tour bus accident that killed Metallica bass player Cliff Burton.
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.
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, and provides conditions that make ice skaters ecstatic: effortless gliding and extremely smooth, if slow, stopping. (One can skate sideways on black ice, using the right blade angle.)
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Black_ice". A list of authors is available in Wikipedia.|