Black ice
From Wikipedia, the free encyclopedia
- For other uses, see Black ice (disambiguation).
Black ice is ice that is transparent. It has different meanings depending on context, but is most often used to refer to highway ice that increases the risk of accidents.
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[edit] On highways
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 also is unusually slick compared to other forms of ice on roadways.[citation needed]
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. 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.
"Black ice" may also be cited as the cause of a car crash when, in fact, the real root cause may have been excessive speed, inattention, or some other fault on the part of the driver.
[edit] Ice skating
In northern New England, black ice traditionally was 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.)
[edit] Maritime black ice
Black ice is a danger for cold-weather fishing trawlers. As ice forms on its superstructure, a boat can become too 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.
