Science Fair Project Encyclopedia
Black ice is a thin coating of 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, so perhaps the term.
It is usually deposited by freezing rain, mist, or fog. Because it contains relatively little entrained 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 roadway forms of ice). 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.
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 wind 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 also refers to the slick layer that forms on roadways from a light rain after a period without any precipitation. The water mixes with the road dust and oil but doesn't wash away. This leaves the road surface as slick as ice, slicker than just being wet with water. This type of road hazard can occur any time of year, but especially in mostly dry summer months.
In either case, while definitely a real phenomenon, "black ice" may 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.
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