Black ice
From Wikipedia, the free encyclopedia
Black (or Glare) Ice is ice frozen without many air bubbles trapped inside, making it transparent. This type of 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.
Contents |
[edit] 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, and is a factor in some car accidents.
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.[1]
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 the cause of "Bridge May Be Icy" warning signs indicating potentially dangerous roadway conditions.
At low temperatures (below 0°F/-18°C), black ice can form on roadways when the moisture in automobile exhaust condenses.[2] Such conditions caused multiple accidents in Minnesota when the temperatures dipped below 0°F for a prolonged period of time in mid-December 2008.[3] Salt's ineffectiveness at melting ice at these temperatures compounds the problem.[4]
Black ice may form even when the ambient temperature is several degrees above the STP freezing point of water 0°C (32°F) 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.[5]
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.
[edit] 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.
[edit] See also
[edit] External links
- Is there really such a thing as black ice?, from the Straight Dope staff
[edit] References
- ^ Nancy Templeman (December 1 1997). "Black Ice Is Dangerous Wintertime Road Hazard". Virginia Polytechnic Institute and State University. http://www.ext.vt.edu/news/releases/120197/blackice.html. Retrieved on 2007-10-10.
- ^ "Is there really such a thing as black ice?". http://www.straightdope.com/columns/read/2016/is-there-really-such-a-thing-as-black-ice. Retrieved on 2008-12-22.
- ^ "Black ice causes treacherous driving conditions in metro". KARE 11 TV. http://m.kare11.com/news.jsp?key=96528&rc=lo. Retrieved on 2008-12-22.
- ^ Ice Melters
- ^ AMS Glossary
