Black ice

From Wikipedia, the free encyclopedia

Black Ice (or glare ice) is ice frozen without many air bubbles trapped inside, making it highly transparent. This type of ice appears in the color of the material beneath it, often wet asphalt or a darkened pond. Because it may be difficult to detect, it presents a significant hazard to automotive traffic, pedestrians, and sailors.

Contents 1 On roads 2 Maritime black ice 3 See also 4 References 5 External links

[edit] On roads

Black ice is a very generic term typically used to describe slippery road conditions. Black ice is sometimes called "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.

Black ice is a term also used in those geographical areas where it rarely rains. When it does rain, the oils from vehicles that have dripped on roadways float on the thin layer of water resulting is a very slippery condition. Also, some of the oils in the asphalt roadways can float on top of the water and make that road even more dangerous.

Because it contains relatively little entrapped air in the form of bubbles, black ice is highly transparent and thus 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 on affected surfaces. Also, in the low rainfall condition geographical area, the black oils that float on the surface of the rainwater on roadways make the surface appear black. This can be especially dangerous on asphalt roads which are black to begin with. ^[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. Road warning signs with the advisory "Bridge May Be Icy" often indicate potentially dangerous roadways.

At low temperatures (below 0°F/-18°C), black ice can form on roadways when the moisture from automobile exhaust condenses on the road surface.^[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 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 extra, unbalanced 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

• Clear ice
• Glaze ice
• Jumble ice
• Verglas

[edit] References

[edit] External links

• Is there really such a thing as black ice?, from the Straight Dope staff