Black ice

For other uses, see Black ice (disambiguation).

Black ice (or glare ice) is commonly a thin, unexpected and invisible coating of ice on a roadway or walkway surface. The thin ice appears as the color of the material beneath it. Minute precipitation amounts are adequate to create slippery, unnoticed and dangerous situations, where travelers have minimal indication of weather changes. Fog, alone, in the absence of any falling precipitation can create a slippery coating to smooth, horizontal surfaces. Because it is always so difficult to see, it represents a universal hazard to automotive traffic, cyclists, motorcyclists, pedestrians, mountaineers and sailors.

On roads and pavements

Black ice on a road.

Black ice is a generic and rather inaccurate term typically used to describe slippery road conditions. Black ice, sometimes called "glare ice" or "clear ice", refers to a thin coating of glazed ice on a road or sidewalk. While not truly black, it is transparent, allowing the usually-black asphalt/macadam roadway to be seen through it, hence the term. It is essentially identical to all other forms of roadway ice, but the lack of noticeable ice pellets, snow, or sleet means that travelers have no obvious sign to reduce speeds. A similar problem occurs on pavements and sidewalks, and is a common cause of slips to pedestrians resulting in personal injury.

The term 'black ice' is sometimes incorrectly used in those geographical areas where it rarely rains. When it does rain, the lubricants that have leaked from vehicles, and the accumulated rubber and brake dust combine with any small amount of water to create slippery conditions. Motorbikes when driven between the tire tracks of larger vehicles (which leave their tracks relatively cleaner) are at special risk from such grimy surfaces.

Because it represents only a thin accumulation, black ice is highly transparent and thus difficult to see (as compared to snow, frozen slush) or thicker ice layers. In addition, it often is interleaved with wet road, which is nearly identical in appearance. For this reason it is especially hazardous when driving or walking on affected surfaces. Deicing with salt (sodium chloride) is effective down to temperatures of about -18°C. Other compounds such as magnesium chloride or calcium chloride have been used for very cold temperatures since the freezing-point depression of their solutions is lower.

In geographic areas with low rainfall, dark or dusty oily residues float to the surface of the rainwater on roadways creating a black surface appearance.

Bridges

Warning sign for ice on bridge

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 bridge pavement temperature to drop more rapidly. Road warning signs with the advisory "Bridge May Be Icy" indicate potentially dangerous roadways above bridge structures.

The I-35W Mississippi River bridge in Minneapolis, Minnesota, was well known for its black ice before it collapsed in 2007 into the Mississippi river. It had caused several pileups during its 40 year life. On December 19, 1985, the temperature reached 30 Degrees below zero. Cars crossing the bridge experienced black ice and there was a massive pile up on the bridge on the northbound side. In February and in December 1996, the bridge was identified as the single most treacherous cold-weather spot in the Twin Cities freeway system, because of the almost frictionless thin layer of black ice that regularly formed when temperatures dropped to 30s Fahrenheit (4 °C to -1 °C) and below. The bridge's proximity to Saint Anthony Falls contributed significantly to the icing problem and the site was noted for frequent spinouts and collisions. It was the only bridge in the United State Interstate system that had its own plumbed saline solution system to address the perpetual icing difficulties. Related corrosion has be cited as a contributing factor to the bridges catastrophic collapse.

The I-35W Mississippi River bridge seen from below in 2006

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. Such conditions caused multiple accidents in Minnesota when the temperatures dipped below 0°F for a prolonged period of time in mid-December 2008. Salt's ineffectiveness at melting ice at these temperatures compounds the problem.

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.

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 cause it to capsize. 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. The ice may be very difficult to remove even when seen, and is especially hazardous in the North Atlantic.

Mountains

Black ice on rocks in the mountains is known as verglas, and is a great hazard for climbers and scramblers. Cold weather is common at high altitudes, and black ice quickly forms on rock surfaces. Loss of traction is as sudden and unexpected as on a pavement or road, but can be fatal if the rock is in an exposed position with a drop below. The ice-axe and crampons are of little use in such circumstances, a belay rope being the only protection which can prevent a fall.

See also

Black ice on a river in the Netherlands

• Congelation ice
• Clear ice
• Freezing-point depression
• Glaze ice
• I-35W Mississippi River bridge
• Jumble ice
• Road traffic accident
• Sodium chloride
• Verglas

External links

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