black ice

Black ice, sometimes called clear ice, refers to a thin coating of glazed ice on a surface.

While not truly black, it is virtually transparent, allowing black asphalt/macadam roadways to be seen through it, hence the term "black ice". The typically low levels of noticeable ice pellets, snow, or sleet surrounding black ice means that areas of the ice are often practically invisible to drivers and thereby do not serve as a good indicator that they should reduce their speeds.

Similar thin invisible layers of ice that form along ships can cause them to become unbalanced. In the mountains, black ice is referred to as verglas (glaze ice) and is a great hazard for climbers.

On roads and pavements

Because it represents only a thin accumulation, black ice is highly transparent and thus difficult to see as compared with snow, frozen slush, or thicker ice layers. In addition, it often is interleaved with wet road, which is nearly identical in appearance. This makes driving or walking on affected surfaces extremely dangerous. Deicing with salt (sodium chloride) is effective down to temperatures of about -18 °C (0 °F). 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.

At low temperatures (below -18 °C), black ice can form on roadways when the moisture from automobile exhaust condenses on the road surface.^[1] Such conditions caused multiple accidents in Minnesota when the temperatures dipped below -18 °C for a prolonged period of time in mid-December 2008.^[2] Salt's ineffectiveness at melting ice at these temperatures compounds the problem.^[3]

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 has left 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.^{[citation needed]} The American Meteorological Society Glossary of Meteorology includes the definition of black ice as "a thin sheet of ice, relatively dark in appearance, [that] may form when light rain or drizzle falls on a road surface that is at a temperature below 0 °C."^[4]

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.

In the United States, road warning signs with the advisory "Bridge May Be Icy" indicate potentially dangerous roadways above bridge structures.

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

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 -34 °C (-30 °F). 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 local freeway system, because of the almost frictionless thin layer of black ice that regularly formed when temperatures dropped below freezing. 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 States Interstate system that had its own plumbed saline solution system to address the perpetual icing difficulties. Related corrosion has been cited as a contributing factor to the bridge's catastrophic collapse.

Ice formation on seagoing vessels

Ice formation is a significant hazard for vessels operating in colder regions and it is caused by seawater spray and water vapour freezing upon contact with the vessel's superstructure. As the formation of ice progresses, the weight of the vessel increases and may ultimately cause a capsizing moment to be created. Furthermore, ice may impede the correct functioning of important navigational instruments onboard, such as radar or radio installations. Different strategies for the removal of such ice have been employed; chipping away the ice or even using fire hoses in an attempt to melt the ice away.

Mountains

Black ice on rocks in the mountains is known as verglas (glaze ice), 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

• Clear ice
• Congelation ice
• Freezing rain
• Freezing-point depression
• Glaze ice
• Jumble ice
• Road traffic accident
• Sand
• Sodium chloride

References

External links

Look up black ice in Wiktionary, the free dictionary.

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