Black ice
Encyclopedia
Black ice, sometimes called glare ice or 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
and is a great hazard for climbers.
, 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 (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. Such conditions caused multiple accidents in Minnesota
when the temperatures dipped below -18 °C 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. 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."
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 Twin Cities
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.
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.
, 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.
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
on a surface.
While not truly black
Black
Black is the color of objects that do not emit or reflect light in any part of the visible spectrum; they absorb all such frequencies of light...
, it is virtually transparent, allowing black asphalt
Asphalt
Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...
/macadam
Macadam
Macadam is a type of road construction pioneered by the Scotsman John Loudon McAdam in around 1820. The method simplified what had been considered state-of-the-art at that point...
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
Verglas
Verglas may refer to:* Verglas Music, a record label which has released works such as Jabberwocky, a 1999 progressive rock album* Verglas, a climbing term referring to a thin coating of glaze ice on rock ....
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 snowSnow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...
, frozen slush
Slush (snow)
Slush, also called slush ice, is a slurry mixture of small ice crystals and liquid water. In the natural environment, slush forms as ice or snow melts. This often mixes with dirt and other materials, resulting in a gray or muddy brown color...
, 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
Deicing
For snow and ice control on roadways and similar facilities, see Snow removalDe-icing is defined as removal of snow, ice or frost from a surface...
with salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
(sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
) is effective down to temperatures of about -18 °C (0 °F). Other compounds such as magnesium chloride
Magnesium chloride
Magnesium chloride is the name for the chemical compounds with the formulas MgCl2 and its various hydrates MgCl2x. These salts are typical ionic halides, being highly soluble in water. The hydrated magnesium chloride can be extracted from brine or sea water...
or calcium chloride
Calcium chloride
Calcium chloride, CaCl2, is a salt of calcium and chlorine. It behaves as a typical ionic halide, and is solid at room temperature. Common applications include brine for refrigeration plants, ice and dust control on roads, and desiccation...
have been used for very cold temperatures since the freezing-point depression
Freezing-point depression
Freezing-point depression describes the phenomenon in which the freezing point of a liquid is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent. This happens whenever a non-volatile solute is added to a pure solvent, such as water...
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. Such conditions caused multiple accidents in Minnesota
Minnesota
Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...
when the temperatures dipped below -18 °C 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
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
point of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
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. 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."
Bridges
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.Twin cities
Twin cities are a special case of two cities or urban centres which are founded in close geographic proximity and then grow into each other over time...
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
Saint Anthony Falls
Saint Anthony Falls, or the Falls of Saint Anthony, located northeast of downtown Minneapolis, Minnesota, was the only natural major waterfall on the Upper Mississippi River. The natural falls was replaced by a concrete overflow spillway after it partially collapsed in 1869...
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 seawaterSeawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...
spray and water vapour freezing upon contact with the vessel's superstructure
Superstructure
A superstructure is an upward extension of an existing structure above a baseline. This term is applied to various kinds of physical structures such as buildings, bridges, or ships...
. 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
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
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 verglasVerglas
Verglas may refer to:* Verglas Music, a record label which has released works such as Jabberwocky, a 1999 progressive rock album* Verglas, a climbing term referring to a thin coating of glaze ice on rock ....
, and is a great hazard for climbers and scramblers
Scrambling
Scrambling is a method of ascending rocky faces and ridges. It is an ambiguous term that lies somewhere between hillwalking and rock climbing. It is often distinguished from hillwalking by defining a scramble as a route where hands must be used in the ascent...
. 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
Crampons
Crampons are traction devices used to improve mobility on snow and ice. There are three main attachment systems for footwear: step-in, hybrid, and strap bindings. The first two require boots with welts, the last adapt to any type....
are of little use in such circumstances, a belay rope being the only protection which can prevent a fall.
See also
- Congelation iceCongelation iceCongelation ice is ice that forms on the bottom of an established ice cover.- Seawater :On seawater, congelation ice is ice that forms on the bottom of an established sea ice cover, usually in the form of platelets which coalesce to form solid ice....
- Clear iceClear iceClear ice refers to a solid precipitation which forms when air temperature is between 0 °C and -3 °C and there are supercooled, relatively large drops of water...
- Freezing rainFreezing rainFreezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet , just above the surface, and then freeze upon impact with any object they encounter. The resulting...
- Freezing-point depressionFreezing-point depressionFreezing-point depression describes the phenomenon in which the freezing point of a liquid is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent. This happens whenever a non-volatile solute is added to a pure solvent, such as water...
- Glaze iceGlaze iceGlaze ice or simply glaze is a smooth, transparent and homogenous ice coating occurring when freezing rain or drizzle hits a surface. It is similar in appearance to clear ice, which forms from supercooled water droplets...
- Jumble iceJumble iceJumble ice is a phenomenon that occurs when ice atop a river or other flowing body of water fractures due to the different flow rates beneath the ice. On a lake, pond, or other stationary body of water, ice forms undisturbed and generally does not move as long as the entire surface of the body of...
- Road traffic accident
- Sodium chlorideSodium chlorideSodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
- SandSandSand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
External links
- Is there really such a thing as black ice?, from the Straight DopeStraight DopeThe Straight Dope is a popular question-and-answer newspaper column published in the Chicago Reader, syndicated in thirty newspapers in the United States and Canada, as well as being available and archived at the .-Newspapers:...
staff