Freezing rain
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Freezing rain is rain maintained at temperatures below freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike a mixture of rain and snow or ice pellets, freezing rain is made entirely of liquid droplets. The raindrops become supercooled while passing through a sub-freezing layer of air hundreds of meters above the ground, and then freeze upon impact with any surface they encounter, including the ground, trees, electrical wires, aircraft, and automobiles.[1] The resulting ice, called glaze ice, can accumulate to a thickness of several centimeters and cover all exposed surfaces. The METAR code for freezing rain is FZRA.
A storm that produces a significant thickness of glaze ice from freezing rain is often referred to as an
Mechanism
Freezing rain is often associated with the approach of a
Freezing rain develops when falling snow encounters a layer of warm air aloft, typically around the 800 mbar (800 hPa; 80 kPa) level, causing the snow to melt and become rain. As the rain continues to fall, it passes through a layer of subfreezing air just above the surface and cools to a temperature below freezing (0 °C or 32 °F or 273 K). If this layer of subfreezing air is sufficiently deep, the raindrops may have time to freeze into ice pellets (sleet) before reaching the ground. However, if the subfreezing layer of air at the surface is very shallow, the rain drops falling through it will not have time to freeze and they will hit the ground as supercooled rain. When these supercooled drops make contact with the ground, power lines, tree branches, aircraft, or anything else below 0 °C, a portion of the drops instantly freezes, forming a thin film of ice, hence freezing rain.[5][6] The specific physical process by which this occurs is called nucleation.[7]
Observations
Surface observations by staffed or automatic stations are the only direct confirmation of freezing rain. One can never see directly freezing rain, rain, or snow on any type of weather radar, whether Doppler or conventional. It is possible, however, to estimate the area covered by freezing rain with radar indirectly.[8][9]
The intensity of the radar echoes (
The presence of this
The height and slope of the brightband will give clues to the extent of the region where melting is occurring. Then, it is possible to associate this clue with surface observations and numerical prediction models to produce output such as the ones seen on television weather programs, where radar echoes are shown distinctly as rain, mixed, and snow precipitations.Effects
At ground level
Freezing rain often causes major power outages by forming glaze ice. When the freezing rain or drizzle is light and not prolonged, the ice formed is thin and usually causes only minor damage (relieving trees of their dead branches, etc.).[10] When large quantities accumulate, however, it is one of the most dangerous types of winter hazard.[11] When the ice layer exceeds approximately 6.4 mm (0.25 in)[citation needed], tree limbs with branches heavily coated in ice can break off under the enormous weight and fall onto power lines. Windy conditions and lightning, when present, will exacerbate the damage. Power lines coated with ice become extremely heavy, causing support poles, insulators and lines to break. The ice that forms on roadways makes vehicle travel dangerous. Unlike snow, wet ice provides almost no traction, and vehicles will slide even on gentle slopes. Because freezing rain does not hit the ground as an ice pellet (called "sleet") but still as a rain droplet, it conforms to the shape of the ground, or object such as a tree branch or car. This makes one thick layer of ice, often called "glaze".
Freezing rain and glaze ice on a large scale is called an
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Glaze on a tree in La Malbaie, Quebec
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Ice on coniferous tree in Tomaszów Mazowiecki, Poland
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Aftermath of freezing rain in Moscow Oblast, Russia, December 2010
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Power outages due to the weight of ice on lines or overhanging tree limbs
Aircraft
Freezing rain is considered to be an extreme hazard to aircraft, as it causes very rapid structural
An aircraft can most easily avoid freezing rain by moving into warmer air. Under most conditions, this would require aircraft to descend, which it can usually do safely and easily even with a moderate accumulation of structural ice. However, freezing rain is accompanied by a temperature inversion aloft, meaning that aircraft are required to climb to move into warmer air, which is a potentially difficult and dangerous task with even a small amount of ice accumulation.
For example, in 1994,
Ghost apples
On one occasion, freezing rain was observed to settle on hanging rotting apples and icing over them immediately, creating a
See also
References
- ^ "Freezing rain". Glossary of Meteorology. Archived from the original on 2019-03-18. Retrieved 2019-12-21.
- ^ National Weather Service Forecast Office, La Crosse, Wisconsin. Significant Weather Phenomena Matrix. Retrieved on 2006-12-08.
- ^ "Cyclones and Fronts: the development of freezing rain". WW2010: The Weather World 2010 Project. University of Illinois. 1999-10-04. Retrieved 2022-02-21.
- ^ David Phillips (2002-12-18). "Ice Storm of 1998". Archived from the original on 26 June 2006. Retrieved 2022-02-21.
- ^ "Cyclones and Fronts: the definition of freezing rain". WW2010: The Weather World 2010 Project. University of Illinois. 1999-10-04. Retrieved 2022-02-21.
- ^ Jeff Haby (2014-02-15). "The Rate at which Rain Freezes in a Freezing Rain Event". Retrieved 2022-02-21.
- ^ "Nucleation". Glossary of Meteorology. American Meteorological Society. Retrieved 2022-02-21.
- ^ ISBN 978-0-12-221420-2.
- ^ a b c "Bright band". Glossary of Meteorology. American Meteorological Society. Retrieved 2022-02-21.
- ^ a b "Glaze Ice". Archived from the original on 2009-07-15. Retrieved 2009-07-18.
- ^ Oblack, Rachelle. "Glaze Ice Definition". Archived from the original on 2008-08-22. Retrieved 2009-07-18.
- ^ Munroe, Susan. "The Canadian ice storm of 1998". Retrieved 2009-07-18.
- ^ a b "Ghost apple: how the polar vortex creates the illusion". BBC. 14 February 2019. Retrieved 21 January 2024.
- ^ "Parked Jeep Leaves Behind Ice Sculpture". NBC News. 19 February 2015. Retrieved 21 January 2024.