Freezing rain
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2008-07-17T04:26:41Z
Kevin
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'''Freezing rain''' is a type of precipitation that begins as [[snow]] at higher altitude, falling from a [[cloud]] towards earth, melts completely on its way down while passing through a layer of air above freezing temperature, and then encounters a layer below freezing at lower level to become [[supercooling|supercooled]]. This water will then [[freezing|freeze]] upon impact with any object it encounters.<ref>Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/browse?s=f&p=39 F.] Retrieved on [[2008-02-17]].</ref> The ice can accumulate to a thickness of several [[centimetre]]s, called [[glaze ice]]. The [[METAR]] code for freezing rain is '''FZRA'''.<ref>National Weather Service Forecast Office, La Crosse, Wisconsin. [http://www.crh.noaa.gov/arx/wx.tbl.php Significant Weather Phenomena Matrix.] Retrieved on [[2006-12-08]].</ref> (see [[freezing drizzle]] for another way of forming ice [[Accretion (atmosphere)|accretion]])
== Mechanism ==
<!--[[Image:Freezing_Rain_formation.PNG|left|thumb|200px|How freezing rain forms, from [[Snow]] through the warm layer and then into the "[[Supercooled]] Stage"]]-->
<!-- Unsourced image removed: [[Image:Freezing_rain.png|thumb|350px|Air mass structure in freezing rain (Source: [[Environment Canada]])]] -->Usually freezing rain is associated with the approach of a [[warm front]] when cold air, at or below freezing temperature, is trapped in the lower levels of the atmosphere as warmth streams in aloft.<ref>University of Illinois. [http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/prcp/zr/cond/cyc.rxml Cyclones and Fronts: the development of freezing rain.] Retrieved on [[2006-12-08]].</ref> This happens, for instance, when a low pressure system moves from the [[Mississippi River]] Valley toward the [[Appalachian Mountains]] and the [[Saint Lawrence River]] Valley of [[North America]], in the cold season, and there is a strong high pressure system sitting further East. The warm air from the [[Gulf of Mexico]] is often the fuel for freezing precipitation.
Freezing rain develops as falling snow encounters a layer of warm air usually around 800 [[millibar|mbar]] (800 [[Pascal (unit)|hPa]])level, then the snow completely melts and becomes rain. As the rain continues to fall, it passes through a thin layer of cold air just above the surface and cools to a temperature below freezing (0 °C). However, the drops themselves do not freeze, a phenomenon called [[supercooling]] (or forming "[[supercooled|supercooled drops]]"). When the supercooled drops strike a ground below 0 °C or anything else below 0 °C (power lines, tree branches, air craft), they instantly freeze, forming a thin film of ice, hence freezing rain.<ref>University of Illinois. [http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/prcp/zr/frz.rxml Cyclones and Fronts: the definition of freezing rain.] Retrieved on [[2007-12-16]].</ref>
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== Observations ==
[[Image:Radar-bright-band.png|left|thumb|400px|Echoes at 1.5 km altitude at the top with strong contamination from the brightband (yellows). The vertical cut at the bottom show that this strong return is only above ground (Source: [[Environment Canada]]).]]
Surface observations by manned or automatic stations are the only direct confirmation of freezing rain. One can never ''see'' directly freezing rain, rain or snow on [[weather radar]]s, [[Doppler effect|Doppler]] or conventional. However, it is possible to estimate the area covered by freezing rain with [[radar]]s indirectly.
The intensity of the radar echoes ([[reflectivity]]) is proportional to the form (water or ice) of the [[precipitation (meteorology)|precipitation]] and its diameter. In fact, rain has much stronger reflective power than snow but its diameter is much smaller. So the reflectivity of rain coming from melted snow is only slightly higher. However, in the layer where the snow is melting, the wet flakes still have a large diameter and are coated with water so the returns to the radar is much stronger.
The presence of this [[weather radar#Bright band|brightband]] indicates that there is a warm layer above ground where snow melts. This could be producing rain on the ground or the possibility of freezing rain if the temperature is below freezing. This [[Artifact (observational)|artifact]] can be located, like on the image at left, with a cross-section through radar data. The height and slope of the brightband will give clues to the extent of the region where melting occurs. Then it is possible to associate this clue with surface observations and numerical models prediction to produce output such as the ones seen on television weather programs that divide radar echoes into rain, mixed and snow precipitations.
== Effects ==
[[Image:Iced power lines.JPG|thumb|200px|Iced power lines threaten power outage to many after a powerful ice storm]]
[[Image:ICERAIN2.JPG|thumb|200px|Ice glaze on crabapple]]
[[Image:Freezing_grass.jpg|thumb|200px|Ice glaze on Grass]]
Freezing rain often causes major power outages. When the ice layer exceeds 0.2", tree limbs with branches heavily coated in ice can break off under the enormous weight and fall onto power lines. Windy conditions, 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 and is still a rain droplet when it makes contact with the ground, the freezing rain conforms to the shape of the ground, making one thick layer of [[ice]], often called glaze. Because [[sleet]] is in pellet form it can be easily moved around, unlike freezing rain which is a continuous layer of ice and cannot be moved around.
Freezing rain and glaze measured on a large scale is called an [[ice storm]]. Effects on plants can be severe, as they cannot support the weight of the [[ice]]. Trees may snap as they are dormant and fragile during [[winter]] weather. Pine trees are also victims of ice storms as their needles will catch the [[ice]], but not be able to support the weight.
One particularly severe ice storm struck eastern [[Canada]] and northern parts of [[New York]] and [[New England]] in January of 1998; for details see [[1998 Ice Storm]].
A severe ice storm caused over $1 billion in damage in the Southern United States in February 1994, primarily in Mississippi, Tennessee, and Alabama.
Freezing rain is also an extreme hazard to aircraft, as it causes very rapid structural icing. Most helicopters and small airplanes lack the necessary [[deicing]] equipment to fly in freezing rain of any intensity, and heavy freezing rain can overwhelm even the most sophisticated deicing systems on large airplanes. Icing can dramatically increase an aircraft's weight, and by changing the shape of its [[airfoils]] also reduce lift and increase drag. All three factors increase stalling speed and reduce aircraft performance, making it very difficult to climb or even maintain level altitude. Icing is most easily evaded by moving into warmer air — under most conditions, this requires aircraft to descend, which can usually be done safely and easily even with a moderate accumulation of structural ice. However, freezing rain is accompanied by a [[temperature inversion]] aloft, meaning that aircraft actually need to ''climb'' to move into warmer air — a potentially difficult and dangerous task with even a small amount of ice accumulation.
In [[1994]], [[ American Eagle Flight 4184]] encountered heavy air traffic and poor weather that postponed the arrival of this flight at [[Chicago]]'s [[O'Hare International Airport]], where it was to have landed en route from [[Indianapolis, Indiana]]. The ATR-72, a twin-engine turboprop carrying 68 people, entered a holding pattern 65 miles southeast of O'Hare. As the plane circled, freezing rain formed a ridge of ice on the upper surface of its wings, eventually causing the aircraft's autopilot to suddenly disconnect and the pilots to lose control. The ATR disintegrated on impact with a field below, killing everyone aboard.
== See also ==
* [[List of ice storms]]
* [[Freezing drizzle]]
* [[Hail]]
* [[Ice pellets]]
* [[Black ice]]
== References ==
{{reflist}}
== External links ==
{{commonscat|Freezing rain}}
*[http://news14.com/printarticle.aspx?ArID=98378 Accumulation vs. damage]
*[http://www.msc-smc.ec.gc.ca/media/icestorm98/index_e.cfm Canadian ice storm of 1998]
*[http://cimms.ou.edu/~cortinas/articles/mwrglakes/glakes.html Climatology of freezing rain]
*[http://www.dumpvideo.net/2007/02/13/ice-storm/ Video of Ice-storm]
*[http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/prcp/zr/prcs/ice.rxml Graphic Explanation]
[[Category:Precipitation]]
[[Category:Weather hazards]]
[[Category:Snow or ice weather phenomena]]
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