Editing Freezing rain

{{Short description|Rain maintained at temperatures below freezing}}
{{Weather}}

'''Freezing rain''' is [[rain]] maintained at temperatures below [[melting point|freezing]] by the ambient [[air mass]] that causes [[freezing]] on contact with surfaces.  Unlike [[rain and snow mixed|a mixture of rain and snow]] or [[ice pellets]], freezing rain is made entirely of [[liquid]] droplets. The raindrops become [[supercooling|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.<ref>{{cite web|work=Glossary of Meteorology |url=http://glossary.ametsoc.org/wiki/Freezing_rain |title=Freezing rain |archive-url=https://web.archive.org/web/20190318184821/http://glossary.ametsoc.org/wiki/Freezing_rain |archive-date=2019-03-18| access-date= 2019-12-21|url-status=live}}</ref> The resulting ice, called [[glaze (ice)|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 [[ice storm]]. Although these storms are not particularly violent, freezing rain is notorious for causing travel problems on roadways, breaking tree limbs, and downing [[power line]]s from the weight of accumulating ice. Downed [[power lines]] cause power outages in affected areas while accumulated ice can also pose significant overhead hazards.  It is also known for being extremely dangerous to aircraft since the ice can effectively 'remould' the shape of the [[airfoil]] and [[flight control surfaces]]. (See [[atmospheric icing]].)<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>

== Mechanism ==
[[File:Ice Storm Chart.svg|thumb|left|upright=0.75|Temperature versus height diagram for different types of [[precipitation]]. The red line shows how freezing rain forms, from [[snow]] through the warm layer and then into the "[[supercooled]] stage".]]
<!--[[File:Freezing Rain skew-t.gif|thumb|[[Skew-T]] sounding taken during a freezing rain event. Temperatures aloft are well above freezing and cool significantly closer to the ground, allowing precipitation to fall as rain and freeze upon reaching the ground.]]-->
Freezing rain is often associated with the approach of a [[warm front]], when subfreezing air (temperatures at or below freezing) is trapped in the lowest levels of the atmosphere while warm air is [[Advection|advected]] aloft.<ref name=U-Ill>{{cite web|publisher=University of Illinois |date=1999-10-04 |title=Cyclones and Fronts: the development of freezing rain |work=WW2010: The Weather World 2010 Project |access-date=2022-02-21 |url=http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/prcp/zr/cond/cyc.rxml}}</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]] during the cold season, with a strong high pressure system sitting further east. This setup is known as [[cold-air damming]], and is characterized by very cold and dry air at the surface within the region of high pressure. The warm air from the [[Gulf of Mexico]] is often the fuel for freezing precipitation.<ref name=DP>{{cite web |url=http://www.msc-smc.ec.gc.ca/media/icestorm98/icestorm98_a_closer_look_e.cfm |author=[[David Phillips (climatologist)|David Phillips]] |title=Ice Storm of 1998 |date=2002-12-18 |access-date=2022-02-21 |archive-url=https://web.archive.org/web/20060626235616/http://www.msc-smc.ec.gc.ca/media/icestorm98/icestorm98_a_closer_look_e.cfm |archive-date=26 June 2006 |url-status=dead}}</ref>

Freezing rain develops when falling snow encounters a layer of warm air aloft, typically around the {{convert|800|mbar|hPa kPa|abbr=on|lk=on}} 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 ({{convert|0|C|F K|disp=or}}). 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 [[Supercooling|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.<ref>{{cite web|publisher=University of Illinois |date=1999-10-04 |title=Cyclones and Fronts: the definition of freezing rain. |work=WW2010: The Weather World 2010 Project |url=http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/prcp/zr/frz.rxml|access-date=2022-02-21}}</ref><ref>{{cite web |url=http://www.theweatherprediction.com/habyhints/210/ |title=The Rate at which Rain Freezes in a Freezing Rain Event |access-date=2022-02-21 |author=Jeff Haby |date=2014-02-15 }}</ref>  The specific physical process by which this occurs is called [[Nucleation#Examples|nucleation]].<ref>{{cite web|url=https://glossary.ametsoc.org/wiki/Nucleation|title=Nucleation |work=Glossary of Meteorology |publisher=[[American Meteorological Society]]|access-date=2022-02-21}}</ref>
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== Observations ==
[[File:Radar-bright-band.png|thumb|left|upright=1.5|Echoes at {{convert|1.5|km|m mi ft|abbr=on}} altitude at the top with strong contamination from the [[Weather radar#Bright band|bright band]] (yellows). The vertical cut at the bottom show that this strong return is only above ground (Source: [[Environment Canada]]).]]
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 effect|Doppler]] or conventional. It is possible, however, to estimate the area covered by freezing rain with [[radar]] indirectly.<ref name="Doviak">{{cite book|last1=Doviak|first1=R. J.|author-link1=Richard Doviak|first2=D. S. |last2=Zrnic|author-link2=Dusan S. Zrnic|title=Doppler Radar and Weather Observations|year=1993|publisher=Academic Press|location=San Diego CA|edition=2nd|isbn=978-0-12-221420-2 }}</ref><ref name=AMS-BB>{{cite web|url=https://glossary.ametsoc.org/wiki/Bright_band |title=Bright band|publisher=[[American Meteorological Society]] |work=Glossary of Meteorology |access-date=2022-02-21}}</ref>

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. In the layer where the snow is melting, however, the wet flakes still have a large [[diameter]] and are coated with water, so the radar returns are much stronger.<ref name="Doviak"/><ref name=AMS-BB/>

The presence of this [[Weather radar#Bright band|brightband]] indicates the presence of a warm layer above ground where snow can melt. This could be producing rain on the ground or the possibility of freezing rain if the temperature is below freezing. The accompanying image shows how such an [[Artifact (observational)|artifact]] can be located with a cross-section through radar data.<ref name="Doviak"/><ref name=AMS-BB/> 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 ==
{{Main|Glaze (ice)}}

=== 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.).<ref name="Glaze Ice">{{cite web|url=http://ecosys.cfl.scf.rncan.gc.ca/perturbation-disturbance/verglas-glaze-ice-eng.asp |title=Glaze Ice |access-date=2009-07-18 |url-status=dead |archive-url=https://web.archive.org/web/20090715235211/http://ecosys.cfl.scf.rncan.gc.ca/perturbation-disturbance/verglas-glaze-ice-eng.asp |archive-date=2009-07-15 }}</ref> When large quantities accumulate, however, it is one of the most dangerous types of winter hazard.<ref>{{cite web|url=http://weather.about.com/od/g/g/glaze.htm|title=Glaze Ice Definition|last=Oblack|first=Rachelle|access-date=2009-07-18|url-status=dead|archive-url=https://web.archive.org/web/20080822014756/http://weather.about.com/od/g/g/glaze.htm|archive-date=2008-08-22}}</ref>  When the ice layer exceeds approximately {{convert|0.25|in|mm|abbr=on|order=flip}} {{Citation needed|date=February 2011}}, 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 [[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.  In February 1994, a severe ice storm caused over $1 billion in damage in the Southern United States, primarily in Mississippi, Tennessee, Alabama, and Western North Carolina, especially the Appalachians. One particularly severe ice storm struck eastern [[Canada]] and northern parts of [[New York (state)|New York]] and [[New England]] in the [[North American ice storm of 1998]].<ref>{{cite web|url=http://canadaonline.about.com/cs/weather/p/icestorm.htm|title=The Canadian ice storm of 1998|last=Munroe|first=Susan|access-date=2009-07-18|archive-date=2009-03-03|archive-url=https://web.archive.org/web/20090303161930/http://canadaonline.about.com/cs/weather/p/icestorm.htm|url-status=dead}}</ref><ref name="Glaze Ice"/>
<gallery mode="packed">
File:Damaged trees in Ljubljana Trg MDB.jpeg|Tree downed by a thick layer of glaze in downtown [[Ljubljana]], [[Slovenia]]
File:Freezing Rain on Tree Branch.jpg|Glaze on a tree in La Malbaie, Quebec
File:Zlodowacenie - Marznący deszcz 6.JPG|Ice on coniferous tree in [[Tomaszów Mazowiecki]], [[Poland]]
File:Bent tree glaze ice Celje.jpg|A birch tree is badly bent under a thick layer of glaze ice in [[Celje]], [[Slovenia]]
File:Ice storm in moscow.JPG|Aftermath of freezing rain in [[Moscow Oblast]], [[Russia]], December 2010
File:Iced power lines.JPG|Power outages due to the weight of ice on lines or overhanging tree limbs
</gallery>

=== Aircraft ===
[[File:Wingice.JPG|thumb|Freezing ice on aircraft wing]]
Freezing rain is considered to be an extreme hazard to aircraft, as it causes very rapid structural [[Atmospheric icing|icing]], freezing necessary components.  Most [[helicopter]]s and small airplanes lack the necessary [[De-ice|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 increase an aircraft's weight but not typically enough to cause a hazard. The main danger comes from the ice changing the shape of its [[airfoil]]s. 
This will reduce [[Lift (force)|lift]] and increase [[Aerodynamic drag|drag]]. All three factors increase [[Stall (fluid dynamics)|stalling]] speed and reduce aircraft performance, making it very difficult to climb or even maintain altitude.

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 [[Inversion (meteorology)|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, [[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 {{convert|65|mi|km|abbr=on|order=flip|disp=or}} southeast of O'Hare. As the plane circled, supercooled cloud droplets, freezing rain or [[freezing drizzle]] 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; all passengers and crew were killed.

=== Ghost apples ===
On one occasion, freezing rain was observed to settle on hanging rotting apples and icing over them immediately, creating a [[Glaze (ice)|glaze]] coating. Because of apples' lower [[freezing point]] than water, under warmer temperature the apples defrosted before the ice did, then the rotting apple mush slipped out of the bottom, leaving icy shells.<ref name="bbc">{{cite web | url =https://www.bbc.co.uk/newsround/47222907|title=Ghost apple: how the polar vortex creates the illusion|publisher=[[BBC]]| date=14 February 2019| accessdate =21 January 2024}}</ref> These icy shells in the form of apples were called ghost apples and were observed in [[Michigan]], United States<ref name="bbc"/> in February 2019. A similar phenomenon was observed in February 2015 in [[Greenville, North Carolina]], when a [[Jeep]] that had backed out of the parking lot left behind an ice shell of its lower front part (with [[Grille (car)|grille]] and [[Bumper (car)|bumper]]) standing on [[icicle]]s.<ref>{{cite web | url =https://www.nbcnews.com/news/us-news/parked-jeep-leaves-behind-ice-sculpture-n309031|title=Parked Jeep Leaves Behind Ice Sculpture|publisher=[[NBC News]]| date=19 February 2015| accessdate =21 January 2024}}</ref>

== See also ==
* [[Black ice]]
* [[Freezing drizzle]]
* [[Hail]]
* [[Ice pellets]]
* [[Ice storm]]
* [[Icing conditions]]
* [[List of ice storms]]

== References ==
{{reflist}}

== External links ==
{{Commons category|Freezing rain}}
*[http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/prcp/zr/prcs/ice.rxml Graphic Explanation]

{{Authority control}}

{{DEFAULTSORT:Freezing Rain}}
[[Category:Precipitation]]
[[Category:Weather hazards]]
[[Category:Snow or ice weather phenomena]]
[[Category:Road hazards]]