Freezing rain

Source: Wikipedia, the free encyclopedia.

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

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.)[2]

Mechanism

supercooled
stage".

Freezing rain is often associated with the approach of a

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.[4]

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

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 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 (

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.[8][9]

The presence of this

artifact can be located with a cross-section through radar data.[8][9]
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

North American ice storm of 1998.[12][10]

Aircraft

Freezing ice on aircraft wing

Freezing rain is considered to be an extreme hazard to aircraft, as it causes very rapid structural

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 airfoils
. This will reduce
drag. All three factors increase 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 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,

Indianapolis, Indiana. The ATR-72, a twin-engine turboprop carrying 68 people, entered a holding pattern 105 km or 65 mi 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

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.[13] These icy shells in the form of apples were called ghost apples and were observed in Michigan, United States[13] 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 and bumper) standing on icicles.[14]

See also

References

  1. ^ "Freezing rain". Glossary of Meteorology. Archived from the original on 2019-03-18. Retrieved 2019-12-21.
  2. ^ National Weather Service Forecast Office, La Crosse, Wisconsin. Significant Weather Phenomena Matrix. Retrieved on 2006-12-08.
  3. ^ "Cyclones and Fronts: the development of freezing rain". WW2010: The Weather World 2010 Project. University of Illinois. 1999-10-04. Retrieved 2022-02-21.
  4. ^ David Phillips (2002-12-18). "Ice Storm of 1998". Archived from the original on 26 June 2006. Retrieved 2022-02-21.
  5. ^ "Cyclones and Fronts: the definition of freezing rain". WW2010: The Weather World 2010 Project. University of Illinois. 1999-10-04. Retrieved 2022-02-21.
  6. ^ Jeff Haby (2014-02-15). "The Rate at which Rain Freezes in a Freezing Rain Event". Retrieved 2022-02-21.
  7. ^ "Nucleation". Glossary of Meteorology. American Meteorological Society. Retrieved 2022-02-21.
  8. ^ .
  9. ^ a b c "Bright band". Glossary of Meteorology. American Meteorological Society. Retrieved 2022-02-21.
  10. ^ a b "Glaze Ice". Archived from the original on 2009-07-15. Retrieved 2009-07-18.
  11. ^ Oblack, Rachelle. "Glaze Ice Definition". Archived from the original on 2008-08-22. Retrieved 2009-07-18.
  12. ^ Munroe, Susan. "The Canadian ice storm of 1998". Retrieved 2009-07-18.
  13. ^ a b "Ghost apple: how the polar vortex creates the illusion". BBC. 14 February 2019. Retrieved 21 January 2024.
  14. ^ "Parked Jeep Leaves Behind Ice Sculpture". NBC News. 19 February 2015. Retrieved 21 January 2024.

External links