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.

Contents

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 lines 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.) [2]

Mechanism

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". Ice Storm Chart.svg
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".

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 advected aloft. [3] 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. [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

Echoes at 1.5 km (1,500 m; 0.93 mi; 4,900 ft) altitude at the top with strong contamination from the bright band (yellows). The vertical cut at the bottom show that this strong return is only above ground (Source: Environment Canada). Radar-bright-band.png
Echoes at 1.5 km (1,500 m; 0.93 mi; 4,900 ft) altitude at the top with strong contamination from the 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 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 (reflectivity) is proportional to the form (water or ice) of the 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 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 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 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 and New England in the North American ice storm of 1998. [12] [10]

Aircraft

Freezing ice on aircraft wing Wingice.JPG
Freezing ice on aircraft wing

Freezing rain is considered to be an extreme hazard to aircraft, as it causes very rapid structural icing, freezing necessary components. 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 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 lift and increase 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, 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 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 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. [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

Related Research Articles

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Frost is a thin layer of ice on a solid surface, which forms from water vapor that deposits onto a freezing surface. Frost forms when the air contains more water vapor than it can normally hold at a specific temperature. The process is similar to the formation of dew, except it occurs below the freezing point of water typically without crossing through a liquid state.

<span class="mw-page-title-main">Hail</span> Form of solid precipitation

Hail is a form of solid precipitation. It is distinct from ice pellets, though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fall in cold weather, while hail growth is greatly inhibited during low surface temperatures.

<span class="mw-page-title-main">Ice</span> Frozen water: the solid state of water

Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 °C, 32 °F, or 273.15 K. As a naturally occurring crystalline inorganic solid with an ordered structure, ice is considered to be a mineral. Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaque bluish-white color.

<span class="mw-page-title-main">Snow</span> Precipitation in the form of ice crystal flakes

Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout its life cycle, starting when, under suitable conditions, the ice crystals form in the atmosphere, increase to millimeter size, precipitate and accumulate on surfaces, then metamorphose in place, and ultimately melt, slide or sublimate away.

<span class="mw-page-title-main">Ice storm</span> Weather event characterized by freezing rain

An ice storm, also known as a glaze event or a silver storm, is a type of winter storm characterized by freezing rain. The U.S. National Weather Service defines an ice storm as a storm which results in the accumulation of at least 0.25-inch (6.4 mm) of ice on exposed surfaces. They are generally not violent storms but instead are commonly perceived as gentle rains occurring at temperatures just below freezing.

<span class="mw-page-title-main">Ice crystal</span> Water ice in symmetrical shapes

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In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates" or falls. Thus, fog and mist are not precipitation but colloids, because the water vapor does not condense sufficiently to precipitate. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called showers.

<span class="mw-page-title-main">Black ice</span> Thin coating of glazed ice on a surface

Black ice, sometimes called clear ice, is a thin coating of glaze ice on a surface, especially on streets. The ice itself is not black, but visually transparent, allowing the often black road below to be seen through it. The typically low levels of noticeable ice pellets, snow, or sleet surrounding black ice means that areas of the ice are often next to invisible to drivers or people walking on it. Thus, there is a risk of slippage and subsequent accident due to the unexpected loss of traction.

<span class="mw-page-title-main">January 1998 North American ice storm</span> Major ice storm that affected Eastern Canada and northeastern US in 1998

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<span class="mw-page-title-main">Rime ice</span> Granular whitish deposit of ice formed by freezing fog

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<span class="mw-page-title-main">Atmospheric icing</span> Weather condition in which water droplets freeze onto objects they come in contact with

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Ice pellets or "sleet" is a form of precipitation consisting of small, hard, translucent balls of ice. Ice pellets are different from graupel, which is made of frosty white opaque rime, and from a mixture of rain and snow, which is a slushy liquid or semisolid. Ice pellets often bounce when they hit the ground or other solid objects, and make a higher-pitched "tap" when striking objects like jackets, windshields, and dried leaves, compared to the dull splat of liquid raindrops. Pellets generally do not freeze into other solid masses unless mixed with freezing rain. The METAR code for ice pellets is PL.

Freezing drizzle is drizzle that freezes on contact with the ground or an object at or near the surface. Its METAR code is FZDZ.

<span class="mw-page-title-main">Glaze (ice)</span> Coating of ice on objects

Glaze or glaze ice, also called glazed frost or verglas, is a smooth, transparent and homogeneous 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. It is a relatively common occurrence in temperate climates in the winter when precipitation forms in warm air aloft and falls into below-freezing temperature at the surface.

<span class="mw-page-title-main">Precipitation types</span> Characters, formations, and phases of water condensed in the atmosphere

In meteorology, the different types of precipitation often include the character, formation, or phase of the precipitation which is falling to ground level. There are three distinct ways that precipitation can occur. Convective precipitation is generally more intense, and of shorter duration, than stratiform precipitation. Orographic precipitation occurs when moist air is forced upwards over rising terrain and condenses on the slope, such as a mountain.

<span class="mw-page-title-main">Outline of meteorology</span> Overview of and topical guide to meteorology

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Rain and snow mixed or sleet is precipitation composed of a mixture of rain and partially melted snow. Unlike ice pellets, which are hard, and freezing rain, which is fluid until striking an object where it fully freezes, this precipitation is soft and translucent, but it contains some traces of ice crystals from partially fused snowflakes, also called slush. In any one location, it usually occurs briefly as a transition phase from rain to snow or vice-versa, but hits the surface before fully transforming. Its METAR code is RASN or SNRA.

<span class="mw-page-title-main">Glossary of meteorology</span> List of definitions of terms and concepts commonly used in meteorology

This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.

<span class="mw-page-title-main">Shower (precipitation)</span> Sudden and brief rain or snowfall

A shower is a mode of precipitation characterized by an abrupt start and end and by rapid variations in intensity. Often strong and short-lived, it comes from convective clouds, like cumulus congestus. A shower will produce rain if the temperature is above the freezing point in the cloud, or snow / ice pellets / snow pellets / hail if the temperature is below it at some point. In a meteorological observation, such as the METAR, they are noted SH giving respectively SHRA, SHSN, SHPL, SHGS and SHGR.

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. 1 2 3 Doviak, R. J.; Zrnic, D. S. (1993). Doppler Radar and Weather Observations (2nd ed.). San Diego CA: Academic Press. ISBN   978-0-12-221420-2.
  9. 1 2 3 "Bright band". Glossary of Meteorology. American Meteorological Society . Retrieved 2022-02-21.
  10. 1 2 "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. 1 2 "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.
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