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Freezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet (or meters), just above the surface, and then freeze upon impact with any object they encounter.[1] The resulting ice, called glaze, can accumulate to a thickness of several centimetres. The METAR code for freezing rain is FZRA. Freezing rain is notorious for causing travel problems on roadways, breaking tree limbs, and downing power lines. It is also known for being extremely dangerous to aircraft since the ice can effectively 'remould' the shape of the airfoil.(See atmospheric icing.) [2]

Contents

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"

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.[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, 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 mbar (800 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 (32 °F)). However, the drops themselves do not freeze, a phenomenon called supercooling (or forming "supercooled drops"). When the supercooled drops strike ground, power lines, tree branches, aircraft, or anything else below 0 °C (32 °F), they instantly freeze, forming a thin film of ice, hence freezing rain.[4]

Observations

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 radars, Doppler or conventional. However, it is possible to estimate the area covered by freezing rain with radars indirectly.

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. 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 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 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

Iced power lines threaten power outage to many after a powerful ice storm
Freezing rain is notorious for breaking tree limbs
Branch fully encapsulated in freezing rain

Freezing rain often causes major power outages. When the ice layer exceeds 0.2 inches (0.5 cm), 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, or object such as a tree branch or car. This makes 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 1998; for details see 1998 Ice Storm. Another 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

References

  1. ^ Glossary of Meteorology. F. Retrieved on 2008-02-17.
  2. ^ National Weather Service Forecast Office, La Crosse, Wisconsin. Significant Weather Phenomena Matrix. Retrieved on 2006-12-08.
  3. ^ University of Illinois. Cyclones and Fronts: the development of freezing rain. Retrieved on 2006-12-08.
  4. ^ University of Illinois. Cyclones and Fronts: the definition of freezing rain. Retrieved on 2007-12-16.

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