Freezing rain

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Freezing rain is the name given to 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, ice pellets, or hail, 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.

Rain liquid water in the form of droplets that have condensed from atmospheric water vapor and then precipitated

Rain is liquid water in the form of droplets that have condensed from atmospheric water vapor and then become heavy enough to fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides suitable conditions for many types of ecosystems, as well as water for hydroelectric power plants and crop irrigation.

Melting point temperature at which a solid turns liquid

The melting point of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at a standard pressure such as 1 atmosphere or 100 kPa.

Air mass a volume of air defined by its temperature and water vapor content

In meteorology, an air mass is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of miles, and adapt to the characteristics of the surface below them. They are classified according to latitude and their continental or maritime source regions. Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry while maritime and monsoon air masses are moist. Weather fronts separate air masses with different density characteristics. Once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character. Classification schemes tackle an air mass' characteristics, as well as modification.

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]

An ice storm is a type of winter storm characterized by freezing rain, also known as a glaze event or, in some parts of the United States, as a silver thaw. 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. From 1982 to 1994, ice storms were more common than blizzards in the U.S., averaging 16 per year. They are generally not violent storms but instead are commonly perceived as gentle rains occurring at temperatures just below freezing.

Airfoil

An airfoil or aerofoil is the cross-sectional shape of a wing, blade, or sail.

Flight control surfaces surface that allows a pilot to adjust and control an aircrafts flight attitude

Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude.

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 advects 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 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.

Warm front

A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold fronts, and move more slowly than the cold fronts which usually follow because cold air is denser and less easy to remove from the Earth's surface. This also forces temperature differences across warm fronts to be broader in scale. Clouds ahead of the warm front are mostly stratiform, and rainfall gradually increases as the front approaches. Fog can also occur preceding a warm frontal passage. Clearing and warming is usually rapid after frontal passage. If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage thundershowers may continue. On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.

In the field of physics, engineering, and earth sciences, advection is the transport of a substance or quantity by bulk motion. The properties of that substance are carried with it. Generally the majority of the advected substance is a fluid. The properties that are carried with the advected substance are conserved properties such as energy. An example of advection is the transport of pollutants or silt in a river by bulk water flow downstream. Another commonly advected quantity is energy or enthalpy. Here the fluid may be any material that contains thermal energy, such as water or air. In general, any substance or conserved, extensive quantity can be advected by a fluid that can hold or contain the quantity or substance.

Mississippi River largest river system in North America

The Mississippi River is the second-longest river and chief river of the second-largest drainage system on the North American continent, second only to the Hudson Bay drainage system. Its source is Lake Itasca in northern Minnesota and it flows generally south for 2,320 miles (3,730 km) to the Mississippi River Delta in the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains all or parts of 32 U.S. states and two Canadian provinces between the Rocky and Appalachian mountains. The main stem is entirely within the United States; the total drainage basin is 1,151,000 sq mi (2,980,000 km2), of which only about one percent is in Canada. The Mississippi ranks as the fourth-longest and fifteenth-largest river by discharge in the world. The river either borders or passes through the states of Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana.

Freezing rain develops when falling snow encounters a layer of warm air aloft, typically around the 800 mbar (800 hPa) 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). 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 (32 °F), a portion of the drops instantly freezes, forming a thin film of ice, hence freezing rain. [4] [5] The specific physical process by which this occurs is called nucleation.

Pascal (unit) SI unit of pressure

The pascal is the SI derived unit of pressure used to quantify internal pressure, stress, Young's modulus and ultimate tensile strength. It is defined as one newton per square metre. It is named after the French polymath Blaise Pascal.

Ice pellets are a form of precipitation consisting of small, translucent balls of ice. Ice pellets are smaller than hailstones which form in thunderstorms rather than in winter, and are different from graupel which is made of frosty white 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 a solid mass unless mixed with freezing rain. The METAR code for ice pellets is PL.

Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid.

Observations

Echoes at 1.5 km (0.93 mi) 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). Radar-bright-band.png
Echoes at 1.5 km (0.93 mi) 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.

Weather radar radar used to locate and monitor meteorological conditions

Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type. Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the intensity of the precipitation. Both types of data can be analyzed to determine the structure of storms and their potential to cause severe weather.

The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.

Radar object detection system based on radio waves

Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object(s). Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed.

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

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.). [6] When large quantities accumulate, however, it is one of the most dangerous types of winter hazard. [7] 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. [8] [9]

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. 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 level 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.

See also

Related Research Articles

Frost coating or deposit of ice that may form in humid air in cold conditions, usually overnight

Frost is a thin layer of ice on a solid surface, which forms from water vapor in an above freezing atmosphere coming in contact with a solid surface whose temperature is below freezing, and resulting in a phase change from water vapor to ice as the water vapor reaches the freezing point. In temperate climates, it most commonly appears on surfaces near the ground as fragile white crystals; in cold climates, it occurs in a greater variety of forms. The propagation of crystal formation occurs by the process of nucleation.

Hail 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 fall generally in cold weather while hail growth is greatly inhibited during cold surface temperatures.

A winter storm is an event in which varieties of precipitation are formed that only occur at low temperatures, such as snow or sleet, or a rainstorm where ground temperatures are low enough to allow ice to form. In temperate continental climates, these storms are not necessarily restricted to the winter season, but may occur in the late autumn and early spring as well. Very rarely, they may form in summer, though it would have to be an abnormally cold summer, such as the summer of 1816 in the Northeastern United States.

Precipitation product of the condensation of atmospheric water vapour that falls under gravity

In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation include drizzle, rain, sleet, snow, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates". Thus, fog and mist are not precipitation but suspensions, 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."

Black ice 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 roads. 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 practically invisible to drivers or people stepping on it. There is, thus, a risk of slippage and subsequent accident due to the unexpected loss of traction.

January 1998 North American ice storm January 1998 storm in North America

The North American Ice Storm of 1998 was a massive combination of five smaller successive ice storms in January 1998 that struck a relatively narrow swath of land from eastern Ontario to southern Quebec, New Brunswick and Nova Scotia in Canada, and bordering areas from northern New York to central Maine in the United States. It caused massive damage to trees and electrical infrastructure all over the area, leading to widespread long-term power outages. Millions were left in the dark for periods varying from days to several weeks, and in some instances, months. It led to 35 fatalities, a shutdown of activities in large cities like Montreal and Ottawa, and an unprecedented effort in reconstruction of the power grid. The ice storm led to the largest deployment of Canadian military personnel since the Korean War, with over 16,000 Canadian Forces personnel deployed, 12,000 in Quebec and 4,000 in Ontario at the height of the crisis.

Rime ice

Rime ice forms when supercooled water liquid droplets freeze onto surfaces. Meteorologists distinguish between three basic types of ice forming on vertical and horizontal surfaces by deposition of supercooled water droplets. There are also intermediate formations.

In aviation, icing conditions are those atmospheric conditions that can lead to the formation of water ice on the surfaces of an aircraft, or within the engine as carburetor icing. Inlet icing is another engine-related danger, often occurring in jet aircraft. These icing phenomena do not necessarily occur together. Many aircraft, especially general aviation aircraft, are not certified for flight into known icing—icing conditions certain or likely to exist, based on pilot reports, observations, and forecasts.

Atmospheric icing occurs when water droplets in the atmosphere freeze on objects they contact

Atmospheric icing occurs when water droplets in the atmosphere freeze on objects they contact. This can be extremely dangerous to aircraft, as the built-up ice changes the aerodynamics of the flight surfaces, which can increase the risk of a subsequent stalling of the airfoil. For this reason, ice protection systems are often considered critical components of flight, and aircraft are often deiced prior to take-off in icy environments.

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

Graupel, also called soft hail or snow pellets, is precipitation that forms when supercooled water droplets are collected and freeze on falling snowflakes, forming 2–5 mm (0.08–0.20 in) balls of rime. The term graupel comes from the German language.

Glaze (ice) smooth, transparent and homogeneous ice coating occurring when freezing rain or drizzle hits a surface

Glaze or glaze ice, also called glazed frost, 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.

Clear ice means a solid precipitation which forms when air temperature is between 0 °C (32 °F) and −3 °C (27 °F) and there are supercooled, relatively large drops of water. A rapid accretion and a slow dissipation of latent heat of fusion favor the formation of a transparent ice coating, without air or other impurities. A similar phenomenon occurs when freezing rain or drizzle hit a surface and is called glaze. Clear ice, when formed on the ground, is often called black ice, and can be extremely hazardous.

Precipitation types

In meteorology, the various types of precipitation often include the character 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, such as a mountain.

Rain and snow mixed

Rain and snow mixed is precipitation composed of rain and partially melted snow. Unlike ice pellets, which are hard, and freezing rain, which is fluid until striking an object, this precipitation is soft and translucent, but it contains some traces of ice crystals, from partially fused snowflakes. In any one location, it usually occurs briefly as a transition phase from rain to snow or vice versa. Its METAR code is RASN.

December 1969 noreaster

The December 1969 nor'easter was a strong winter storm that mainly affected the Northeastern United States and southern Quebec between December 25 and December 28, 1969. The multi-faceted storm system included a tornado outbreak, record snow accumulations, a damaging ice storm, and flooding rains.

Glossary of meteorology Wikimedia list article

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

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.
  5. "THE RATE AT WHICH RAIN FREEZES IN A FREEZING RAIN EVENT" . Retrieved 2015-12-30.
  6. "Glaze Ice". Archived from the original on 2009-07-15. Retrieved 2009-07-18.
  7. Oblack, Rachelle. "Glaze Ice Definition". Archived from the original on 2008-08-22. Retrieved 2009-07-18.
  8. Munroe, Susan. "The Canadian ice storm of 1998" . Retrieved 2009-07-18.
  9. "Glaze Ice". Archived from the original on 2009-07-15. Retrieved 2009-07-18.