Freezing rain

Freezing rain is the name given to rain precipitation maintained at temperatures below freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike sleet, 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.

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."

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.

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.

Other Languages
Afrikaans: Vriesreën
العربية: مطر متجمد
български: Леден дъжд
català: Pluja gelant
dansk: Isslag
Deutsch: Eisregen
eesti: Jäävihm
euskara: Euri izoztu
한국어: 우빙
hrvatski: Ledena kiša
Bahasa Indonesia: Pembekuan hujan
latviešu: Atkala
lietuvių: Lijundra
Limburgs: Iezel
magyar: Ónos eső
Bahasa Melayu: Hujan beku
Nederlands: IJzel
Nedersaksies: Iesel (weer)
日本語: 雨氷
norsk nynorsk: Underkjølt regn
português: Chuva congelada
Simple English: Freezing rain
slovenščina: Poledica
svenska: Frysande regn
українська: Крижана буря
文言: 雨冰
粵語: 凍雨
žemaitėška: Apskards
中文: 冻雨