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Freezing rain
Freezing rain is the name given to rain that falls when surface temperatures are below freezing. Unlike a mixture of rain and snow, ice pellets (both of which are sometimes called "sleet"), or hail, freezing rain is made entirely of liquid droplets. The raindrops become supercooled while passing through a sub-freezing layer of air many hundreds of feet above the ground, and then freeze upon impact with any surface they encounter.〔Glossary of Meteorology. (F. ) Retrieved on 2008-02-17.〕 The resulting ice, called glaze, can accumulate to a thickness of several centimeters. 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 not violent storms, freezing rain is notorious for causing travel problems on roadways, breaking tree limbs, and downing power lines from the weight of accumulating ice. 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.)〔National Weather Service Forecast Office, La Crosse, Wisconsin. (Significant Weather Phenomena Matrix. ) Retrieved on 2006-12-08.〕
== Mechanism ==
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.〔University of Illinois. (Cyclones and Fronts: the development of freezing rain. ) Retrieved on 2006-12-08.〕 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 (). 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 , they instantly freeze, forming a thin film of ice, hence freezing rain.〔University of Illinois. (Cyclones and Fronts: the definition of freezing rain. ) Retrieved on 2007-12-16.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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