Freezing rain

Freezing rain is the name given to rain that falls when surface temperatures are below freezing. 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, and automobiles. The resulting ice, called glaze, 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.)

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. 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.

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Wikipedia