What is freezing rain?

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What is freezing rain?

ENLARGE

Orange leaves and trip of tree branch are coated in ice

Freezing rain covers everything with a sheet of ice.

When water particles fall from clouds and reach the surface as precipitation, they do so primarily as rain, snow, freezing rain or sleet. The many types of precipitation occur because water can change phase as it falls.

In northern temperate latitudes, precipitation usually begins falling from a cloud as ice particles. The temperature between the cloud bottom and the ground then determines the precipitation type:

  • If the air temperature is above freezing all the way to the ground, the ice melts into liquid droplets — rain.
  • If the air temperature stays below freezing all the way to the ground, the ice crystals never melt and snow falls.
  • If the precipitation particles melt and then fall through a layer of cold air near the ground, we get ice storms, which usually form either sleet or freezing rain.

Sleet occurs when the layer of subfreezing air at the surface is deep enough for a raindrop to freeze into translucent balls of ice. Sleet bounces when it hits the surface, so it does not coat objects with a sheet of ice.

Freezing rain forms when a thin layer of cold air near the surface super-cools the melted precipitation, which then freezes on contact with cold objects on the ground. Freezing rain covers everything in a sheet of ice, creating shimmering landscapes, but also treacherous roads. Heavy, coated tree branches can break and destroy power lines.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.