What is sleet?

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What is sleet?
ENLARGE

Millions of tiny balls of ice lay on a wooden deck.

When precipitation droplets refreeze before hitting the ground, you get sleet.

Sleet is translucent balls of ice that are frozen raindrops. The most common forms of precipitation are rain, snow, freezing rain, and sleet.

In Wisconsin, precipitation usually begins as ice particles in a cloud. The temperature conditions below the cloud base determine if the precipitation ends up as rain, snow, freezing rain, and sleet. The altitude of the melting line, or the height where the air temperatures are at freezing, determines the type of precipitation. If the freezing line is above the cloud base, than the ice particles will melt as they fall and we will have rain. If the ground is at, or very near freezing, then the precipitation will be snow, as the ice crystals won’t have enough time to melt.

In freezing rain and sleet, the melting line is between the ground and the cloud base, and the ground temperature is below freezing. So, the ice crystals melt and become liquid drops with a temperature near freezing.

The main difference between getting freezing rain or sleet is the height of the melting layer and the temperature of the surface. Freezing rain occurs when the liquid drops hit the ground and freeze on contact. Sleet occurs when there is enough time for the liquid drops to refreeze before they hit the ground.

A mixture of rain and snow is not sleet, even though that term is often used in common parlance. A mixture of rain or snow is a term used to describe either wet snow or a mixture of snow and rain.

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.