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Ice pellets

earth science Maturity 5-7 Next article →

Ice pellets are a specific form of precipitation. They consist of small, hard, and translucent balls of ice.

Sleet (ice pellets)
Sleet (ice pellets)

While they may look like other frozen weather, they are distinct. They are not the same as graupel, which is a soft hail made of opaque rime. They are also different from a slushy mixture of rain and snow. Unlike liquid raindrops that make a dull splat, ice pellets often bounce. When they strike objects like windshields or dried leaves, they make a high-pitched "tap."

Sleet on the ground
Sleet on the ground

The formation of ice pellets requires a very specific temperature profile in the atmosphere.

SleetSounding
SleetSounding

This process begins with snowflakes falling from a cold layer high in the sky. As they fall, they encounter a layer of air that is above the freezing point. This warm layer causes the snowflakes to melt partially or completely. In French, this is called neige fondue, which literally means "melted snow." After melting, the water continues to fall toward the ground. It then enters a second sub-freezing layer of air located near the surface. This cold layer causes the water to re-freeze into hard pellets before it hits the ground.

The thickness of this bottom cold layer is very important for the type of weather produced. If the sub-freezing layer is deep enough, the water has time to become solid ice pellets. However, if the cold layer is too small, the water will not have time to re-freeze. In that case, the precipitation becomes freezing rain instead. This rain stays liquid until it hits a surface, where it then freezes.

SleetSounding
SleetSounding

These temperature patterns are most common in front of a warm front during the cold season. They can also appear behind a cold front or near a stationary front.

Terminology for this weather varies depending on where you live in the world. In the United States, the official term used by the National Weather Service is sleet. However, the word sleet has a different meaning in many Commonwealth countries, such as Canada. In those places, sleet usually describes a mixture of rain and snow. Because of this confusion, Environment Canada avoids the word sleet entirely. Instead, they use the specific terms "ice pellets" or "wet snow." Weather experts also use the METAR code PL to identify ice pellets, though the code was PE before November 1998.

In most parts of the world, ice pellets occur for only brief periods. They usually do not accumulate in large or troublesome amounts. However, there are notable exceptions in certain geographic regions. In the eastern United States and southeastern Canada, large accumulations can happen. This occurs when warm air flows north from the Gulf of Mexico. This warm air can overrun cold, dense air at the surface for many hundreds of miles. When this happens for an extended period, significant amounts of ice pellets can build up.

Sleet on the ground
Sleet on the ground

Dealing with an accumulation of ice pellets is different from dealing with snow.

Sleet on the ground
Sleet on the ground

If you have the same volume of ice pellets as you do snow, the pellets are much heavier. This extra weight makes the ice more difficult to clear away from roads or paths. Additionally, ice pellets take a much longer time to melt than fresh snowfall. This is because they have less surface area compared to the same volume of snow. Because they do not melt quickly, they can remain on the ground for a long time.

To understand the physical scale of these objects, we can compare them to common items.

Sleet (ice pellets)
Sleet (ice pellets)

For example, ice pellets are often very small when held next to a U.S. penny. A penny has a diameter of 19.05 mm. Seeing them next to such an object helps show how tiny and numerous they are. Understanding these small pieces of ice helps scientists and weather services predict how much weight will fall on structures and how long the ice will last. This connection between temperature layers and physical properties is a key part of studying weather systems.

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File:SleetSounding.svg
SleetSounding.svg
File:Sleet on the ground.jpg
Sleet on the ground.jpg
File:Sleet (ice pellets).jpg
Sleet (ice pellets).jpg
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