Sleet: Definition & Formation

Instructor: Megan Wahl

Megan has taught middle school science and developed curriculum for k-higher ed. She has a master's degree in Educational Technology.

In this lesson you will learn more about a weather phenomenon called sleet. You will know how to identify sleet and why sleet happens as a weather event.

Winter Weather Conditions

If you've ever lived in a climate with cool weather, you are probably familiar with sleet. Not quite rain and not quite snow, sleet forms during the winter months and in periods of cold temperatures. It looks like tiny ice pellets falling from the sky.

Sleet falls in the form of ice pellets
closeup image of ice pellets

When sleet falls, it covers all outside surfaces. Children hoping for a snow day are happy to see sleet because it makes roads and sidewalks slick, often forcing schools to close. You might also be able to tell it's sleeting by using your ears. You can often hear sleet coming down, striking roofs, gutters, cars, and any other objects exposed outside.

What is Sleet?

Sleet is defined as pellets of ice that fall as a form of precipitation. Sleet forms in the atmosphere from frozen raindrops or partially frozen snowflakes. According to the National Weather Service, heavy sleet is defined as an accumulation of ice pellets covering the ground at a depth of one-half inches or more.

How Does Sleet Form?

If you stand on the grass and look up in the sky, you see clouds and the sun during the day, and the moon, stars, and clouds at night. It seems like the sun, moon, and stars are close, but in reality they are thousands and millions of miles above the surface of the Earth. The outer edges of Earth are quite far away -- it's about 400 miles from where we stand on Earth to where space begins. The space between our grass yards and outer space is the atmosphere. This is where gases mix together, including water vapor that evaporates from surface water. Water is constantly moving from Earth's surface to the atmosphere, and then back to Earth as precipitation.

Our atmosphere has several layers -- five, to be exact. The bottom layer called the troposphere is the layer of the atmosphere closest to the ground where we live. It is the thickest at the equator, about 12 miles or 20 kilometers, and thinner at the poles, about 4 miles or 6 kilometers. The troposphere is the part of the atmosphere where precipitation forms and weather occurs. Temperatures at Earth's surface and temperatures in the atmosphere determine which type of precipitation will form. In general, the higher you go in the atmosphere, the cooler the temperatures are. Precipitation can take the form of rain, snow, sleet, or hail, depending on those temperatures.

Sleet
raindrops freeze into sleet in cooler surface temperatures

Sleet falls during cold temperatures. Snow flakes form in the atmosphere and fall down towards the ground. If they pass through a warm layer of air, the snow flakes melt and turn into liquid. As they pass through the warm air and move down toward cold surface temperatures, the liquid refreezes into pellets of ice, called sleet.

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