Many times we are asked, "What's the difference between freezing rain and sleet?" Below we answer that question and address the key atmospheric differences in their formation.
Freezing rain vs. sleet
Sleet is frozen precipitation that falls as ice pellets that you may see bouncing off your windshield, roof or the ground. Depending on the intensity and duration, sleet can accumulate much like you see with snow.
Freezing rain on the other hand, falls just like normal rain only it freezes on contact with roads, trees, power lines and other structures since temperatures are 32 degrees or below at the surface. Even light accumulations may cause dangerous travel, while heavier amounts can be damaging.
So what in the atmosphere causes these two different types of wintry precipitation?
In the sleet scenario, snow falling aloft goes through a warm layer where it melts into rain. After exiting this warmer layer, the raindrops then refreeze into pellets of ice as they fall into a deep and or very cold layer of sub-freezing air near and just above the surface of the earth.
For freezing rain, the setup essentially remains the same except for one key difference.
Snow falling aloft goes through a warm layer where the flakes melt into rain. The difference is the warm layer is much deeper and extends closer to the ground making the layer of sub-freezing air near the earth's surface much thinner. As a result, the raindrops do not have time to refreeze into ice pellets and instead adhere to various surfaces such as cars, trees and power lines.
Snow melts in a warm layer aloft in both cases. The depth of the cold layer near the surface determines whether raindrops form into sleet (left) or freeze on contact to surfaces (right).
No warm layer for snow
You may have guessed already, but for snow the warm layer aloft is absent and the snowflakes make it all the way to the ground.
Snow can fall when surface temperatures are above freezing in a relatively shallow layer at the surface. In situations like this, the snow will not have enough time to melt before reaching the ground – though it will be quite wet with large flakes, the result of wet snowflakes sticking to one another.
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By Dan Heims, president, Terra Nova Nurseries, Inc.
Photographs courtesy of Terra Nova Nurseries, Inc.
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