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What's The Deal With Sleet?

by Steve Templeton

KMOV.com

Posted on February 12, 2008 at 3:51 PM

Updated Wednesday, Nov 4 at 3:30 PM

A few people at the station were asking the question, "If we're getting so much ice South of St. Louis, why aren't there more power outages?" Kent and I addressed that on the evening shows Monday night and even did a brief explainer of how sleet forms. But I'll go into a bit more detail here.

To answer the question above, keep in mind that sleet and freezing rain are both ice. But they form in different ways.

It's freezing rain that can cause power outages because it falls as liquid water that freezes shortly after contact with objects like trees, power lines or the ground. That's how power lines can get a coating of ice and break under the weight of that ice. While freezing rain falls as rain and turns into ice, sleet is already ice as it falls.

Sleet develops because at some point, a thousand or more feet above the ground, a rain drop freezes on it's way down to earth. Now it has become sleet. Sleet will bounce off of power lines and trees and settle on the ground as small ice pellets, creating slick driving and walking conditions.

So that helps to explain the difference between sleet and freezing rain, and why 3" of sleet didn't cause massive power outages in Southern Missouri yesterday. However, how does this stuff form in the first place?

It's all because of a warm, above frrezing layer of air, thousands of feet above the ground. This warm air melts snow into rain drops. These raindrops now fall down towards the ground where temperatures are much colder. Yesterday for instance it was 15 to 20 degrees at the surface. So, these raindrops freeze into sleet as they encounter the colder air near the surface. If the cold air is shallow enough, the rain drops don't have a chance to freeze and they stay a liquid. But once this liquid hits a cold object like the ground, trees or power lines it freezes and that of course is freezing rain.

I hope this helps clear that up, and now it's on to clearing up the ice. Upper 40s on Thursday should help!

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