Rare 'roll cloud' glides over Chicago

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A rare sight appeared in the sky over Chicago today. (WLS)

A rare "roll cloud" rolled over Chicago Tuesday.

As ABC7 Meteorologist Cheryl Scott explains, a roll cloud is a low, horizontal tube-shaped cloud that formed by winds changing speed and/or direction when the air temperature reverses its state. That results in warm air on top of cold air.

The shear in the atmosphere sets up a rolling motion, similar to the way a rolling pin is used in baking.

Earlier Tuesday, Scott said, a cold front moved into Chicago allowing for the spectacle.

Thanks to viewer Amy King for sharing her video with ABC7 Eyewitness News.

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weatheramazing videoChicago - Downtown
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