Tuesday, April 28, 2015

Shelf Clouds: Not Tornadoes, But Still Scary and Ominous

A shelf cloud, also known as an arcus or arc cloud, may be the most frequently submitted cloud photo to ourweather.com photo gallery and Facebook page. Based on their appearance, we certainly understand why. 
Shelf clouds are typically seen at the leading edge of a thunderstorm or squall line of thunderstorms. While menacing in appearance, shelf clouds are not tornadoes or wall clouds.
What you're seeing in a shelf cloud is the boundary between a downdraft and updraft of a thunderstorm or line of thunderstorms.
Rain-chilled air descends in a thunderstorm's downdraft, then spreads laterally when reaching Earth's surface. Warmer, more moist air is lifted at the leading edge, or gust front, of this rain-cooled air. When this warm, moist air condenses, you see the shelf cloud.

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