Shelf Clouds: A Menacing Sight!

Jon Erdman Published: Apr 15, 2013, 8:53 AM EDT weather.com

http://www.weather.com/news/shelf-cloud-photos-20130412

Freeport, Ill.

iWitness Weather

A huge shelf cloud rolls in just east of Freeport, Ill. in 2010. (Photo credit: iWitnessWeather/danros2)

Most of you would probably say a tornado is the most frightening cloud formation.
Perhaps close behind is what you see above.
Resembling an advancing army, shelf clouds are typically seen at the leading edge of a squall line of thunderstorms.
What you're seeing in a shelf cloud is the boundary between a thunderstorm (or line of thunderstorms') downdraft and updraft.
Rain-chilled air descends in a thunderstorm, then spreads laterally when reaching the 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.
As the shelf cloud passes, you'll feel an abrupt wind shift in both direction and speed, followed within minutes by heavy rain or hail.
A more rare variety of this type of cloud, called a roll cloud, resembles a giant rolling pin in the sky.  (See the second photo in the slideshow above)
Unlike a shelf cloud, the roll cloud is completely detached from its parent thunderstorm.