Shelf Clouds: A Menacing Sight!

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

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.

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