MAMMATUS OVER MICHIGAN

I have had the opportunity on a few occasions to see these unique clouds with my own two eyes. They only stick around for a short time after a thunderstorm but they are well worth seeing if you have the chance. This picture does not even do them justice as to how cool they are to see.

http://www.spaceweather.com/

Quote :

On July 22nd a rain squall blew through Iron Mountain, Michigan, and left some strange-looking clouds in its wake. Laura Tappy snapped this picture:

"There was heavy rain for about five minutes," describes Tappy. "When it ended, the sky turned an eerie yellow color and these mammatus clouds appeared."
Named for their resemblance to a cow's underbelly, mammatus clouds sometimes appear at the end of severe thunderstorms when the thundercloud is breaking up. Researchers have called them an "intriguing enigma," because no one knows exactly how and why they form. The clouds are fairly common but often go unnoticed because potential observers have been chased indoors by the rain. If you are one of them, go back outside when the downpour stops; you could witness a beautiful mystery in the sky.