Spaceweather.com, 7/24/13
MAMMATUS OVER MICHIGAN: July 22, 2013, Laura Tappy.
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.
