Fr/spaceweather.com

MAMMATUS OVER MINNESOTA: On May 10th, a severe storm captured national attention when it dumped golf-ball-sized hail on a Minnesota Twins baseball game. “I missed the hail,” reports John Rogers of New Hope, Minnesota, “but I got a nice view of the clouds that formed after the storm passed.” He snapped this picture in waning twilight at 8:30 pm local time:

These are mammatus clouds. Named for their resemblance to a cow’s underbelly, they 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, dash outside when the downpour stops; you could witness a beautiful mystery in the sky.