Mammatus clouds are benign, but their striking appearance might make you think otherwise. According to the University of Illinois, they're "usually seen after the worst of a thunderstorm has passed," so if you see these hanging overhead, you can rest easy with the knowledge that relief is near.
The imagery in the clip below is stunning, and descriptions of these clouds void of pictures are simply a disservice. So check out the clip below for the visual marvel of some of Earth's rarest cloud formations, and let us know which is your favorite. When the video's over, be sure to share it with your friends on Facebook!
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Source: http://expanse.collectivepress.com/these-rare-cloud-formations-might-make-you-rethink-whats-possible-in-this-world