These eerie mammatus clouds appeared over a high school graduation ceremony in Pekin, Ill., on May 22, 2011, as part of the tornado outbreak that produced the devastating Joplin tornado the same day. (Credit: iWitness Weather/Candi Carter Kupris)
While the mechanisms behind the formation of most clouds are well-understood, that is not the case for the mammatus cloud.
These clouds, often seen behind departing thunderstorms, appear to "hang" from the sky, suggesting that perhaps they are formed by sinking air, rather than the rising air that produces most clouds.
But a 2006 review in the Journal of the Atmospheric Sciences noted that there are at least ten different theories about why and how mammatus clouds form, and found shortcomings with practically all of those theories.
Nonetheless, you can rest assured that as frightening as they appear and despite the rough company they keep, mammatus clouds in and of themselves are not dangerous and do not produce severe weather.