Asperatus clouds are so rare they managed to escape classification until 2009. Ominous and stormy as they appear, these clouds often break up rather quickly, without producing a storm. As with most other undulating cloud types, these clouds are formed when turbulent winds or colliding air masses whip up the bottoms of the cloud layer into fancy shapes and formations. More common in the plains of the United States (try Iowa), asperatus clouds are at their weird and swirly best during the morning or midday hours after a thunderstorm.
Photograph by B.J. Bumgarner