A newly-discovered cloud formation, first identified last year, has appeared again this week after a massive storm over Missouri. The clouds, called "Undulatus asperatus," create a roiling, churned pattern that looks almost like the wake from a ship.

According to the National Weather Service:

A storm system moving across the Ozarks on June 7th produced widespread interest for more than just the thunder and rainfall it produced. As the system moved across the region, the balance of instability aloft and a relatively stable low level airmass helped to produce what was likely a potentially new form of clouds now under review by the World Meteorological Society or WMO . . . Undulatus asperatus (or alternately, asperatus) is a rare, newly recognized cloud formation . . . The clouds are most closely related to undulatus clouds. Although they appear dark and storm-like, they tend to dissipate without a storm forming. The ominous-looking clouds have been particularly common in the Plains states of the United States, often during the morning or midday hours following convective thunderstorm activity. As of June 2009[update] the Royal Meteorological Society is gathering evidence of the type of weather patterns in which undulatus asperatus clouds appear, so as to study how they form and decide whether they are distinct from other undulatus clouds.

photos via Sam Cogley and Hollywood (Thanks, Josh Hamblin!)