Undulatus asperatus (or alternately, asperatus) is a rare, newly recognized cloud formation, that was proposed in 2009 as the first cloud formation added since cirrus intortus in 1951 to the International Cloud Atlas of the World Meteorological Organization. The name translates approximately as roughened or agitated waves.

Margaret LeMone, a cloud expert with the National Center for Atmospheric Research has taken photos of asperatus clouds for 30 years, and considers it a likely new cloud type. On June 20, 2006 Jane Wiggins took a picture of asperatus clouds from the window of a downtown office building in Cedar Rapids, Iowa. In 2009 Gavin Pretor-Pinney, founder of the Cloud Appreciation Society, came across the image and began working through the Royal Meteorological Society to promote the cloud type. Wiggins’ photograph was posted on the National Geographic website on June 4, 2009.

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 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.
source