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.
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 2009the 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: National Weather Service