According to International Cloud Atlas, Asperitas are defined as ... Well-defined, wave-like structures in the underside of the cloud; more chaotic and with less horizontal organization than the variety undulatus.
Asperitas is characterized by localized waves in the cloud base, either smooth or dappled with smaller features, sometimes descending into sharp points, as if viewing a roughened sea surface from below.
Varying levels of illumination and thickness of the cloud can lead to dramatic visual effects. Occurs mostly with Stratocumulus and Altocumulus.
This cloud formation was first popularized and proposed as a type of cloud in 2009 by Gavin Pretor-Pinney of the Cloud Appreciation Society.
Although they appear dark and storm-like, they almost always 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. Read more
Video: Asperatus and Other New Cloud Types Added For the First Time in 30 Years
This update is the first in 30 years for the International Cloud Atlas, which the WMO calls "the global reference for observing and identifying clouds." A new cloud species has been added to the atlas called volutus, more commonly known as a roll cloud by meteorologists. Cloud species are subdivisions of the 10 basic cloud "genera," the WMO says.
Asperatus Clouds Over New ZealandNASA - February 27, 2013
What kind of clouds are these? Although their cause is presently unknown, such unusual atmospheric structures, as menacing as they might seem, do not appear to be harbingers of meteorological doom. Known informally as Undulatus asperatus clouds, they can be stunning in appearance, unusual in occurrence, are relatively unstudied, and have even been suggested as a new type of cloud.
Whereas most low cloud decks are flat bottomed, asperatus clouds appear to have significant vertical structure underneath. Speculation therefore holds that asperatus clouds might be related to lenticular clouds that form near mountains, or mammatus clouds associated with thunderstorms, or perhaps a foehn wind -- a type of dry downward wind that flows off mountains. Such a wind called the Canterbury arch streams toward the east coast of New Zealand's South Island. The above image, taken above Hanmer Springs in Canterbury, New Zealand, in 2005, shows great detail partly because sunlight illuminates the undulating clouds from the side.
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