Asperatus Cloud Formation, New Zealand

Asperatus (aka undulatus asperatus) cloud formations are very rare worldwide – so rare they were not proposed as a separate cloud classification until 2009. The name translates as roughened or agitated waves. It is thought that asperatus clouds' choppy undersides may be due to strong winds disturbing previously stable layers of warm and cold air.

Graeme And Gerson, an MSc student at the Department of Meteorology, Reading University, studied weather records and used a computer model to simulate the cloud formation. In doing so, Graeme found that asperatus clouds form in the sort of conditions that produce mamma clouds (also known as mammatus); asperatus clouds form when the winds up at the cloud level cause the cloud to shear into wave-like forms known as undulatus.

If accepted as a new classification, asperatus will be the first new classification in the World Meteorological Organization's International Cloud Atlas since cirrus intortus in 1951.

This image, captured by Merrick Davies, is of a rather turbulent sky over Hanmer Springs, South Island, New Zealand. Such cloud formations have been seen all over Britain, as well as in the plains of the USA.

Photo: Merrick Davies

For more cloud photos, and to send your own cloud photos, check out the cloud appreciation society: The Royal Meteorological Society is gathering evidence of the type of weather patterns in which undulatus asperatus clouds appear.

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