Cloud of the Day – Asperatus

Asperatus, also known as undulatus asperatus and altocumulus undulatus asperatus, is biblical. It’s the kind of cloud that makes people think that the world might be at the mercy of supernatural forces. They might fall to their knees and beseech their wrathful gods for mercy. Other people might look at them and say, “So, that’s where van Gogh got it from.”

Photo credit - Ken Prior

Photo credit – Ken Prior

Asperatus, loosely meaning “roughened waves,” is thought to form under the same kind of conditions as mammatocumulus, only with winds strong enough to shear the mammatus bulges into wave-like undulatus forms. This cloud hasn’t yet been officially named and added to the World Meteorological Organization’s definitive International Cloud Atlas. The Atlas was most recently published in 1975. The last time a cloud was added was 1951. The jury is out on whether asperatus will be added to the Atlas, and no one expects it to be soon if it is.

Note: The World Meteorological Organization has added asperatus — renamed asperitas — to the International Cloud Atlas (see this Green Comet post) in its fifth edition published in 2017.

Photo credit - Agathman - CC-BY

Photo credit – Agathman – CC-BY

The Cloud Appreciation Society has been very important in the discovery of this new cloud type, especially its founder, Gavin Pretor-Pinney. The society has thousands of members who send in beautiful photographs of clouds, and it has a nice selection of asperatus. Because of the way their site is set up, I can’t link directly, so you’ll have to search on “asperatus.”

Photo credit - Ave Maria Mõistlik - CC-BY-SA

Photo credit – Ave Maria Mõistlik – CC-BY-SA

Asperatus is not a harbinger of stormy weather, more often appearing as the weather abates.

Photo credit - NASA - PD

Photo credit – NASA – PD


3 comments for “Asperatus

  1. February 20, 2015 at 22:18

    Biblical is a good word really.
    Is it more likely to appear in upper latitudes?

    • February 20, 2015 at 22:38

      One article mentioned the American plains as a place to see them. That makes sense because they get some big thunder storms on the Prairies, and thunder storms are a good source of mammatus. I’d guess that any place that boils up big thunder clouds would be a candidate.

  2. February 22, 2015 at 10:43

    Magnificent! I can see why van Gogh was impressed enough to paint the cloud.

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