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