Turn an ocean upside down and hang it over a quiet neighbourhood, and you get something like these skies over Nebraska. Posted to Reddit in July 2014 by the user jebleez under the caption “The clouds were like a rolling ocean tonight”, the photographs show a cloud base heaving in dark folds and troughs. Today meteorologists have an official name for this look: asperitas.
Asperitas, the newest cloud in the atlas
The World Meteorological Organization’s International Cloud Atlas describes asperitas as “well-defined, wave-like structures in the underside of the cloud”, more chaotic and less horizontally organised than the related variety undulatus. It appears mainly with stratocumulus and altocumulus, two common layer clouds.
The name has an unusual history. Gavin Pretor-Pinney, founder of the Cloud Appreciation Society, began collecting members’ photographs of these turbulent cloud bases and in 2009 proposed them as a new type, first under the name undulatus asperatus, from the Latin for roughness. One of the best-known early pictures was taken over Cedar Rapids, Iowa, in June 2006. In March 2017 the WMO added asperitas to the new edition of the International Cloud Atlas as a supplementary feature. It was the first cloud formation added since cirrus intortus in 1951.
How wave-shaped cloud bases form
Pretor-Pinney worked with Giles Harrison, an atmospheric physicist at the University of Reading, to find out what shapes asperitas. Their study, published in 2017 in the Royal Meteorological Society journal Weather and reported by Eos, matched sighting photographs with weather-model data on temperature, humidity and wind.
Their conclusion: the waves are most likely made by atmospheric gravity waves. When a parcel of air is pushed upward, gravity pulls it back down and it overshoots, setting up an oscillation that ripples through the layer. Inside a cloud, those oscillations sculpt the base into the rough, chaotic swell seen in these photos, and the pattern can last for hours. The clouds usually sit low, around 1,000 to 2,000 metres up, and are probably made of water droplets.
Why the Great Plains see asperitas so often
Many of the most striking asperitas photographs come from the American Midwest: Iowa in 2006, and Lincoln, Nebraska, in July 2014, where the photographer Alex Schueth filmed a time-lapse of the clouds rolling over the city. The Plains get frequent summer thunderstorms, and asperitas often forms in the morning or around midday after a night of storm activity.
Despite their menacing look, these clouds almost always fade away without a storm developing. They are a leftover, not a warning sign. For a real warning, watch the official forecast and radar, and treat any thunder you hear as the moment to head indoors.
Photographing a sky like this
The best asperitas pictures share a few habits. Include something on the ground, such as roofs, trees or power lines, so the viewer can read the size of the waves. Shoot at a low sun angle, when side light picks out every fold, and expose for the bright gaps so the dark troughs keep their texture.
For more weather on our pages, compare our storm-cloud gallery, look closer at a rimed snow crystal under a microscope, or see the Grand Canyon’s cloud inversions.
Related reading
- Twenty Dramatic Cloud Images: Earth’s Skies and One Planetary Comparison
- Richard Clarkson’s Cloud: Turning Storm Light into an Object
- Eight Cloud Forms: A Gallery of Structure and Light
- Lenticular Clouds: How a Still Shape Can Contain Moving Air
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