From a 12th-floor office in Cedar Rapids, Iowa, Jane Wiggins photographed gray clouds folded into uneven ridges above the city. The Cloud Appreciation Society collected her image alongside other photographs showing a similarly roughened sky.
The formation now has an official name: asperitas. Its lower surface has a wrinkled, irregular relief, with rounded swells, hollows and occasional sharp projections. Light passing through thicker and thinner parts of the cloud exaggerates the contrast, making a broad layer look deeply sculpted.
In the World Meteorological Organization’s classification, asperitas is a supplementary feature seen mostly on stratocumulus and altocumulus. Those names identify the host cloud; asperitas describes the unusual shape of its underside.
The arrangement helps distinguish it from other cloud displays. Undulatus tends to form more orderly bands. Mammatus has hanging pouches. Asperitas can have some rounded shapes too, but its waves vary in shape and alignment across the cloud base.
Gavin Pretor-Pinney, the Cloud Appreciation Society’s founder, proposed a classification in 2008, initially using the name asperatus. As the proposal developed, the name became asperitas, a Latin noun meaning roughness. The revised International Cloud Atlas, launched on March 23, 2017, included it among five new supplementary features.
What could be making the waves
Recognizing the pattern has proved easier than establishing a single explanation for it. A study led by atmospheric scientist Giles Harrison, published in Weather in 2017, combined reported sightings with satellite images, atmospheric-model profiles and laser measurements of cloud height.
In three European cases, the roughened cloud bases sat well below the altitude where temperatures reached freezing. The researchers inferred that these examples were made of liquid droplets. The atmospheric-model profiles indicated stable layers within the clouds and changes in wind with height, conditions that could help waves persist and become distorted.
Their proposed explanation involved atmospheric gravity waves. In stable air, a pocket of air pushed up or down can bob around its original height. A suitable layer within a cloud can trap that wave motion, while winds changing speed or direction with height bend the resulting pattern. Nearby thunderstorms or a weather front offered possible sources of the disturbances in the cases studied.
A later study explored a different starting point. In research published in Physical Review Fluids in 2022, S. Ravichandran and Rama Govindarajan simulated droplets settling from cloud into drier air below. Evaporation cooled the air below the cloud enough for denser pockets to sink. With an intermediate amount of wind shear, the simulations produced elongated, inclined structures resembling asperitas.
That was an idealized model, rather than a reconstruction of a measured cloud. Its authors allowed that different events might develop through different processes. Both studies give changing winds an important shaping role, but they don’t establish one recipe that explains every sighting.
Asperitas has been observed near storms and fronts, as well as in calmer settings. The feature alone doesn’t tell an observer that severe weather is about to arrive. Forecasts and warnings remain the guide. The National Weather Service advises anyone who hears thunder to shelter in a substantial building or an enclosed, hard-topped metal vehicle.
The photographs remain useful because they can record where and when the pattern appeared. For the 2017 study, clusters of time-stamped sightings let researchers compare the cloud display across a region, then check it against nearby instruments and atmospheric data. A cloud watcher’s view from the ground could be connected to the winds, moisture and cloud height surrounding it.
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