asp · Special · 21 October 2026 · By The Cumulus Desk

Asperitas: the newest cloud in the atlas

Asperitas is the most recently added cloud supplement, formally recognised by the World Meteorological Organization in 2017 after a decade of citizen-science campaigning. It is a dramatic, chaotic, wave-like underside on a layer cloud, and it is worth knowing both for its own sake and because its history shows how cloud classification actually works.

Asperitas

Band: Special
Altitude: 1.5 km – 5 km
Code: asp
Seasonal window: Asperitas is reported year-round. Reports are most common in temperate mid-latitudes where strong shear and moist mid-level layers coincide, which is most often in spring and autumn. (Mar–Nov)
Published: 21 October 2026
By: The Cumulus Desk

Weather clue

Asperitas usually appears in a moist, sheared layer and often near the end of a convective episode. If it is followed by thickening cloud and falling pressure, expect rain; if the layer breaks up within the hour, the airmass is drying.

How to observe it

Identify the parent cloud layer first — asperitas is a supplement, not a genus.
Photograph with the sun low and to one side; a high sun flattens the structure completely.
Record whether mammatus is present nearby, whether precipitation followed within the hour, and the surface wind.
Submit the observation with a date, time, location and a landmark reference to the Cloud Appreciation Society or a national meteorological archive.

What it looks like

Asperitas is defined by its underside: a layer cloud whose base has been replaced by a chaotic, undulating, wave-like surface, as if you were looking at the underside of a choppy sea from below. The waves are not organised into regular rows the way altocumulus undulatus is; they are disordered, with overlapping scales and steep local gradients.

It is usually seen on altocumulus or stratocumulus, and it is often accompanied by mammatus nearby. When lit from a low angle, the relief is dramatic; when the sun is high, it can look like nothing more than a messy grey sheet.

How it got its name

Asperitas was not identified by a research programme. It was documented by amateur observers, in particular the Cloud Appreciation Society, which collected thousands of photographs from around the world over several years and submitted the case for recognition to the WMO. It was added to the International Cloud Atlas in March 2017, alongside a set of other new entries including volutus, fluctus, flammagenitus, homogenitus and cataractagenitus.

It is a good example of how the atlas is maintained in practice: systematic observation by a distributed network of observers, followed by formal review. The Latin word asperitas means roughness or unevenness, and it is the first cloud name added to the atlas that came from a public campaign rather than from a research institution.

What causes it

The honest answer is that the mechanism is not fully settled. Several explanations have been proposed and the current assessment is that more than one may operate.

Shear instability. The most widely supported explanation is that asperitas forms where there is strong wind shear across a thin, moist layer, producing Kelvin-Helmholtz-like billows that become disordered as they interact with each other.

Gravity waves from below. Another proposal is that the wave structure is imposed from underneath, either by an updraught from convection below or by a gravity wave generated by distant terrain, and that the cloud simply makes an existing wave pattern visible.

Mammatus-related descent. Because asperitas is often seen near mammatus, some researchers suggest the two share a common origin in organised descent within the cloud layer, with asperitas being the more sheared and more chaotic version.

Observing it usefully

Because the mechanism is unsettled, careful observational records have real scientific value. The Cloud Appreciation Society and the WMO both accept and archive reports.

The most useful observations record the parent cloud type, the height estimate, the wind direction and speed at the surface, whether any precipitation occurred within the following hour, and whether mammatus was present. Photographs taken at a low sun angle show the structure far better, and a photograph with a fixed landmark lets other observers estimate scale.

Rules of thumb

Asperitas was formally added to the International Cloud Atlas in March 2017.
It is defined by a chaotic, wave-like underside on a layer cloud, not by regular undulations.
Its mechanism is still not settled; shear instability and imposed gravity waves are both candidates.
It often appears near mammatus and on altocumulus or stratocumulus layers.

Look-alikes
The other cloud	Comparison — Asperitas
波状高积云 · Altocumulus undulatus	Undulatus forms regular, parallel bands with a consistent wavelength; asperitas is disordered, with overlapping scales and no consistent wavelength.
悬球状云 · Mammatus	Mammatus hangs downward as discrete rounded pouches; asperitas is a continuous, chaotic undulating surface rather than separate pouches.

The two or three genera this one is most often confused with, and the single feature that separates them.

Observation questions

Why is asperitas not a cloud genus?

Because it describes a structure on the underside of an existing cloud layer rather than a distinct cloud type with its own formation mechanism. In the classification it is a supplementary feature, which is why it is always recorded together with a parent cloud such as altocumulus or stratocumulus.

How did a citizen observation become an official cloud name?

The Cloud Appreciation Society collected thousands of photographs from observers worldwide over roughly a decade and submitted the case to the World Meteorological Organization, which reviewed it and added asperitas to the International Cloud Atlas in March 2017.