Asperitas (from the Latin for "roughness") is a supplementary cloud feature officially recognised by the World Meteorological Organization (WMO) in 2017, the first addition to the International Cloud Atlas in over 30 years. It has a spectacular appearance: the cloud base displays chaotic, turbulent undulations resembling the surface of a rough sea viewed from below.
Formation and characteristics
Asperitas forms on the underside of stratocumulus or altocumulus layers when strong variations in wind shear and gravity waves exist at the interface between air layers with different speeds, temperatures, or humidities. Unlike undulatus clouds (regular, parallel undulations), asperitas displays irregular, multi-directional, turbulent-looking undulations.
Its official recognition was largely driven by the Cloud Appreciation Society, which compiled photographs of this formation from around the world. Although visually dramatic and even threatening, asperitas is not usually associated with severe weather: it can appear before or after storms, but also in situations of relative stability.
It is observed at all latitudes, including Spain, where it can appear in association with moderate instability and mid-level cloud layers. It is a reminder that cloud classification continues to evolve. See also: cumulonimbus, nimbostratus.