Asperitas clouds

Height of base: 4,000 - 10,000 ft

Shape: Undulating waves

Latin: aspero - make rough or uneven

What is asperitas cloud?

Asperitas (formerly referred to as Undulatus Asperitas) is a distinctive, but relatively rare cloud formation that takes the appearance of rippling waves. These wave-like structures form on the underside of the cloud to makes it look like a rough sea surface when viewed from below.

How does asperitas form?

The way in which asperitas clouds form is somewhat a mystery, yet there is much debate and confusion over how the wave-like clouds come into existence. It is hypothesised that their appearance is associated with the aftermath of convective thunderstorms, though they have also been sighted in relatively calm environments. One theory does suggest that they are formed when mammatus clouds descend into areas of the sky where wind direction changes with height causing the wave-like movement.

What is clear however is that atmospheric conditions must be unstable to form a wavy cloud base like that seen with asperitas.

What weather is associated with asperitas formations?

Though the formation itself does not produce rainfall, asperitas have been linked to thunderstorms, occurring afterwards. Though the likely unstable atmospheric conditions required to form the wavy cloud base could also allow the growth of convective rain clouds, meaning that asperitas could be accompanied by other, precipitation-producing clouds.

The 'newest' cloud type

The addition of this previously undocumented cloud formation to the World Meteorological Organisation's International Cloud Atlas was first proposed by the Cloud Appreciation Society in 2008. Between then and 2015 the case was supported by members of the public sending in images of the dramatic cloud formations.

At the WMO's 17th Congress in 2015, the classification was accepted, making asperitas the first new cloud type in over 50 years, since cirrus intortus in 1951.

What clouds are associated with asperitas?

It is not yet defined what cloud type gives its home to asperitas, but the WMO began work on deciphering the cloud's mysteries in 2008. Full information will most likely be published in the next edition of the International Cloud Atlas.

In the timelapse video below, you can see the full effect of Asperitas and its wave-like appearance: