Asperitas – Underwater Skies

Imagine looking up and seeing the sky behave like the underside of a choppy ocean. That’s the dramatic effect of asperitas clouds — chaotic, wave-like formations that ripple across the base of Stratocumulus or Altocumulus clouds, resembling turbulent water seen from below.

Asperitas is what we call a supplementary feature — a distinctive formation that appears as part of a larger cloud type. The name comes from the Latin for ‘roughness,’ and it’s easy to see why, as Heather Cormons discovered from the top of a mountain outside Shenandoah National Park, near Lydia, Virginia, US.

‘These clouds were rolling and changing shape in wavelike formations as they moved through the sky,’ said Heather. ‘I had never seen anything like it, and I wondered if they might be asperitas clouds. There was no storm or rain we could detect.’

While asperitas often accompany thunderstorm clouds, known as Cumulonimbus, they can also appear in calmer skies. Their origins remain a bit mysterious, but we believe they most likely form when several atmospheric ingredients come together – gravity waves, wind shear, and a stable layering of air:
Gravity waves are ripples rising and falling air that flow through the atmosphere, often around storm clouds or along weather fronts.
Wind shear refers to sudden changes in wind speed or direction with altitude.
Atmospheric stability is when warmer air sits above cooler air, suppressing vertical movement.

When gravity waves enter a stable region of cloud, they can become ducted, or trapped, propagating through the cloud as a result. It is thought these gravity waves can then become distorted by wind shear, resulting in the turbulent, chaotic undulations we call asperitas.

Asperitas was officially added to the World Meteorological Organization’s International Cloud Atlas in 2017 — thanks in large part to the Cloud Appreciation Society and its members, who had campaigned for some years to have this striking formation recognised.

It became the first new cloud classification to be recognised since the 1950s — a testament to the power of curious observers with their eyes on the sky.

Altocumulus asperitas spotted in the sky outside of Shenandoah National Park, near Lydia, Virginia, US by Heather Cormons. View this in the photo gallery.