Mark Anstey (Member 63,396) was travelling home along the River Mersey in Liverpool, England when he noticed a regular series of ripples, like those in the wake of a boat. But these weren’t waves on the water. They’d formed way above, in a patch of Altocumulus clouds.

Fluctus, also known as Kelvin-Helmholtz wave clouds, appear like breaking waves, curling over themselves in vortices. Fluctus are rare, short-lived, and highly prized by cloudspotters. The features can form in various cloud types – Cirrus, Altocumulus, Stratocumulus, Cumulus, Stratus, and even fog – and usually last only a minute or two. They are caused by wind shear, when faster-moving air across the top of the cloud causes undulations to develop that it lifts into a series of curls. They might resemble breaking waves, but no ‘Ferry Cross the Mersey’ caused these undulations to form.