photo #9361253

Valerie James caught a row of Altocumulus fluctus cloud features in shadow over Bentworth, Hampshire, England. The distinctive fluctus feature also goes by the not-very-snappy name ‘Kelvin-Helmholtz wave cloud’. This is in reference to a pair of Victorian scientists, Lord Kelvin and Hermann von Helmholtz, who studied vortex dynamics.These ocean breakers of the sky develop in wind shear along the stable boundary between two differing layers of air. Usually, a layer of denser, cooler air finds itself below one of less dense, warmer air, with the winds in each differing considerably. Any cloud that develops around this stable boundary between airflows can become ruffled as it is blown along, and the tops of the undulations can, for a few brief moments, be curled over into a procession of vortices. Since fluctus clouds are rare and fleeting, they inevitably make waves whenever they appear.