The breaking-wave cloud features known as fluctus form as a result of wind shear. When cloud develops at the boundary between a layer of cold air below and warmer air above, undulations can be lifted up and curled over by winds flowing much faster in the layer above than in the one below.

Sheryl R. Garrison (Member 51,711) spotted these Stratus fractus fluctus clouds in Waterton Lakes National Park, Canada. ‘Fast and fleeting fluctus forms a fanciful mountain wave al fresco,’ she wrote. ‘Say that three times as fast as you can. That’s about how long it lasted.’