These dark waves rolling in over Chilliwack, British Columbia, Canada on a layer of Stratocumulus are the cloud features known as fluctus.

Spotted by Philip Tingey (Member 45,142), they are also known as Kelvin-Helmholtz wave clouds, after two physicists who studied the fluid dynamics behind the phenomenon.

Fluctus form in shearing winds. A faster airstream flowing over a slower one can cause undulations in any cloud layer that has formed in between. If the air is stable and the wind speeds are just right, the tops of the undulations can be curled over, revealing the usually invisible motion of the atmosphere. Fluctus can form over mountains, like Philip’s have here, where the lower winds are slowed as they interact with the raised terrain, but the higher winds can flow unimpeded.

Fluctus forming in a row of waves like this are rare, and they’re usually short lived. Philip knew he’d have to be quick to catch them, so he ran for his camera rather than his surfboard.