Here’s a special kind of cloud known to scientists as a Kelvin Helmholtz cloud. These clouds look like breaking ocean waves, with the rolling eddies seen at the top of the cloud layers usually evenly spaced and easily identifiable. Kelvin Helmholtz clouds are named for Lord Kelvin and Hermann von Helmholtz, who studied the physics of the instability that leads to this type of cloud formation.
A Kelvin Helmholtz instability forms where there’s a velocity difference across the interface between two fluids: for example, wind blowing over water.
You’ll often see the characteristic wave structure in this type of cloud when two different layers of air in our atmosphere are moving at different speeds. The upper layers of air are moving at higher speeds and will often scoop the top of the cloud layer into these wave-like rolling structures.
The clouds often form on windy days, when there’s a difference in densities of the air, for example, during a temperature inversion. They’re often good indicators of atmospheric instability and the presence of turbulence for aircraft.
It’s widely believed that these waves in the sky inspired the swirls in van Gogh’s masterpiece Starry Night.
Bottom line: Kelvin Helmhotz clouds – also called billow clouds – form when two different layers of air in our atmosphere are moving at different speeds.
Eleanor Imster has helped write and edit EarthSky since 1995. She was an integral part of the award-winning EarthSky radio series almost since it began until it ended in 2013. Today, as Lead Editor at EarthSky.org, she helps present the science and nature stories and photos you enjoy. She also serves as one of the voices of EarthSky on social media platforms including Facebook, Twitter and G+. She and her husband live in Tennessee and have two grown sons.