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| | Earth on May 29, 2014

I saw a cloud that looked like ocean waves. What is it?

Van Gogh clouds! Like breaking ocean waves. They are called Kelvin Helmholzt clouds, aka billow clouds or shear-gravity clouds.

View larger. |  Photo credit: Paul Chartier

View larger. | EarthSky Facebook friend Paul Chartier captured these Kelvin Helmholtz clouds on Saturday, May 17, 2014 at Tupper Lake, New York, in the Adirondack Mountains. Photo by Paul Chartier

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.

View larger. |  Photo credit: Risa Bender

View larger. | EarthSky Facebook friend Risa Bender wrote in late May 2014: “This curious cloud pattern appeared east of Dallas last night!” Photo by Risa Bender

Cindy Gurmann caught these Kelvin Helmholzt clouds near New York, New York in March, 2015.

Cindy Gurmann caught these Kelvin Helmholzt clouds near New York, New York in March, 2015.

Helio de Carvalho Vital submitted this photo to EarthSky.  It shows Kelvin-Holmholtz clouds over Rio de Janeiro, Brazil.

Helio de Carvalho Vital submitted this photo to EarthSky. It shows Kelvin Holmholtz clouds over Rio de Janeiro, Brazil.

Helio de Carvalho Vital caught this Kelvin Helmholtz effect in virga - rain that falls from a cloud but doesn't reach the ground - in Rio de Janeiro, Brazil on May 15, 2015.

View larger. | Helio de Carvalho Vital also submitted this photo to EarthSky. It’s a Kelvin Helmholtz effect in virga, or rain that falls but doesn’t reach the ground. He caught it in Rio de Janeiro, Brazil on May 15, 2015.

Kelvin Helmholtz clouds seen over San Francisco.These clouds, sometimes called

Kelvin Helmholtz clouds seen over San Francisco. These clouds, sometimes called “billow clouds,” are produced by instability, when horizontal layers of air brush by one another at different velocities. Photo via Wikimedia Commons.

Earth isn't the only planet with Kelvin Helmholtz clouds.  Here they are on Saturn; Jupiter has them, too.  Image via Wikimedia Commons.

Earth isn’t the only planet with Kelvin Helmholtz clouds. Here they are on Saturn; Jupiter has them, too. Image via Wikimedia Commons.

Bottom line: Kelvin Helmhotz clouds – also called billow clouds – form when two different layers of air in our atmosphere are moving at different speeds.