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EarthSky // Today's Image Release Date: May 29, 2014

Kelvin Helmholtz clouds

It looks like someone painted the sky with breaking ocean waves. They are called Kelvin Helmholzt clouds, aka as billow clouds or shear-gravity clouds.

View larger. |  Photo credit: Paul Chartier

View larger. | 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. They 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.

How Kelvin Helmholtz clouds form. 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 exmaple, during a temperature inversion.

These clouds are often good indicators of atmospheric instability and the presence of turbulence for aircraft.

Thank you to Paul Chartier and Risa Bender for sharing your photos with us!

View larger. |  Photo credit: Risa Bender

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

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. It’s widely believed that these waves in the sky inspired the swirls in van Gogh’s masterpiece Starry Night. 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.

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