BBC: Stunned sky-watchers in the US state of Wyoming have snapped photos of a rare cloud formation crashing across the horizon like ocean surf.
The billowy phenomenon was visible on Tuesday above the crest of the Bighorn Mountains from the city of Sheridan.
Known as Kelvin-Helmholtz instability, they form when a faster stream of air moves above rising air below.
BBC Weather’s Matt Taylor says the pictures are one of the most stunning and epic examples of Kelvin-Helmholtz clouds he has ever seen.
“Part of the beauty of Kelvin-Helmholtz clouds is that they really show up the fluidity of the atmosphere,” he said.
“How, like waves in the ocean, the atmosphere moves and responds to the environment around it. The air is effectively rising up and tumbling over on itself.”
The cloud formation is named after scientists Lord Kelvin and Hermann von Helmholtz, who studied the physics behind the phenomenon.
Also known as fluctus clouds, they are seen as a possible inspiration for Van Gogh’s painting Starry Night.
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