Stunned sky-watchers in the US state of Wyoming have snapped photos of a rare cloud formation crashing across the horizon like ocean surf.
"This was special and I immediately knew I needed to capture it," said local Rachel Gordon.
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.
Ms Gordon, who told BBC News she took the images from her parents' back door before posting them to the Facebook page Wyoming through The Lens, said: "It was an awe-inspiring moment.
"I'm just glad others can enjoy the experience now, too."
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."
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Wonderful world of clouds
The cloud formation is named after scientists Lord Kelvin and Hermann von Helmholtz, who studied the physics behind the phenomenon.
The UK-based Cloud Appreciation Society describes such formations as the crown jewel in many cloud spotters' collections.
Also known as fluctus clouds, they are seen as a possible inspiration for Van Gogh's painting Starry Night.
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