Rare and distinct cloud formations that resemble rippling ocean waves were recently spotted in the sky over New Hampshire.

The cloud formations, known as Undulatus Asperitas, have wave-like structures that form on the underside of the cloud to make it look like a rough sea surface when viewed from below, the UK Met Office noted.

Crystal Lee snapped photos of the cloud formations as she traveled through the Coös County town of Gorham.

While the process behind what forms Asperitas formations is largely unknown, theories suggest that their appearance is associated with the “aftermath of convective thunderstorms, though they have also been sighted in relatively calm environments,” according to the Met Office.

“Asperitas clouds can be stunning in appearance, unusual in occurrence, and are relatively unstudied,” NASA said of the unusual formation. “Speculation, therefore, holds that Asperitas clouds might be related to lenticular clouds that form near mountains, or Mammatus clouds associated with thunderstorms.”

The cloud formation was added to the Meteorological Organisation’s International Cloud Atlas just a few years ago.

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