The clouds were formed as a result of a phenomenon known as Kelvin-Helmholtz instability
The surf was up Tuesday afternoon along the crest of the Bighorn Mountains. Sky surf, that is.
Those looking westward from Sheridan, Wyo., witnessed exceptional examples of giant cloud waves as they curled and crashed over the snowy landscape. The wave clouds were crafted by a phenomenon known as Kelvin-Helmholtz instability, which occurs as a result of differences in air density in adjacent layers of the sky. Moisture and strong winds are required for these clouds to trace their wavelike shape.
The Bighorn Mountains span northern Wyoming and southern Montana and stretch into the High Plains. They are independent from the Rockies to their west but magnificent in their own right.
Kelvin-Helmholtz wave clouds form in environments characterized by strong winds aloft and rising channels of instability off the ground. As the instability channel ascends, it is pushed over by the strong winds above, spinning up eddies.
In this case, it seems likely that the terrain of Bighorns helped force the air to rise through a process known as orographic lift. Some pockets of instability may have developed as the sun heated the ground and rose into much colder air aloft.
A time lapse video of the scene appears to show the waves propagating southward along the spine of the Bighorn Mountains.
Tuesday evening’s wave train was big enough to be seen from weather satellites in space!
Kelvin-Helmholtz instability is not only found in the atmosphere but also in other fluids both on Earth and in space. Such wave clouds in the atmosphere aren’t uncommon, particularly near mountain peaks, but tend to be rather fleeting. Pristine examples are rare.
Tuesday’s spectacle in Wyoming was exceptional for its size and textbook structure. It is reminiscent of another mountain display of Kelvin-Helmholtz waves in 2015 in Breckenridge, Colo.
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