Four cloud waves known as Kelvin-Helmholtz instabilities, or fluctus, break across the skies of the US state of Missouri. (NWS AWC: Shawn McCauley)
The name makes it sound like something best avoided, but fluctus — or Kelvin-Helmholtz instabilities — are amazing atmospheric formations that mimic breaking ocean waves.
In a stunning photo taken by pilot Shawn McCauley over the US state of Missouri, four distinct crests can been seen rising from a thick bed of cloud, curling as if part of a set of large, surging waves.
And according to experts, the sky waves form exactly like those in the ocean, appearing briefly atop clouds before breaking.
“These clouds look like breaking waves in the ocean, because it is the same physical process,” the US National Weather Service Aviation Weather Centre said.
These wave-like patterns form when layers of moving fluid interact.
The lighter, warmer layer of fluid floats over the top of the denser, cooler layer in a process called fluid stratification.
The difference in density means the layers move at different velocities, with the heavier layer moving more slowly than the lighter layer.
Because there’s a velocity difference at the boundary between the fluids, the faster-moving fluid tries to ‘pull’ the heavy fluid with it, into the upper layer.
But the denser fluid sinks back down, forming ‘rolling’ waves.
The WMO’s International Cloud Atlas said the Kelvin-Helmolz wave occurs mostly with Cirrus, Altocumulus, Stratus and Cumulus clouds.
The cloud wave formations are not only seen on earth.
In 2004, the Cassini space probe photographed a large stretch of fluctus along the edge of Saturn’s atmosphere.
NASA said they, “should be fairly common on the gas-giant planets, given their alternating jets and the different temperatures in their belts and zones”.
Fluctus, the short-lived wave formation, on the top surface of clouds. (International Cloud Atlas: June Gronseth)