When Amy Hunter looked up at the overcast sky over Virginia’s picturesque Smith Mountain, what she saw looked like a surreal painting. Rolling waves tumbled across the sky. The clouds mysteriously formed perfect crests, as if the sea had floated up to the atmosphere.
Hunter snapped a photo of the peculiar cloud formation and shared it in a Facebook group for pictures of Smith Mountain. Most group members said they had never seen such an incredible phenomenon before – but there was a reason the rare waves had formed in the clouds.
A woman spotted a rare cloud formation over Smith Moutain in Virginia and shared the perplexing photo with a Facebook group.
They’re Kelvin-Helmholtz waves, which usually form on windy days when there is a velocity difference across the interface between two fluids, such as wind blowing over water, according to EarthSky.org.
When there is a difference of air densities, and the upper air is moving at a higher speed, the clouds will whip up into wave-like shapes.
The waves are named after Lord Kelvin and Hermann von Helmholtz, who studied the physics behind this cloud formation. The presence of Kelvin-Helmholtz waves, could mean there is an atmospheric instability – which could mean turbulence for aircraft.
These odd clouds have been stopping people in their tracks for centuries – they are even believed to have inspired Van Gogh’s “Starry Night,” according to EarthSky.
The Kelvin–Helmholtz instability (after Lord Kelvin and Hermann von Helmholtz) can occur when there is velocity shear in a single continuous fluid, or where there is a velocity difference across the interface between two fluids.
An example is wind blowing over water: The instability manifests in waves on the water surface. More generally, clouds, the ocean, Saturn’s bands, Jupiter’s Red Spot, and the sun’s corona show this instability.
For some short enough wavelengths, if surface tension is ignored, two fluids in parallel motion with different velocities and densities yield an interface that is unstable for all speeds. Surface tension stabilises the short wavelength instability however, and theory predicts stability until a velocity threshold is reached. The theory, with surface tension included, broadly predicts the onset of wave formation in the important case of wind over water.