Bizarre Kelvin-Helmholtz Waves Appear Over Alabama
While driving through Birmingham, Alabama, Redditor alison_bee couldn’t help but notice the bizarre, repetitive wave shapes appearing in the clouds near the horizon. While these strange cloud formations look otherworldly, they’re an example of what’s called Kelvin-Helmholtz instability — which is a pretty awesome name for a spectacular phenomenon.
Here’s how Wikipedia describes what you’re seeing:
[…] when velocity shear is present within a continuous fluid, or when there is sufficient velocity difference across the interface between two fluids. One example is wind blowing over a water surface, where the wind causes the relative motion between the stratified layers (i.e., water and air). The instability will manifest itself in the form of waves being generated on the water surface.
The example of a strong wind blowing over water is, I think, particularly illustrative. In this case, it seems that cooler, denser air close to the ground is being pushed by warmer air above it. At least, that’s what I think is happening.
Heres what Redditor and meteorologist zensunnioracle had to say:
Meteorologist here. These are indeed Kelvin-Helmholtz waves. What is happening is that the nocturnal near-surface layers (lowest 50-100m) of the atmosphere are much more stable than the layers above it in the mornings. Until the ground heats up due to daytime heating, the surface layers stay more stable than the air over it. Kelvin-Helmholtz waves occur when the wind shear between the layers destabilizes the topmost portion of that stable layer, and entrains the air into the unstable layer. What you see is stable air being lifted, cooled, and condensed so that this process becomes visible, though this commonly happens many places without being visible.
As spectacular as these waves are here on Earth, the same forces create similar patters on the gas giant planets like Saturn and Jupiter. While those are some truly enormous waves, these pictures from alison_bee should show that the Earthbound variety aren’t to be sneezed at either.
Someone also shot a video of the line of wavy clouds.