They’re a weird sight to see in the sky… if you’re lucky enough to see them. This week, it’s a brief primer on “Kelvin-Helmholtz clouds” or “KH clouds” or “fluctus clouds,” lots of names for the same thing.
* Back in 1868, Hermann von Helmholtz not only had one of the best scientist names to date but also realized a weird thing about fluid dynamics. In his words: “Every perfect geometrically sharp edge by which a fluid flows must tear it asunder and establish a surface of separation.” In plain English: “wind shear,” where fluid moving at different speeds or directions (or both) causes eddies and spin-ups. Think of it like a paddle pushed through the water, causing rotation on either side. It’s the same thing we look at for tornado potential, albeit with much different results. Anyway, a few years later, Lord Kelvin did the math on the whole thing, while figuring out how ocean wind waves worked.
* Let’s start with two fluid masses going in the completely opposite direction of each other and yes, air is a fluid, just not a liquid. That flow will always have little disturbances and friction, and it starts scooping one mass over the other. Voila: your waves are formed.
* You need ideal conditions on windy days to get these going, and the cloud type itself doesn’t actually matter. Cirrus, cumulus and stratus, all of them are part of a fluid environment. They’re also a good sign of turbulence way up there, so as tempting as it might be to fly closer, pilots tend to avoid such a rocky atmosphere.
* These wavy sights are usually short-lived and on fairly small scales, though some larger systems passing over mountain ranges can get enough turbulence going.
* Remember Lord Kelvin with his ocean waves? There’s your clue that this phenomenon isn’t just relegated to the sky. Fast air moving over slow water whips up those surface waves. You can even see KH instabilities on other planets, like in the Great Red Spot of Jupiter. That massive storm is a subject for another day.
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