Looking up in the sky and imagining shapes or animals with cloud formations has been with us ever since we could walk. Some of these objects actually hold fluid dynamic science behind their shape. Ever look up and notice a strange cloud that looks very similar to a wave that could be seen on a lake or the ocean? This cloud formation is actually a rare type of cloud that holds some of the more scientifically advanced atmospheric dynamics within it.
Named after William Thomson Kelvin and Hermann von Helmholtz, Kelvin-Helmholtz instability can occur when there is velocity shear in a continuous fluid. This instability can occur when there is a velocity difference across the interface between two fluids. To think about this in a visible sense, think about wind blowing over water. The instability that the wind creates manifests in waves that are produced on the surface of the water. Helmholtz studied the dynamics of two fluids of different densities when a small disturbance, like a wave, was introduced at the boundary of the connecting fluids. Helmholtz noticed how the formation of these wave-like structures were almost always because of different velocities interacting with one another. This can happen with any fluid, most notably, with clouds. When these Kelvin-Helmholtz waves are spotted they tend to look like ocean waves traversing across the sky.
To explain how Kelvin-Helmholtz clouds form one will have to think about shear. When two different layers of air in the atmosphere are moving at different speeds, this is called shear. When the upper layer of air is moving at a higher speed than the low-level air, the air tends to scoop the top of an existing cloud layer into wave-like rolling clouds. Due to the amount of shear and difference between air levels, windy days are often the best days to spot these clouds, and can also indicate aircraft turbulence due to the shear associated with the formation of these clouds.
If one of these clouds ever traverses across the sky above, just know that atmospheric and fluid dynamics is at work with ample amount of shear to create such a cool looking cloud.
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©2019 Weather Forecaster Alec Kownacki