The Definition of a Kelvin-Helmholtz Cloud

Look up on a windy day and you might see a Kelvin-Helmholtz cloud. Also known as a billow cloud, a Kelvin-Helmholtz cloud looks like rolling ocean waves in the sky. They are formed when two air currents of varying speeds meet in the atmosphere and they make a stunning sight. What Are Kelvin-Helmholtz Clouds? Kelvin-Helmholtz is the scientific name for this impressive cloud formation. They are also known as billow clouds, shear-gravity clouds, KHI clouds, or Kelvin-Helmholtz billows. Fluctus is the Latin word for billow or wave and this may also be used to describe the cloud formation, though that most often occurs in scientific journals. The clouds are named for Lord Kelvin and Hermann von Helmholtz. The two physicists studied the disturbance caused by the velocity of two fluids. The resulting instability causes the breaking wave formation, both in the ocean and the air. This became known as Kelvin-Helmholtz Instability (KHI). Kelvin-Helmholtz instability is  not found on Earth alone. Scientists have observed formations on Jupiter as well as Saturn and in the suns corona.   Observing and Effects of Billow Clouds Kelvin-Helmholtz clouds are easily identifiable though they are short-lived. When they do occur, people on the ground take notice. The base of the cloud structure will be a straight, horizontal line while billows of waves appear along the top. These rolling eddies on the top of the clouds are usually evenly spaced. Quite often, these clouds will form with cirrus, altocumulus, stratocumulus, and stratus clouds. On rare occasions, they may also occur with cumulus clouds.   As with many distinct cloud formations, billow clouds can tell us something about atmospheric conditions. It indicates instability in air currents, which may not affect us on the ground. It is, however, a concern for aircraft pilots as it forecasts an area of turbulence. You may recognize this cloud structure from Van Goghs famous painting, The Starry Night. Some people believe that the painter was inspired by billow clouds to create distinct waves in his night sky. The Formation of Kelvin-Helmholtz Clouds Your best chance for observing billow clouds is on a windy day because they form where two horizontal winds meet. This is also when temperature inversions -- warmer air on top of cooler air -- occur because the two layers have different densities. The upper layers of air move at very high speeds while the lower layers are rather slow. The faster air picks up the top layer of the cloud its passing through and forms these wave-like rolls. The upper layer is typically drier because of its velocity and warmth, which causes evaporation and explains why the clouds disappear so quickly. As you can see in this Kelvin-Helmholtz  instability animation, the waves form at equal intervals, which explains the uniformity in the clouds as well.