Fluctus Cloud Feature: Kelvin-Helmholtz Waves, Curls

Description & Characteristics. Popularly known as Kelvin-Helmholtz instability or KH wave clouds, the ‘fluctus’ cloud feature can be found amongst five cloud types: cirrus, altocumulus, cumulus, stratus, and stratocumulus. Translated from latin, meaning wave, the five types of fluctus clouds are respectively abbreviated as ‘Ci flu’, ‘Ac flu’, ‘Cu flu’, ‘St flu’, and ‘Sc flu’. Fluctus cloud formations are easily identified through their distinctive curled, crashing wave appearance.

Named after physicists Lord Kelvin and Hermann von Helmholtz, Kelvin-Helmholtz instability in clouds is caused by speed shear, which means the wind is moving at a different speed in the upper layer of a cloud than it is in the lower layer. This causes instability in the cloud with the end result forming a cloud that resembles a crashing wave. Kelvin Helmholtz instability isn’t only found in clouds. This phenomena can happen deep in the ocean or in clear air. It can happen as wind blowing over water, or even in the atmosphere of other planets. It’s thought that Van Gogh’s Starry Night oil on canvas was inspired by Kelvin-Helmholtz wave clouds.

The fluctus cloud feature is a definite cloudspotting favorite. They’re rare, short-lived, and when you see an especially prominent fluctus formation, they can be spectacular. They can generally be found on windy days, and are equally uncommon in each cloud type they can be found in.