In certain circumstances, impressive wave-shaped clouds can be seen in the sky. These are formed due to wave motion in the air and are called Kelvin-Helmholtz waves, after the two physicists who described these clouds in the 19th century. Formed when there is sufficient wind shear, the waves become visible as clouds develop, provided there is sufficient moisture in the air.
Waves driven by wind
The atmosphere is a fluid and actually behaves in a similar manner to water. If two layers of air with different wind speeds therefore lie on top of one another (which is referred to as shear), friction appears at the boundary between them. This produces waves. If the two layers have stable stratification, no spontaneous vertical movements can develop. Accordingly, the waves become stronger and can ultimately break like waves on the shore.
Visualisation of the waves
If there is sufficient moisture at the boundary between the two air layers, as well as sufficient wind shear, clouds can form that make the waves visible. These waves generally first appear on stratus (low cloud) or altostratus (mid-altitude cloud) but can also occur in the case of cirrus or cumulus clouds. According to the WMO International Cloud Atlas, these cloud types are also known as fluctus. The following example shows Kelvin-Helmholtz waves in a thin stratus cloud.
In the lowlands, the waves can be observed on the low stratus or fog layer, for example, which generally marks the upper boundary of the cold air pool. Below, you can see a great example of Kelvin-Helmholtz waves on the upper boundary of an expansive low stratus layer at the foot of the Jura mountains.
A short-lived phenomenon
Fluctus clouds are generally short-lived and rare, as the conditions in which they form also only prevail for a short time. If you see clouds like these, we would be delighted if you could share a photo of them with us via the weather observations in our MeteoSwiss App.
