Some natural phenomena are so rare that not everyone has a chance to witness them. Even more thrilling, sometimes these phenomena can be caused by trivial, well-known processes. For example, wave clouds, a rate type of cloud that looks exactly as it is called — in the form of swirling air, very similar to sea waves that come one after another. In this lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you will learn more about what are wave clouds and how do they form. But we will need to start with a bit of background.
Imagine the sea. It's sunny, the water is warm, and you float on soft, tender waves. But where do these waves come from? Sighing, you leave the beach, and go home and open your physics textbook.
Air and water are two different mediums with different densities, and water is considerably heavier than air. Seawater moves very slowly, while the wind speed can be very significant. Due to the density difference, churning movements are formed where the air and water meet, creating turbulence. This is how what we know as sea waves are formed. The faster the wind, and the longer it blows, the taller the waves become. This mechanism is called Kelvin–Helmholtz instability.
The top of the wave can move faster than its base. This happens mainly because the bottom of the wave slides along the sea bed and lower the water level, slowing down. If the top of the wave outruns its bottom by too great a degree, the wave will topple over.
The exact same thing can happen in the atmosphere. For that, we need two layers of air with different densities and wind speeds. The top level must be less dense than the bottom one: for instance, because the top layer is warmer. The wind speeds also need to be different, but the faster wind doesn't necessarily need to be on top.
The formation of wave clouds. Valerya Milovanova / Windy.app
This can happen in weather fronts, stream currents, or simply close to the ground, where land elevations and irregularities slow down the lower layer of air. But the essential requirement is that the lower air layer has some clouds, which would "draw" the wave, making it visible to us. At the same time, the higher level can't have any clouds. If there are no clouds, or clouds in both layers, the waves will remain invisible to the observer.
These wave clouds are also sometimes referred to as Kelvin–Helmholtz clouds. Let's sum up what is needed for us to see them:
Kelvin–Helmholtz clouds are very rare. Each one of the three requirements happens in the atmosphere pretty often, but it's not common at all for them to happen, and all at once.
By the way, Kelvin–Helmholtz instability can be observed not only on Earth, but also on other planets, and even on stars. For example, they are very visible in Jupiter's atmosphere, especially close to the Great Red Spot. The only difference is that two layers of gas are next to each other in this case, and not in horizontal layers.
Kelvin–Helmholtz clouds most often appear when it's windy. The wind speed difference is also a common sign of a weather front approaching, meaning the weather will get worse. The turbulence that causes these clouds is hazardous for aviation, making wave clouds a warning sign for both pilots and air traffic controllers.
Text: Eugenio Monti, a meteorologist and a climatologist
Cover photo: Tessa Rampersad / Unsplash