Cloud-watchers can talk all about the interesting shapes they’ve noticed over the years. They may have been gazing out the car window during a road trip, or tanning on the beach, or on a morning walk. Then they notice a cloud formation that looks like a dolphin, or a boat, or letters of the alphabet. But few are fortunate enough to see Kelvin-Helmholtz clouds, which look like a pattern of ocean waves in the sky.
What are Kelvin-Helmholtz clouds?
Kelvin-Helmholtz clouds, also known as fluctus clouds, is a rare phenomenon that’s easy to miss because it disappears so quickly. But its distinct water-like waves makes it a favorite among cloud-watchers. In fact, some theorize that its artistic form might have inspired Vincent Van Gogh’s painting Starry Night.
The history
Kelvin-Helmholtz clouds are named after Lord Kelvin and Hermann von Helmholtz, and moreover, after their discovery of Kelvin-Helmholtz instability. The two had studied the physics — the fluid dynamic, in particular — behind the atmospheric instability that creates unique clouds.
The Kelvin-Helmholtz instability
Kelvin-Helmholtz instability occurs when there’s friction velocity in one continuous fluid. It also happens when there’s a velocity difference on the interface between two layers of fluid. It doesn’t just occur in clouds; it can appear in clear air, in the ocean, and even in the atmosphere of other planets.
What creates fluctus clouds?
The unique waving shape occurs when two different air layers move at different speeds in the atmosphere. Since the upper layer of air is less dense, it moves at a higher speed, and “pushes over” the tops of clouds. This creates the shape of crashing waves. The atmosphere often splits into layers when air densities change, like during temperature inversions. The upper layer is typically drier than the lower one, which leads to quicker evaporation. This is why this cloud formation can dissipate rapidly, often lasting only for a couple minutes.
Turbulence ahead
Kelvin-Helmholtz clouds may be beautiful to view while on solid ground, but it’s less wonderful for people in aircrafts. To pilots, they mean instability and turbulence. However, people in planes generally don’t spot fluctus clouds because they are covered by other clouds. More often than not, there won’t be any indicators of instability because of clear skies.
How to spot Kelvin-Helmholtz clouds
You’ll need keen eyes, and unfortunately, a lot of luck. However, certain circumstances increase the odds of the clouds forming. EarthSky suggests looking out for windy days because of the different densities in the air such as when warm air is flowing over cool air. For that reason, fluctus clouds are more likely to appear around sunrise or sunset, which is when lower clouds are cool and upper clouds are warm.
Read More: The Last Places for Unspoiled, Starry Nights on Earth
Which types feature Kelvin-Helmholtz clouds?
Keep in mind, Kelvin-Helmholtz clouds are a type of formation, not type of clouds on their own. They tend to appear on these five cloud types: cirrus, altocumulus, cumulus, stratus, and stratocumulus. But it’s equally rare to find fluctus clouds in each one, according to the International Cloud Atlas.
Cirrus
Cirrus clouds are made of ice crystals so they aren’t the typical puffy, cotton-ball shape. Instead, they create forms similar to bony fish skeletons, spider webs, commas, or other fibrous-looking things. Alternatively, they can appear clumped together or entangled, according to What’s This Cloud.
Altocumulus
Altocumulus clouds are usually clustered together in a pattern that may look like fish scales. But they can also form a wide variation of shapes and sizes, from castle towers, to a spool of wool, to blanketing the entire sky.
Cumulus
Cumulus clouds are the default cloud when it comes to doodling and cartoons. They look like popcorn or pom poms, but their puffy nature lends itself to imagination. Cloud-watchers could spy a number of different shapes in a sky full of cumulus clouds.
Stratus
People think of stratus clouds when they picture dreary, cloudy days, because they paint the sky entirely grey. They can float to the ground to create fog. Sometimes they block the sun completely, and sometimes sun rays could still shine through. Stratus clouds aren’t popular, but if you’re on the hunt for Kelvin-Helmholtz clouds, you may be less disapointed to see a dreary, foggy day.
Stratocumulus
As the name shows, stratocumulus clouds are a blend of stratus and cumulus clouds. Therefore, they appear like a layer of puffy clouds, usually joined together. Being a cross between two types of clouds, stratocumulus forms are widely diverse and dramatic. They can come in many shapes and sizes and can include some interesting features, such as fluctus clouds.
Read More: Stunning Iridescent Clouds Snapped Above Skies of Siberia’s Belukha Mountain