Asperitas clouds
What do you think clouds and playing musical instruments have in common? We can all agree that both are very beautiful! But also, music and clouds can be caused by the same process—vibrational motion. Today we’re going to tell you about asperitas clouds: rare, impressive, and definitely the most musical.
Asperitas clouds. Photo: Cody Chan / Unsplash
Not really clouds
Everyone knows that there are different types of clouds. There are cumulus, nimbus, and stratus. These names characterize the appearance, shape, and conditions of a cloud’s formation, and all clouds of the same type have similarities. But sometimes clouds can have unusual features. That is, a pair of clouds may have the same origin and generally the same shape, but look different. And undulatus asperitas, the Latin name for asperitas clouds, are exactly the shape that layered cumulus or highly layered clouds sometimes take.
How asperitas clouds appear
The exact process by which asperitas clouds form is currently unknown. We have a rough idea of what conditions lead to their appearance, but even if these conditions are met, asperitas clouds do not always appear.
To begin with, a stratocumulus (layered) or altocumulus (high) cloud must appear. This usually occurs in anticyclones, when there is stable stratification (i.e. vertical air movements are impeded). When the surface of the earth warms up in anticyclones, small cumulus clouds can form, which are called cumulus humilis, or fair-weather clouds.
If there are clouds like this in the sky this afternoon, it probably won’t rain today. Photo: Kosam Chandpuri / Unsplash
If the atmosphere is highly stratified (which is to say that different layers of air have very different temperatures or wind speeds), cumulonimbus clouds, and as they grow upward, they may hit the boundary between different layers, and begin to roll apart, as if they had been rolled out by a rolling pin. When all this happens at an altitude of about a kilometer, stratocumulus laconus clouds are formed, and when it happens at an altitude of about five kilometers, altocumulus clouds are formed.
This is what you get when you roll cumulus clouds out with a rolling pin. Photo: Nicole Geri / Unsplash
We’ve dealt with cloudiness. But how does this signature roughness of asperitas clouds appear?
First of all, let’s think about the stratification (layers) of the atmosphere—whether it is stable or unstable. With unstable stratification, a random displacement of one particle of air in an upward or downward direction can happen, leading to further mixing of the different layers of air (creating a convection current). In stable stratification, on the other hand, the air particle tends to return to its original position as quickly as possible, and all the changes are quickly reversed.
The way that air in stable stratification moves back into place is similar to the physics of playing a guitar. When you pluck a string, you hear a loud sound. Then the string continues to vibrate for a while, until it finally stops. In the same way, a layer of the atmosphere that has been displaced in an upward or downward motion by some force vibrates, and then stops. And if when playing the guitar, sound waves spread as a result of vibrations, gravitational waves also propagate when the air vibrates. We do not hear them, but they «vibrate» the atmosphere for tens of kilometers from the place of their appearance, and affect many properties of the surrounding air.
With this in mind, it will be easier to compare the atmosphere not with a guitar, but instead, a drum or a cymbal. You may have already seen the so-called Chladni figures. To create them, you can pour sand on a cymbal, and then hit it. Since the metal plate oscillates unevenly, the sand moves away from the areas with the strongest oscillations, and concentrates where the oscillations are minimal. As a result of the vibrations, various fanciful patterns are formed.
Something similar happens in the atmosphere, but instead of sand, there are clouds. Under the influence of gravitational waves, some parts of the clouds are compacted, and others become thinner. If the cloud layer is not very dense, the sun shines through the thinner areas.
Where and when to observe them
Asperitas clouds are one of the rarest weather phenomena. There’s a reason why they were only included in the International Cloud Atlas in 2017! Before the advent of cameras and smartphones for the general public, no meteorologist had ever been able to see them.
Asperitas clouds appear in temperate latitudes when there is an anticyclone or other high-pressure zone over an area.
An anticyclone over Europe on a Windy.app map. In this case, the appearance of asperitas clouds is possible inside the 1019 hPa isobar, as well as to the north and east of it.
The unevenness of the underlying surfaces will be an advantage: the presence of mountains, hills, lakes or bays can help the clouds’ formation. And also, the forces on the atmosphere of wind shear or jet currents can contribute (which you can track on the aerologic chart in our appendix). If you have all of these factors in one place, you can look up at the sky more often, and hope for a miracle!
What weather can we expect?
To an observer on the ground, asperitas clouds are completely harmless. Despite their intimidating appearance, there are no dangerous weather phenomena associated with them, and they don’t even produce rain. But rough clouds can be dangerous to aviation. They require a very variable and unstable atmosphere, so an airplane flying through these clouds will experience strong turbulence! So we hope everyone can see these clouds someday, but only from below!
Text: Eugenio Monti, a meteorologist and a climatologist
Cover photo: Manny Becerra / Unsplash
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