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Picture the ocean on a dark and windy day. Rather than smooth rolling waves, the surface is rough
and choppy, with deep troughs and frothy crests. Now, imagine that stormy ocean flipped upside down
and blanketing an entire sky. It may sound crazy, but that image is a pretty close
approximation of asperitas clouds, an extremely rare cloud type that was only recognized by the
World Meteorological Organization in 2017. “Asperitas,” which translates to “rough and
uneven,” is one of the rarest cloud forms on record – and the only one to have been
formally added to the WMO’s Cloud Atlas in almost 70 years. Given how recently they were discovered,
not much is known about how or when asperitas clouds form (although we have some theories.) So,
what makes asperitas clouds different? Why haven’t we noticed them before? And could their
discovery provide a blueprint for identifying extremely rare weather events in the future?
I’m Alex McColgan, and you’re watching Astrum.
Join me today as we look at spectacular, high-definition images of asperitas clouds,
learn what scientists think could be giving them their dramatic appearance, and explore the
exciting implications of this unusual convergence of citizen science and trailblazing research.
In 2006, Jane Wiggins, an amateur photographer located in Cedar Rapids, Iowa,
took a photo from her office window that she described as
“looking like Armageddon.” She sent the image to the Cloud Appreciation Society, which posted
it on their website. The photo immediately set off shockwaves among cloud watchers,
who suspected that it could be evidence of a previously unknown cloud formation.
Other amateur hobbyists using the CloudSpotter mobile app started sharing their images with each
other, sharing weather conditions to look out for, to help others see it for themselves. This caught
the eye of the Royal Meteorological Society, which used dozens of photos collected by the Cloud
Appreciation Society to prepare a case that they sent to the World Meteorological Organization.
After several years of debate, the WMO made an announcement: for the first time since 1951,
they were adding a totally new type of cloud to the Cloud Atlas, giving asperitas clouds
the unique distinction of being the only cloud type to have been discovered as a result of
digital crowdsourcing. But why was this rare? And who decides what makes a cloud form unique?
Let’s start with a primer. Clouds, essentially, are liquid droplets or solid particles suspended
in the atmosphere of a planetary body. While on other planets, clouds can be made of methane,
ammonia or even sulfuric acid, here on Earth, the vast majority contain liquid or frozen water.
(And personally, I’m glad we don’t have many of the sulfuric acid variety.)
Clouds form when saturated air reaches its dew point, the temperature at which,
under a given set of barometric conditions, water vapour condenses into liquid due to cooling air or
increased saturation from an adjacent water source. There are five primary cloud types:
stratus, cumulus, stratocumulus, cumulonimbus and cirrus, each of which are subdivided into
different genera based on altitude. However, in addition to these primary categories,
there is a range of accessory clouds (which are clouds that become detached from the original
genus-cloud) as well as supplementary features. All of these are formally classified in the WMO
Cloud Atlas: the international manual for cloud identification, which uses a Linnean nomenclature
that has been in place since the early 19th Century. The WMO is responsible for updating
the Cloud Atlas, but revisions are extremely rare. The last new formation was cirrus intortus,
added way back in 1951. Now, Asperitas clouds are thought to be mostly a supplementary feature of
stratiform clouds, which are low elevation layered clouds without vertical development,
although asperitas have also been observed to be stratocumulus or altocumulus. Interestingly,
while the bases of stratiform clouds are generally flat, they tend to develop
significant features at the top due to wind and temperature changes. This is extremely common,
and something you will no doubt be familiar with if you’ve looked out the window of a plane. What
makes asperitas clouds unusual is their highly developed structures at the bottom. In 2017,
researchers from the Cloud Appreciation Society, University of Reading and McGill University
published a paper attempting to describe the features and causes of asperitas cloud formation.
According to the authors, asperitas clouds are defined by their rough, somewhat wavelike cloud
base, that lacks the smooth undulating shape of the (possibly related) undulatus cloud type. Given
these semi-wavelike characteristics, the authors hypothesize that asperitas clouds are likely
created by weak oscillatory activity. There are a few potential causes of this. One is orography,
which is the wavelike movement caused by the flow of air over mountainous terrain.
However, there could be other reasons, such as horizontal wind shear; convection, which is the
exchange of matter or temperature between fluid bodies; or atmospheric gravity waves. (By the way,
in case you’re wondering, a gravity wave isn’t the same thing as a gravitational wave. As much as I
love spacetime physics, we’re not talking about massive ripples in the spacetime fabric here!)
A gravity wave occurs at the border between one fluid and another, when a force upsets
the equilibrium and gravity tries to restore it, which produces a back-and-forth movement,
or wave orbit. Some familiar examples of gravity waves include tsunamis and the changing tides.
You have probably seen the result of gravity waves affecting clouds too. These are undulatus clouds.
I’m sure you’ve noticed cloud formations like this before, from a ground view.
The view from space is equally spectacular. But asperitas clouds have an appearance of roughness
that indicates unusual instability at the cloud base. These aren’t smooth and wavelike, like the
undulatus cloud type we just looked at, but have a formation that is chaotic, like the stormy ocean
we talked about earlier. While stratiform clouds often have what is known as Kelvin-Helmholtz
instability at the top, an instability that comes from a velocity shear within fluids, this
instability is more unusual at the base of clouds. So, what might be the cause? We don’t know yet,
but the authors of the 2017 paper from earlier suggest that this instability may propagate
downward from the top of the cloud, which could happen if the cloud is thin enough. Perhaps there
are different reasons each time. The scientist who captured this timelapse said that here, gravity
waves were moving through an inversion, underneath an elevated region of instability. You’ve probably
seen an inversion before, where clouds or even smog is trapped under a layer of hotter air.
Thunderstorms were in the forecast, but the thunderstorms could not pull their "inflow
air" from near the surface due to the inversion, so instead, they pulled from above the inversion.
The gravity waves moving through the inversion created enough lift that a strong updraft was
formed and it moved in phase with the peak of the gravity wave. This enhanced lift from the updraft
increased the amplitude of the wave and made the asperitas cloud here very visually impressive.
Regardless of how it happens, this internal instability appears to disrupt
the weak oscillatory activity at the base, resulting in the rough, choppy appearance.
So, when you think about it, the winds and currents causing the choppiness of the ocean
on a stormy day aren’t that unlike the chaotic forces scientists think are producing instability
in an asperitas cloud. In both cases, complex fluid dynamics are in play,
giving rise to eerily similar characteristics. Nature may not repeat itself, but it often rhymes!
So, there you have it, a primer on one of the newest and least understood cloud types on record.
If this interests you, you might want to keep watching the sky and taking photos of anything
interesting you see. Who knew we’d still be discovering new cloud types? What else
out there don’t we know about? And who knows, you could be part of the next scientific breakthrough!
Isn’t it amazing what actually goes on in the world around us that maybe we do take for granted!
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