Get your head in the clouds: a catalog of strange and spectacular meteorological phenomena
Embark to a wonderful journey among the clouds
Even now, I consider Clouds to be one of my best article. Part because of the twist on the concept of clouds themselves, part because these aggregations of ice and water are fascinating. The more you know about them, the better it gets. Follow me as we explore the vast sky, from the mesosphere to the ground.
A primer on clouds
A cloud is, just like the wind, a physical manifestation of the forces at play in the atmosphere of a planet. It is a mass of small droplets and frozen shards suspended high above the ground. On our planet, the particles that compose most clouds are water, either as liquid or ice. It doesn't have to be water though, as demonstrated by the sulfur dioxide clouds of Venus, the ammonium and ammonium hydrosulfide clouds hovering Jupiter and Saturn, or methane clouds covering Urenus, Neptune and even Titan, one of Saturn's moon.
Formation
Clouds may appear out of thin air, but there are rules that govern their creation. On Earth, water droplets are omnipresent, and when we look up the sky, there are billions of minuscule ice crystals over our head. So why isn't the sky constantly covered by a thick blanket of clouds like in the Divine Ordeal? This is due to another physical phenomenon worthy of an article for itself: the dew point.
The dew point is a temperature threshold of any body of air, under which it becomes saturated with water vapour. This threshold is not constant, but rather depends on the pressure and moisture of the air. Since the pressure is more or less constant for a given atmospheric layer, the only parameter driving the formation of clouds are the relative humidity and temperature of the air. Either the air is cooled beyond the dew point, or a source of moisture raises this point to the ambient temperature.
Now, let's take a top-down approach. I will be your guide for this descent, as we will slowly glide down the atmosphere and explore the diversity of clouds existing on our beloved planet. Brace yourself, the jumps might sometimes be dizzying.
Let's go down exploring
Noctilucent clouds
We're lucky that this side of the planet is nearing night. The first clouds that we get to sight are located in the mesosphere, at the very frontier of space. While space begins at 100km above the ground, the Noctilucent clouds hover between 50 and 80km! A vertigineous height if you consider that the Cirrus, classified as high clouds, are found between 7 and 14km up the air, and even the highest clouds painstakingly reach 25km. They are so high even weather ballons cannot reach them, which shrouded their formation in mystery.
At this altitude, the air is almost barren of moisture and the temperature is freezing cold: -126°C! And yet enough ice crystals rise and give birth to these electric blue filaments that ripple the night sky. Their pale hue is indiscernable at daytime, as they have a very thin ice layer that completely blends in the rest of the sky. After twilight however, when everything is dark and the lower clouds are all black and grey, noctilucent clouds are high enough that the curvature of Earth allows them to reflect the Sun's light, giving them their bright colour.
To witness such a wondrous show outside of our ship, you should head toward the nearest pole, without crossing the polar circle, preferably during summer. They are also easier to observe when the atmosphere is saturated with tiny particles from a Dantesque eruption sending ashes into the mesosphere or a spatial dust cloud that our planet happens to cross. Planetary-scale pollution can extend the area and frequency in which they appear, as gas emissions reach the mesosphere and oxidyze, leading to an increase in humidity.
Altitude: 50-80km
Named after: Latin for "Night Shining"
Rarity: Very rare, only visible around dawn or dusk.
To look further: Wikipedia page Astrum's video
Nacreous Clouds
Let's take a giant leap as we descend to the low stratosphere and swiftly head North, in a region that is still in the middle of its twilight. This is the only time and place to observe one of the most vibrant clouds. They form between 15 and 25km up at temperatures lower than -85°C, when the Sun has passed below the horizon by just a few degree, and their peculiar ice crystals diffract the last or first rays of sunlight.
Nacreous clouds have a wonderful pearlescent colour that is at its brightest right after the Sun passed the horizon, but continue to shine for up to two hours after that. This is due to very small and spherical crystals of the same size, diffracting the sunlight like a disco ball in the nightclub imaginable.
Altitude: 15-25km
Named after: Nacre, or mother of pearl
Rarity: Rare, visible in specific conditions.
To look further: Nacreous clouds on cloudatlas Wikipedia page
Enough with staggering heights. We are now entering the realms of regular clouds: the cumulus, altocumulus and cirrus, some names you might have already heard somewhere. As they refer to large families of clouds, we won't explore them in-depth. As a rule of thumb, you can remember that:
- Cumulus and Stratus are low-level clouds (0-2 km)
- Altocumulus are mid-level clouds (2-8 km)
- Cirrus are high-level clouds (3-18 km)
They are mostly indicators of the height of the clouds, and to a lesser extent their composition. Note that it is true when they are used as prefix. For example, cirrocumulus are high-level clouds of the cirrus family. Yes, cloud naming is confusing and largely uninspired. Don't blame me.
Lacunosus
Trypophobs beware! The lacunosus clouds are celestial nightmares to them. They are a peculiar formation that appear in other cloud types, giving a honeycomb texture to their underside. A lacunosus is identified by the numerous holes in a cloud, rather than the cloud itself. The gaps are created when a layer of cooler air ends up above a warmer one. As the cold air is denser, it sinks down through the cloud while the warm air rises up, lifting up the borders of the gap.
You'd be lucky to actually witness such a formation, as they tend to be very short-lived and may easily be missed. The exact cause of their existence is still unclear, only that they can be unsettling to the unsuspecting onlooker.
Altitude: All three cloud levels
Named after: Latin for "Full of holes"
To look further: What's this cloud article
Fallstreak Holes
Let's continue with holes, but take things a step bigger. Fallstreak holes, also called cavums, are witnesses of a disturbance in clouds. They look like, and they are, a cloud collapsing on itself. A cloud is, at its core, an unstable structure which relies on the eclectic physics of the high troposphere to exist.
It is not uncommon for a cloud to be made of supercooled droplets, which is water remaining in its liquid form despite being well below 0°C. To freeze, water needs some solid particles to act as icing nuclei, catalyzers to enable the reaction. The cloud droplets are ripe for freezing, but they cannot due to the undisturbed air. However, should these specific particles collide with the cloud, they will begin to agglutinate on these newfound nuclei and create a chain reaction. The droplets in the vicinity will freeze in turn, and join the rapidly expanding icicle.
At some point, the icy formation will get heavier than the rest of the cloud and fall, which is the most common image of fallstreak holes: a large breach in a cloud ceiling, with a stranded formation slightly below, falling to its doom. Indeed, this heavier cloud will never reach the ground and evaporate far before, only a few hundred meters in the descent.
Now, what kind of particles are fit to inflict such a gaping wound to a majestic cloud? Actually, a lot. It can be ice crystals falling from a Cirrus cloud above, dust particles carried from the ground below or, as we just did, a vessel breaking through the cloud. The particles emitted from a regular plane or our shuttle are just as good nuclei as any. I'd wager than we would soon see one such hole form overhead.
Altitude: 500-15,000m
Named after: Holes
Rarity: Common, especially near aircraft bases
To look further: Wikipedia page
Virga
With a shape very similar to what can be found in the center of the Fallstreak Holes, virgae are our next stop. Essentially, virgae are rain or snow clouds, not that different in that function than other precipitation cloud. What makes them special is that theirs never reach the ground. It happens when a layer of warm and dry air is below the cloud, evaporating the rain long soon after its creation. Virgae can be long, vertical strands like a waterfall suspended in the air, or display sharp angles when the wind blows at different speeds along the descent.
The best place to observe this phenomenon is continental arid regions, as their dry air prevent most precipitation from reaching the ground, but not the formation of cloud up above. Such regions on Earth are the plains of North America, the Middle East, Australia and North Africa. On other planets, sulfuric rain virgae have been spotted on the surface on Venus, and snow virgae on Mars.
Altitude: All cloud levels, high enough for the precipitation to evaporate
Named after: Latin for Rod, Staff
Rarity: Common in dry regions
To look further: Cloud Appreciation Society article
Kelvin-Helmholtz Instability/Fluctus
On your left, you can witness a most unique cloud. Fluctus are a consequence of what smart people in lab coat call the Kelvin-Helmholtz instability. They bear a striking likeness to ocean waves, though these are created by a different mechanism. The Kelvin-Helmholtz instability is acused by a shear in wind velocity at the interface of two layers of air. If the upper layer of warmer air moves faster than the colder air below, the clouds formed at that very height will break in wave pattern, the intensity of the roll depending on the speed difference. Note that while beautiful, these clouds are also indicators of turbulences potentially dangerous for planes.
Savour the sight of these swirling clouds, because the term "instability" is not given lightly. This phenomenon is usually short-lived, lasting no more than a couple of minutes. While rare, they can appear at about any height, even at ground level. All you need is a shearing between two air layers. But it goes even further, as if these clouds can appear on any planet that has an atmosphere and gas worlds, they have also been sighted on the surface of the Sun and even in the Orion Cloud! They are not only one of the most fascinating, they are also very easy to put in your worlds and story to enrich your skies.
Altitude: Anywhere that has air, up to 12,000m.
Named after:Physicists Lord Kelvin and Hermann von Helmholtz.
Rarity: Incredibly rare, as it needs clouds to form at a very specific height and is ephemereal.
To look further: Fluctus on What's This Cloud, Wikipedia article (the french version has a lot more info)
Roll Clouds
While we observe it from above, to see this one in the most impressive way possible, it is better to be on the ground. There, you can witness the full glory of these low-hanging cloudy tubes, stretching infinitely from one horizon to the other. They are like infinite cheese wheels moving to up to 60 km/h, with an unsettling rotation. Indeed, the clouds rotate against the travel direction, in a motion that seem both familiar and alien. Their surface texture range from silky smooth to gravel rough.
They are a type of Arcus, threatening storm precursors. These low clouds are at the base of thunderclouds and form a few minutes before the tempest begins to rage on. However, once the heavy clouds subside, the Arcus continues his quiet life, becoming a peaceful roll cloud. When the conditions are right, roll clouds can form in a cloudless weather, near a coast where a cold see breeze meet warmer currents.
One very famous roll cloud that you might have heard of before is the Morning Glory. This exceptional formation can only be seen in one single place on Earth: Northern Queensland, Australia. Due to the peculiar geology of the gulf of Carpentaria, astounishingly long roll clouds appear in the morning, spanning up to 1,000 km in length. These roll clouds, which can show up in group of up to ten, are not over 200 meters above ground, with an impressive diameter of 1-2 km. It is so humongous that it is very easily noticed from space.
Altitude: 100-1500m
Named after: Cinnamon rolls, probably
Rarity: Depending on the location. Frequent in some places, nonexistent in others.
To look further: Arcus clouds and Morning Glory wikipedia article.
Undulatus and Radiatus
Beware: the music is quite loud
As we saw previously with the ephemeral Kelvin-Helmholtz Instabilities, the interaction between the currents in the atmosphere can lead to pretty impressive phenomenon. A tamer version of this are the undulatus, that you probably already saw without taking note of them. They form when the air stream undulates, due to a shearing caused by, for example, the topology of the terrain underneath. Most of the time, they are invisible unless a cloud display the light perturbations. It results in a wavy pattern, that can even be extrapolated to look like Roll Clouds in some extreme cases.
Undulatus form perpendicular to the direction of the wind, which is how the ripple effect is formed. When clouds form parallel to the flow, they are of the Radiatus variety. They form surprisingly straight lines, going around neatly ordered with visible gaps between them. In cumulus clouds, this formation is kown as cloud street and is a bliss for any glider pilot, as it indicates with precision long lines of lifting air.
Altitude: Any height
Named after: The undulations and radiation effect they display
Rarity: Quite common
To look further: EarthSky article
Lenticularis
We've seen clouds falling, rolling and soon dashing, but what if I told you there exist a type of cloud that stay still at all times? If you see, most of the time over a high relief, an unmoving flying disc amongst a frantic flock of clouds carried by a strong wind, it might be a UFO, but most likely a lenticularis cloud. But you can easily get fooled, and it is a recurrent hypothesis that some UFO reports were due to this strange cotton saucer.
Lenticularis clouds are usually caused by a moist air flowing over an elevated ground. When the atmosphere is stable, which means that any airstream moving vertically will either sink back down or rise back up to its original level. This is due to temperature variations with altitude, and can cause wave-like patterns when the flow rise up to get over the peak then dips back at its original level. If said peak is high enough, the air will cool to the dew point and lenticularis clouds appear at the apex of the wave.
Thus, the air in a lenticularis cloud is not fixed. Instead, the cloud is continually renewed, its back degrading as the air gets warmer and the front welcoming new cool air. This phenomenon can be seen when observing them for long enough, as they often display crests or bumps running along the current. When it happens on multiple layers at once, the lenticularis clouds can pile up to form even weirder constructs.
Altitude: All cloud layers, though most noticeable at mid-level
Named after: Latin for "lentil"
Rarity: Common above elevated ground
To look further: Wikipedia article
Pileus
We are now in the realm of accessory clouds. They accompany main clouds and add tantalizing features to them. The first of them, the pileus, have a lot in common with lenticularis clouds. They, too, form when a stable airstream has to go above an obstacle before coming back down. However, a pileus doesn't avoid a mountain, but another cloud! Indeed, when a convection cloud develop upwards and hit against a moist airstream above, the fluffy smooth pileus appear for a moment. It is in striking contrasts with the rougher texture of the cumulus below, which is rising rapidly and absorb the pileus in a matter of minutes. They are thus forerunners of bad weather, as a convection cloud in a moist area is a good recipe for rain.
As any warm and sudden updraft can cause a pileus to appear, some have been witnessed atop of the ash cloud caused by a volcanic eruption. And, when all the right conditions are met, they can have the tantalizing hue of nacreous clouds (while they are different: in this case, we call them iridescent clouds).
Altitude: mid-level, above cumulus
Named after: Latin for "cap"
Rarity: Very fleeting, hard to observe
To look further: Cloud Appreciation Society aricle
Incus
Impressive timelapse of the formation of the Incus!
Now, let's get closer to a thunderstorm for our next series of clouds. Well, I say get closer but to observe our next feature, you will need to step back a little. Like kilometers worth of steps. If WorldAnvil had a cloud mascot, it would be this one. These anvil-shaped clouds form above cumulonimbus clouds, the big bad guy of storm clouds. The surface of an incus covers hundreds of square kilometers, making them larger than most other clouds. While they are majestic, they also imply heavy rain, strong winds and lightnings below, so it is best to admire them from afar.
The incus is a canopy of ice crystals, very different from the storm below yet an unmistakable mark. It is formed when the cumulonimbus grows upwarrd, creating a cumulus congestus tower. The water droplets will gradually freeze as they rise in the atmosphere, until they hit a temperature inversion. Then, the unrelenting cloud cannot develop higher and spreads in all directions, giving this typical anvil shape.
Altitude: Above storm clouds, up to 23,000m
Named after: Latin for "anvil"
Rarity: Always present in mature thunderstorms
To look further: Wikipedia article
Horseshoe Vortex
We feasted our eyes on the Incus, now let's dive straight into the storm. If sighting a Fluctus requires a very good timing and specific condition, capturing a horseshoe vortex needs sheer luck and a keen eye. They are both rare and fleeting, their lifetime barely reaching the minute. The horseshoe forms in the vicinity of storm clouds, in regions of rotating air called vortices. Such a vortex is usually vertical, the rotation perfectly illustrated by roll clouds. When an updraft is sent spinning upon reaching a brutal change in the horizontal wind speed, a horseshoe vortex cloud may appear.
It is very rare for all the conditions of a cloud formation to be achieved within the vortex. But when that glorious incidence happen, a gentle crescent will develop in the upper arc of the vortex. The cloud will rotate on itself, and usually stretch until it is torn appart and vanish, its memory only engraved in the eventual witness' eyes. The best place to observe them is quite dangerous, as much as the vicinity of a Supercell Storm can be.
Altitude: 500-1500m
Named after: Its peculiar form.
Rarity: Extreme
To look further: Wikipedia article
Funnel
I hope you haven't been too shaken up by the winds and that it was worth it. We're out of trouble for now, though that might not last for long: the funnel cloud has a violent birth, in the middle a supercell storm. When many air currents entwine into a coordinated, but raging system, you can see a cloud finger descending from the storm's base, like a deity ready to strike an impure ground. A funnel can form in two ways: either when the air is sinking from a cloud, dragged down by the weight of the rain, or when it is sucked upward to contribute to the massive updraft that feeds the storm.
The former is rather tame, the pulled down air forming vortices that never are destructive by nature. These can form landspouts or waterspouts, cousins of tornadoes with only a fraction of their power. Beware however, as it is hard to distinguish which phenomenon is happening until it is too late, and the other option is not as kind.
The latter situation, when air is flowing vertically, can herald real tornadoes. The air expands and cools as it rises and rotate in a vortex. If enough moisture condense, the funnel appears, much larger and menacing than its little brother. And should the tip of this gigantic finger touch the ground, a tornado will be born.
Altitude: Below storm clouds
Named after: Self-explanatory
Rarity: uncommon, requires a storm.
To look further: Wikipedia article
Mammatus
No the sky did not just got covered in pustules or terminid's eggs. These cloud formations, which appears mostly on the underside of Incus we saw earlier, are still today a mysterious occurence. The lumps, typically 1 or 2 km across, are formed by pockets of cold air sinking down below the clouds. However, the reasons and the exact mechanisms are unknown, none of the many hypotheses being able to close the debate.
They come in different textures and size, and may even appear in high clouds, though their most common occurence is in storm clouds. As such, they are often considered to be harbinger of the storm, even if they don't develop directly under a severe weather. One thing is for sure, their otherworldly look is enough to give chills to everyone seeing them for the first time!
Altitude: All levels, mostly below storm clouds
Named after: Latin for breasts
Rarity: Rare, near storms
To look further: Wikipedia article Pecos Hank video
Undulatus Asperitas
We near the end of our tour, and face the final boss of all clouds. Like an upside down raging ocean, they are perhaps the most terrifying sight the sky has to offer. Forming just above the ground, between 200 and 1000m, they have rarely been sighted and even more rarely recorded, and acknowledged by the World Meteorological Organization in 2017. This is due to the extreme condition they need to appear, a fragile balance of wind shear, perturbations and temperature.
Regular undulatus clouds are smooth, a uniform white with barely any rough patch. Asperatus, on the other hands, are nothing but roughness and bumps at their base, which highlights unusual instability in the cloud. This irregularity is what gives it its "stormy ocean" looks. This instability may have multiple causes, one of which may be an inversion trapping a thunderstorm and forcing it to pull its inflow air from above, rather than from the surface, causing massive disruption in the cloud's structure.
Altitude: 200-1000m
Named after: Latin for "rough, uneven"
Rarity: Extreme
To look further: Astrum video
Worldbuilding clouds
This section will come later, along with an example. Stay tuned for advices on how to use clouds in your work!
References
Les plus étranges nuages de la Terre (certains ne devraient pas exister) [French video by Balade Mentale]
The Cloud Collector's Handbook
https://cloudappreciationsociety.org/shop/cloud-collectors-handbook/
Cloud Appreciation Society
https://cloudappreciationsociety.org/
You didn't promise too much. Really great cloud formations that remind you to give clouds more space in worldbuilding, stories and RPs. Thanks for the research.