Cloud Types
Just like wind, a cloud is a physical manifestation of forces at play in the planet's atmosphere. Clouds are masses of small water droplets and frozen shards suspended high above the ground, forming or dissipating largely due to dew point - the temperature threshold of any body of air, under which it becomes saturated with water vapour.
Noctilucent clouds
Located in the mesosphere, 50-80km high, at the very frontier of space. At this altitude, the air is almost barren of moisture and the temperature is -126°C! Noctilucent clouds are visible as electric blue filaments rippling across the night sky. Their pale hue is indiscernable during daytime, as they have a very thin, almost transparent, ice layer that completely blends in with the atmosphere. After twilight, when the world is dark and the lower clouds become black and grey, noctilucent clouds are high enough that the planet's curvature allows them to reflect sunlight, giving them their color.
Altitude: 50-80km
Name means "Night Shining"
Rarity: Very rare, only visible around dawn or dusk.
Nacreous clouds
Form between 15 and 25km above ground level at temperatures lower than -85°C, when the Sun has passed below the horizon by just a few degrees allowing their peculiar ice crystals to diffract the last or first rays of sunlight. Nacreous clouds have a wonderful pearlescent colour that is at its brightest right after the sun passes the horizon, but continue to shine for up to two hours afterward.
Altitude: 15-25km
Named after: Nacre, or mother of pearl
Rarity: Rare, visible only in specific conditions.
Cumulus, altocumulus and cirrus clouds refer to large families of clouds.
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)
Lacunosus clouds
Peculiar formations that appear in other cloud types, giving a honeycomb texture to their underside. A lacunosus is identified by numerous gaps or holes in a cloud created when a layer of cooler air ends up above a warmer one. As cold air is denser, it sinks down through the cloud while warmer air rises up, lifting up the borders of the gap.
Altitude: All cloud levels
Name meansr: "Full of holes"
Rarity: Not rare, but fleeting
Fallstreak holes
Fallstreak holes, also called cavums, are evidence of a disturbance in clouds. They look like, and are, a cloud collapsing on itself. It is not uncommon for a cloud to be made of supercooled droplets, where water remains in liquid form despite being below 0°C. To freeze, water needs some solid particles to act as icing nuclei, catalyzers to enable the reaction. These cloud droplets are ripe for freezing but cannot due to undisturbed air. However, should these specific particles collide with ice crystals falling from a Cirrus cloud above or dust particles carried from the ground below, they begin to agglutinate on these newfound nuclei and create a chain reaction. Droplets in the vicinity freeze in turn, and join a rapidly expanding icicle. At some point, the icy formation gets heavier than the rest of the cloud and falls, 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 - where it quickly evaporates.
Altitude: 500-15,000m
Name means: "Holes"
Rarity: Common
Roll clouds
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 seems both familiar and alien. Their surface texture ranges 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. However, once the heavy clouds subside, the Arcus continues his quiet life, becoming a peaceful roll cloud.
Altitude: 100-1500m
Named after: cinnamon rolls
Rarity: Depending on the location. Frequent in some places, nonexistent in others.
Undulatus and Radiatus
Undulatus 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 displays light perturbations resulting in a wavy pattern. 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 neatly ordered lines with visible gaps between them.
Altitude: Any height
Named after: undulations
Rarity: Quite common
Lenticularis
Appears like an unmoving flying disc amongst a frantic flock of clouds carried by strong winds. Lenticularis clouds are usually caused by moist air flowing over elevated ground in a stable atmosphere. Initially appears as wave-like patterns as airflow ascends and descends. If it peaks high enough, the air cools to the dew point and lenticularis clouds appear at the apex of these 'waves.' 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 wind current.
Altitude: All cloud layers, though most noticeable at mid-level
Name mean: "lentil"
Rarity: Common above elevated ground
Pileus
An accessory cloud accompanying other clouds, adding features to them. 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 develops upwards against a moist airstream above, the fluffy smooth pileus appears in striking contrasts with the rougher texture of cumulus clouds below, which usually rise rapidly and absorb Pileus clouds in a matter of minutes. These 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.
Altitude: mid-level, above cumulus
Name means:"cap"
Rarity: Very fleeting, hard to observe
Incus
These anvil-shaped clouds form above cumulonimbus clouds, the big bad 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 lightning below, so it is best to admire them from afar. The incus is a canopy of ice crystals, very different from the storm below. It is formed when the cumulonimbus grows upward, creating a cumulus congestus tower. Water droplets 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 it the typical anvil shape. Altitude: Above storm clouds, up to 23,000m
Named means: "anvil"
Rarity: Always present in mature thunderstorms
Horseshoe Vortex
A horseshoe forms in the vicinity of storm clouds, in regions of rotating air called vortices, and is a rare and fleeting phenomenon with a lifespan barely reaching a minute. 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 - a gentle crescent in the upper arc of the vortex. The cloud will rotate on itself and usually stretch until it is torn apart.
Altitude: 500-1500m
Named after: Its peculiar form.
Rarity: Extreme
Funnel / Torando
A funnel cloud has a violent birth, in the middle a supercell storm. When many air currents entwine into a coordinated but raging system, one can see a cloud finger descending from the storm's base. 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 condenses, a funnel appears, and should the tip of this gigantic finger touch the ground, a tornado is born.
Altitude: Below storm clouds
Named after: Self-explanatory
Rarity: uncommon, requires a storm.
Mammatus
These cloud formations appear mostly on the underside of Incus clouds. They appear as lumps, typically 1 or 2km across, and are formed by pockets of cold air sinking down below the clouds. They come in different textures and size and may even appear in higher clouds, though their most common occurence is in storm clouds. As such, they are often considered to be harbingers of a storm, even if they don't develop directly under 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
Name means: "breasts"
Rarity: Rare, near storms
Undulatus Asperitas
Like an upside down raging ocean, these are perhaps the most terrifying sight the sky has to offer. These clouds form just above ground level (between 200 and 1000m) due to extreme conditions of wind shear and temperature. Regular undulatus clouds are a smooth uniform white. Asperatus, on the other hand, have rough ridges and furrows at their base, highlighting unusual instability in the cloud. This irregularity is what gives it its "stormy ocean" appearance. This instability may have multiple causes, one of which is the inversion of a thunderstorm forcing it to pull its inflow air from above, rather than from the surface, causing disruptions in cloud structure.
Altitude: 200-1000m
Name means: rough, uneven
Rarity: Extreme
Noctilucent clouds
Located in the mesosphere, 50-80km high, at the very frontier of space. At this altitude, the air is almost barren of moisture and the temperature is -126°C! Noctilucent clouds are visible as electric blue filaments rippling across the night sky. Their pale hue is indiscernable during daytime, as they have a very thin, almost transparent, ice layer that completely blends in with the atmosphere. After twilight, when the world is dark and the lower clouds become black and grey, noctilucent clouds are high enough that the planet's curvature allows them to reflect sunlight, giving them their color.
Altitude: 50-80km
Name means "Night Shining"
Rarity: Very rare, only visible around dawn or dusk.
Nacreous clouds
Form between 15 and 25km above ground level at temperatures lower than -85°C, when the Sun has passed below the horizon by just a few degrees allowing their peculiar ice crystals to diffract the last or first rays of sunlight. Nacreous clouds have a wonderful pearlescent colour that is at its brightest right after the sun passes the horizon, but continue to shine for up to two hours afterward.
Altitude: 15-25km
Named after: Nacre, or mother of pearl
Rarity: Rare, visible only in specific conditions.
Cumulus, altocumulus and cirrus clouds refer to large families of clouds.
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)
Lacunosus clouds
Peculiar formations that appear in other cloud types, giving a honeycomb texture to their underside. A lacunosus is identified by numerous gaps or holes in a cloud created when a layer of cooler air ends up above a warmer one. As cold air is denser, it sinks down through the cloud while warmer air rises up, lifting up the borders of the gap.
Altitude: All cloud levels
Name meansr: "Full of holes"
Rarity: Not rare, but fleeting
Fallstreak holes
Fallstreak holes, also called cavums, are evidence of a disturbance in clouds. They look like, and are, a cloud collapsing on itself. It is not uncommon for a cloud to be made of supercooled droplets, where water remains in liquid form despite being below 0°C. To freeze, water needs some solid particles to act as icing nuclei, catalyzers to enable the reaction. These cloud droplets are ripe for freezing but cannot due to undisturbed air. However, should these specific particles collide with ice crystals falling from a Cirrus cloud above or dust particles carried from the ground below, they begin to agglutinate on these newfound nuclei and create a chain reaction. Droplets in the vicinity freeze in turn, and join a rapidly expanding icicle. At some point, the icy formation gets heavier than the rest of the cloud and falls, 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 - where it quickly evaporates.
Altitude: 500-15,000m
Name means: "Holes"
Rarity: Common
Roll clouds
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 seems both familiar and alien. Their surface texture ranges 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. However, once the heavy clouds subside, the Arcus continues his quiet life, becoming a peaceful roll cloud.
Altitude: 100-1500m
Named after: cinnamon rolls
Rarity: Depending on the location. Frequent in some places, nonexistent in others.
Undulatus and Radiatus
Undulatus 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 displays light perturbations resulting in a wavy pattern. 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 neatly ordered lines with visible gaps between them.
Altitude: Any height
Named after: undulations
Rarity: Quite common
Lenticularis
Appears like an unmoving flying disc amongst a frantic flock of clouds carried by strong winds. Lenticularis clouds are usually caused by moist air flowing over elevated ground in a stable atmosphere. Initially appears as wave-like patterns as airflow ascends and descends. If it peaks high enough, the air cools to the dew point and lenticularis clouds appear at the apex of these 'waves.' 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 wind current.
Altitude: All cloud layers, though most noticeable at mid-level
Name mean: "lentil"
Rarity: Common above elevated ground
Pileus
An accessory cloud accompanying other clouds, adding features to them. 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 develops upwards against a moist airstream above, the fluffy smooth pileus appears in striking contrasts with the rougher texture of cumulus clouds below, which usually rise rapidly and absorb Pileus clouds in a matter of minutes. These 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.
Altitude: mid-level, above cumulus
Name means:"cap"
Rarity: Very fleeting, hard to observe
Incus
These anvil-shaped clouds form above cumulonimbus clouds, the big bad 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 lightning below, so it is best to admire them from afar. The incus is a canopy of ice crystals, very different from the storm below. It is formed when the cumulonimbus grows upward, creating a cumulus congestus tower. Water droplets 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 it the typical anvil shape. Altitude: Above storm clouds, up to 23,000m
Named means: "anvil"
Rarity: Always present in mature thunderstorms
Horseshoe Vortex
A horseshoe forms in the vicinity of storm clouds, in regions of rotating air called vortices, and is a rare and fleeting phenomenon with a lifespan barely reaching a minute. 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 - a gentle crescent in the upper arc of the vortex. The cloud will rotate on itself and usually stretch until it is torn apart.
Altitude: 500-1500m
Named after: Its peculiar form.
Rarity: Extreme
Funnel / Torando
A funnel cloud has a violent birth, in the middle a supercell storm. When many air currents entwine into a coordinated but raging system, one can see a cloud finger descending from the storm's base. 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 condenses, a funnel appears, and should the tip of this gigantic finger touch the ground, a tornado is born.
Altitude: Below storm clouds
Named after: Self-explanatory
Rarity: uncommon, requires a storm.
Mammatus
These cloud formations appear mostly on the underside of Incus clouds. They appear as lumps, typically 1 or 2km across, and are formed by pockets of cold air sinking down below the clouds. They come in different textures and size and may even appear in higher clouds, though their most common occurence is in storm clouds. As such, they are often considered to be harbingers of a storm, even if they don't develop directly under 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
Name means: "breasts"
Rarity: Rare, near storms
Undulatus Asperitas
Like an upside down raging ocean, these are perhaps the most terrifying sight the sky has to offer. These clouds form just above ground level (between 200 and 1000m) due to extreme conditions of wind shear and temperature. Regular undulatus clouds are a smooth uniform white. Asperatus, on the other hand, have rough ridges and furrows at their base, highlighting unusual instability in the cloud. This irregularity is what gives it its "stormy ocean" appearance. This instability may have multiple causes, one of which is the inversion of a thunderstorm forcing it to pull its inflow air from above, rather than from the surface, causing disruptions in cloud structure.
Altitude: 200-1000m
Name means: rough, uneven
Rarity: Extreme
Source: Real-Life Worldbuilding: Inspiration Next Door, WorldAnvil world created by Rumengol
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