Most of us stopped looking up somewhere around the end of childhood. Which is a shame, because the sky occasionally does things that look less like weather and more like a rendering error — waves breaking upside down, a perfect circular hole punched through an overcast layer, a sheet of cloud glowing electric blue an hour after sunset.
Every one of these has a name and a cause. Below are 30 of the strangest, grouped by what they actually look like rather than counted down, so that if you photographed something odd this morning you can find it quickly. A few entries are common clouds rather than rare ones — cumulonimbus, cirrus, contrails — and they earn their place because the genuinely rare features on this list form on them. You cannot identify mammatus without knowing an anvil when you see one.
What Makes a Cloud “Rare”?
A cloud forms when moist air cools until water vapour condenses onto microscopic particles — dust, sea salt, pollen — called condensation nuclei. That much happens over most of the planet every day.
What makes a formation unusual is almost always one of three things. Extreme altitude: noctilucent clouds form around 50 miles up, in air roughly a hundred-thousandth the density of sea level. Unusual dynamics: a specific wind shear, a mountain in exactly the right place, or two air layers sliding past each other at different speeds. Optical geometry: several entries here are not really clouds at all, but light doing something particular as it passes through ice crystals at a precise angle.
The official reference is the World Meteorological Organization’s International Cloud Atlas, digitised and substantially revised in March 2017 — its first major update since 1987. That revision added around a dozen new classifications, several of which appear below.
Quick Identification Guide
Six formations account for most misidentifications. The first four are lumpy or wavy cloud undersides that get confused with one another constantly; the last two are low, dark structures ahead of a storm that people frequently mix up.
| If it looks like… | It’s probably | Key tell | Rarity |
|---|---|---|---|
| Smooth hanging pouches, like bubble wrap | Mammatus | Rounded lobes, usually after the storm passes | Uncommon |
| A rough, choppy sea seen from underneath | Asperitas | Chaotic, no repeating pattern | Rare |
| Neat repeating waves, all curling the same way | Kelvin-Helmholtz | Regular breaking crests; gone in minutes | Rare |
| Even, tidy ripples like sand on a beach | Undulatus | Regular, parallel, much flatter | Common |
| A solid low wedge attached to the storm | Shelf cloud | Attached to the cloud base, arrives before rain | Uncommon |
| A detached horizontal tube, rolling | Roll cloud | Completely separate from any cloud base | Very rare |
Wave and Ripple Clouds
1. Asperitas
Asperitas is the most dramatic cloud most people will ever see, and until recently it officially did not exist. Its underside looks like the surface of a storm-tossed ocean turned upside down — sculptural, chaotic, and lit from beneath in a way that makes photographs look manipulated.
It owes its existence to one photograph and one persistent amateur. On 20 June 2006, Jane Wiggins shot the formation from an office window in Cedar Rapids, Iowa, and sent it to the Cloud Appreciation Society. In 2009, the society’s founder Gavin Pretor-Pinney began working with the Royal Meteorological Society to argue it was a genuinely new type. The World Meteorological Organization eventually agreed, and in March 2017 asperitas became official — the first cloud added to the International Cloud Atlas since cirrus intortus in 1951. The name is Latin for “roughness.”
Formally it is a supplementary feature of altocumulus and stratocumulus rather than a type in its own right. What separates it from ordinary undulatus is disorder: undulatus ripples are regular and parallel, asperitas is turbulent and irregular with no repeating pattern. Despite looking apocalyptic, it usually breaks up without producing a storm — which is exactly what happened when a widely photographed display rolled over northern California and the Santa Cruz coast in late July 2026.
2. Kelvin-Helmholtz Waves
A row of perfectly formed breaking waves hanging in the air — the most literal thing the sky does.
They form when two air layers slide past each other at different speeds. The faster upper layer drags across the slower one below, and the boundary rolls over into a line of crests: the same instability that lets wind raise waves on water. It is named for Lord Kelvin and Hermann von Helmholtz, who described the physics in the nineteenth century.
They are also a turbulence warning for pilots, and catching one is mostly luck — they typically hold their shape for a minute or two before collapsing. It is often suggested that the swirling sky of Van Gogh’s The Starry Night was inspired by them; it is a lovely idea that art historians cannot actually confirm. (If that appeals, we have catalogued the 100 most famous paintings of all time.)
3. Lenticular Clouds
Smooth, lens-shaped, often stacked in tiers like a pile of plates, lenticular clouds are responsible for a real share of historical UFO reports. It is easy to see why: sharply defined, symmetrical, and — most unnervingly — they do not move.
They form when stable, moist air is pushed up and over a mountain range, setting up a standing wave downwind. Air rises and cools at the wave crest, condensing into cloud; it descends and warms in the trough, and the cloud evaporates. Air rushes through at speed while the cloud stays anchored to the wave, which is why it appears frozen in place for hours.
Glider pilots hunt them deliberately, because the wave lift beneath can carry a sailplane to extraordinary altitude. Powered aircraft avoid the area for a different reason: the same wave system generates violent rotor turbulence beneath the smooth cloud.
4. Morning Glory Clouds
A single tube of cloud, up to 600 miles long, rolling across the sky shortly after dawn at around 35 mph. Sometimes several arrive in sequence, like breakers coming ashore.
The Morning Glory is a solitary atmospheric wave. It has been recorded elsewhere, but there is only one place on Earth where it appears predictably: the Gulf of Carpentaria in northern Queensland. Around Burketown, between late September and early November, sea breezes from opposite sides of the Cape York Peninsula collide overnight and launch the wave westward.
Glider pilots travel from across the world to surf its leading edge, riding the updraft along a cloud hundreds of miles long. It is one of very few weather phenomena reliable enough to plan a trip around — the same specific-place pull behind our guide to the world’s best love lock locations.
5. Roll Clouds (Volutus)
A long horizontal tube that appears to rotate slowly about its own axis, entirely detached from any other cloud. That detachment is the identifying feature, and what separates it from a shelf cloud, which is always attached to a storm base.
They form along the leading edge of a cool outflow — a thunderstorm gust front or a sea breeze — where advancing cold air wedges under warmer air and sets up a rolling circulation. Classified as volutus in the 2017 Atlas update. Rare enough that many people go a lifetime without seeing one; the Morning Glory above is the most famous roll cloud in the world.
6. Undulatus (Rippled Clouds)
Regular parallel ripples running across the sky like sand ridges on a tidal flat. Unda is Latin for wave, and undulatus is the tidy, well-behaved cousin of asperitas — similar cause, vastly more organised result. Gentle wind shear between two air layers sets up a regular oscillation at the boundary. Genuinely common worldwide, and beautiful enough that most people photograph it assuming they have caught something rare.
7. Mackerel Sky
Rows of small rounded cloudlets covering the sky in a pattern that does genuinely resemble the scales along a mackerel’s flank. The name is old — sailors and farmers used it long before anyone classified clouds formally.
It is produced by either cirrocumulus (high, icy, fine-grained) or altocumulus (lower, thicker, larger elements). The traditional rhyme — “mackerel sky, mackerel sky, never long wet, never long dry” — encodes a real signal: the pattern indicates instability aloft, which often precedes a change in weather within a day.
8. Banner Clouds
A banner cloud streams horizontally from a sharp mountain summit like a flag, staying attached to the peak while appearing to pour continuously downwind. The Matterhorn produces the most photographed examples on Earth.
The mechanism is a pressure drop: as wind accelerates around a steep isolated peak, pressure falls sharply on the leeward side, air is drawn upward, and it cools enough to condense. It needs a genuinely sharp, isolated summit, which is why only a handful of mountains worldwide make good ones.
Glowing, Coloured and Optical Clouds
9. Noctilucent Clouds
The highest clouds on Earth, and the only ones that shine after dark. Noctilucent clouds form around 50 miles up in the mesosphere — roughly seven times the altitude of a cruising airliner.
The name means “night shining,” which is slightly misleading: they generate no light. They are simply so high that they stay in direct sunlight long after the ground has fallen dark, catching the sun from below the horizon and glowing electric blue against a genuinely black sky.
They need extraordinary cold — around −200°F and below — and something to condense onto, thought to be meteoric dust. Look between roughly 50° and 70° latitude in summer, when the sun sits 6–16° below the horizon. They were first recorded in 1885, and NASA observations indicate they have grown brighter and more frequent, which researchers link to rising water vapour in the mesosphere.
10. Nacreous (Polar Stratospheric) Clouds
Also called mother-of-pearl clouds, and the comparison is exact: soft pastel bands of pink, green and gold with a satiny quality no other cloud has.
They form in the stratosphere, roughly 9 to 16 miles up, and require temperatures below about −108°F — conditions found almost exclusively over polar regions in deep winter. Their exceptionally tiny, uniform ice particles diffract sunlight into those characteristic bands.
They are also destructive. Their particle surfaces host reactions that convert stable, harmless chlorine compounds — what industrial CFCs eventually become in the stratosphere — into reactive forms that destroy ozone once sunlight returns in spring. Polar stratospheric clouds are central to why the ozone hole forms over Antarctica specifically. They do not carry CFCs upward, as is sometimes claimed; they provide the surfaces on which chlorine chemistry is switched on.
11. Iridescent Clouds (Irisation)
Patches of soft rainbow colour — usually pink, green and violet — smeared across a cloud edge like oil on water. Unlike a rainbow, the colours are irregular and blotchy rather than arranged in arcs.
Irisation happens when sunlight diffracts around water droplets that are both very small and unusually uniform in size, which is why it appears most often on thin, newly forming cloud edges. It occurs within roughly 40° of the sun and is commonest close in, so it is frequently missed simply because people do not look near the glare. Block the sun with your hand and check the surrounding cloud edges — it is far more common than most people realise.
12. Circumhorizontal Arc (“Fire Rainbow”)
A broad horizontal band of pure spectral colour lying flat across high cirrus, like a section of rainbow that has been ironed straight. The popular name “fire rainbow” is doubly wrong — no fire, not a rainbow — but it has stuck.
It is an ice halo. Sunlight enters the flat vertical side face of plate-shaped ice crystals in cirrus and exits through the bottom, splitting into colours on the way. The geometry is strict: the sun must be higher than 58° above the horizon. That makes it impossible above about 55° latitude — Moscow never sees one — and increasingly marginal as you go north. In southern England, around 51°N, the sun clears 58° for only a few weeks around midsummer, so they are possible but genuinely rare; across the southern and central United States they are a reasonably common summer sight.
13. Fog Bow
A rainbow with the colour drained out — a pale white arc standing in fog or mist, sometimes with the faintest blush of red on the outer edge and blue within. The physics matches a rainbow, but the droplets are far smaller: in fog they are tiny enough that diffraction smears the separated colours back into white. Best seen with the sun low and behind you, looking into a fog bank, and often accompanied by a glory.
14. Glory and the Brocken Spectre
Concentric coloured rings surrounding the shadow of your own head, cast onto cloud or fog below you. From an aircraft window it appears as rings around the plane’s shadow — a reliable sight on the shaded side of a flight above cloud.
A glory is backscattered light returning almost exactly the way it came, from droplets directly opposite the sun. Because the geometry depends on the observer’s own position, each person sees only their own, which historically gave the effect a mystical reputation.
When the rings surround a hugely elongated shadow cast onto fog from a mountain ridge, it is called a Brocken spectre, after the peak in Germany’s Harz Mountains where it is frequently reported. The looming figure is your own shadow, distorted because fog offers no depth cues.
Storm and Severe-Weather Clouds
A note before this section: several formations below are visible signs of severe weather. Photograph them from shelter, not from an open field, and treat any official warning for your area as the authority rather than what the sky appears to be doing.
15. Mammatus Clouds
Rows of smooth rounded pouches hanging beneath a cloud base, lit bronze if you catch them near sunset. The name comes from the Latin mamma, for the obvious reason.
Most clouds have flat bases because they form where rising air reaches its condensation level. Mammatus invert that: pockets of cold, dense, cloud-laden air sink into clearer air below, and the descending lobes stay visible because they are saturated. They are a supplementary feature rather than a type, most often seen on the underside of a cumulonimbus anvil.
Worth correcting a common belief: mammatus generally appear after the worst of a storm has passed, on the trailing anvil. They indicate severe convection nearby rather than danger directly overhead.
16. Shelf Clouds (Arcus)
A low dark wedge stretching across the horizon and advancing like a wall — among the most intimidating things the sky produces, and among the most photographed.
A shelf cloud forms on the leading edge of a thunderstorm’s cold outflow. Dense rain-cooled air spills out ahead of the storm and wedges beneath warmer, moister air, forcing it up and condensing it into a smooth layered ramp. The underside often looks ragged and turbulent while the top is deceptively smooth.
Unlike a roll cloud it stays attached to the storm base. It usually means gusty outflow winds are minutes away, sometimes strong enough to bring down branches and power lines — so this is one to watch from indoors.
17. Funnel Clouds (Tuba)
A rotating cone descending from a cloud base and tapering to a point in mid-air. Classified as tuba, a funnel cloud is a tornado that has not touched down — and the moment it reaches the surface it stops being a funnel cloud and becomes a tornado.
The visible funnel is condensation, not debris. Rotating air at the vortex core is at lower pressure, which cools it enough for moisture to condense and make the rotation visible. Plenty of funnels never reach the ground — and, importantly, some rotating columns do reach the ground while staying invisible until they pick up dust.
This is the one entry on the list that carries a specific instruction: a funnel cloud means tornado-capable conditions overhead. Take shelter and report it to local authorities rather than watching to see whether it descends.
18. Waterspouts
A rotating column connecting cloud to water. Waterspouts split into two quite different categories, and the distinction matters.
Fair-weather waterspouts form from the water upward under developing cumulus in light winds, and are comparatively weak and short-lived. Tornadic waterspouts are tornadoes that formed over water or moved onto it, and carry a tornado’s full destructive potential. The Florida Keys are the most active area in the world — the National Weather Service there estimates somewhere between 50 and 500 a year, a range that says a great deal about how hard they are to count.
A persistent myth holds that waterspouts suck up seawater. They do not, in any meaningful volume: the visible column is condensed freshwater cloud, with sea spray confined to the base.
19. Anvil Clouds (Cumulonimbus Incus)
The flat-topped, sideways-spreading crown of a mature thunderstorm, shaped like a blacksmith’s anvil — the signature of a storm that has hit its ceiling.
The updraft rises until it reaches the tropopause, where temperature stops falling with height. Unable to climb further, it spreads horizontally, and the anvil is ice crystals blown downwind by high-altitude winds. A storm powerful enough to punch past the tropopause produces an overshooting top — a dome bulging above the anvil, and a reliable indicator of a severe storm.
20. Pileus (Cap Clouds)
A smooth silky hood perched on top of a rising cumulus tower. Pileus are short-lived, sometimes under a minute, and frequently iridescent.
They form when a vigorously rising cumulus shoves the moist air directly above it upward faster than it can flow around, condensing a thin layer across the top. Because they need a strong updraft, a pileus is a good early sign that the cloud beneath is growing fast. They also appear above volcanic eruption columns and large explosions, for the same reason.
21. Cumulonimbus
The thunderstorm cloud itself, and the largest single structure in the lower atmosphere — a big one spans the full depth of the troposphere, from a base a few thousand feet up to an anvil near 60,000 feet. Not rare, but included because almost every entry in this section is a feature attached to one. The scale is worth registering: a large cumulonimbus holds hundreds of thousands of tonnes of water.
Holes, Streaks and Vortices
22. Fallstreak Holes (Cavum)
A clean circular or elliptical hole punched through an otherwise unbroken cloud layer, usually with wispy ice streamers trailing down through the middle. They look artificial enough to generate a steady trickle of UFO reports.
They begin with supercooled water — droplets still liquid well below freezing, waiting for something to trigger crystallisation. An aircraft climbing or descending through the layer cools the air over its wings enough to set freezing off, and a chain reaction spreads outward. The new ice crystals grow at the expense of surrounding droplets, become heavy, and fall out, leaving a hole with the fallstreak visible inside it.
The 2017 Atlas update gave them the formal name cavum. They are commonest near airports, which is a strong hint about the cause.
23. Virga
Rain that never arrives — trailing wisps hanging beneath a cloud base, curving as they fall and fading out in mid-air.
It happens when rain or snow falls into a deep layer of dry air and evaporates on the way down. The characteristic hook shape comes from wind shear dragging the streaks sideways as they descend. Deserts and high plains produce the most dramatic examples, and it is a familiar sight over the American Southwest.
It is not purely decorative. Evaporation cools the air it falls through, and that cooled air can descend fast — a microburst, capable of producing damaging winds at the surface and a serious ongoing hazard to aircraft on approach.
24. Von Kármán Vortex Streets
A double row of alternating spirals trailing hundreds of miles downwind of an island, visible only from orbit. They look mathematical, because essentially they are.
When a steady wind carrying a low marine cloud deck meets a tall isolated island, the obstruction sheds vortices alternately from each side, producing a staggered wake — the same effect that makes a flag ripple or a wire hum, scaled up to hundreds of miles. It is named for the aerodynamicist Theodore von Kármán.
The reliable producers are steep volcanic islands sitting in persistent trade winds beneath a stable cloud layer: the Canary Islands, Madeira, the Juan Fernández Islands off Chile, and the Aleutians. NASA’s Earth Observatory publishes new examples regularly.
25. Actinoform Clouds
Vast radial structures of marine stratocumulus spreading like the spokes of a wheel or the veins of a leaf across as much as 180 miles of ocean. Nobody knew they existed until satellites went up.
They were discovered in the 1960s by the earliest TIROS weather satellites, because they are simply too large to perceive from the ground or an aircraft — standing underneath one you would see ordinary overcast. The name comes from the Greek for “ray.”
Sixty years on, the mechanism is still not fully explained. They form over cool subtropical oceans, often where stratocumulus is producing drizzle, and they matter well beyond curiosity value: marine stratocumulus decks reflect a great deal of sunlight, so their behaviour is a live question in climate modelling.
Clouds We Make Ourselves
26. Contrails (Homogenitus)
The white lines aircraft leave behind. Jet engines emit water vapour and soot into air that is extremely cold and often already near saturation; the vapour condenses and freezes onto the soot almost immediately.
Whether a contrail persists is a useful weather signal. In dry upper air it evaporates within seconds; in humid upper air it lingers and spreads, sometimes broadening into sheets of artificial cirrus — which is why persistent, spreading contrails often precede a warm front. The 2017 Atlas formalised human-made clouds as homogenitus, and clouds subsequently altered by human activity as homomutatus.
Their climate effect is real, and worth stating precisely rather than dramatically. The concern is radiative forcing: contrail cirrus traps outgoing heat. The most-cited assessment puts contrail cirrus at roughly 57 mW/m² of effective radiative forcing against about 34 mW/m² for aviation’s cumulative CO₂ — so somewhat larger, though estimated with considerably lower confidence. That is a warming problem, not an air-quality one; claims that contrails threaten breathable air have no basis.
27. Pyrocumulus (Flammagenitus)
A cloud born from fire. Intense heat from a wildfire or volcanic eruption drives a violent updraft; moisture — from combustion, from the vegetation itself, from surrounding air — rises with it and condenses onto abundant smoke particles into a dirty grey-brown cauliflower tower above the plume.
Named flammagenitus in the 2017 Atlas. When one grows tall enough to become a full thunderstorm it is a pyrocumulonimbus, and these are genuinely dangerous: they generate erratic winds that drive fire spread unpredictably, and dry lightning that starts new fires miles away. The Australian bushfires of 2019–20 produced an outbreak of them powerful enough to inject smoke into the stratosphere, where it circled the Southern Hemisphere for months and was tracked by satellite — the largest such event on record at the time.
28. Steam Devils
Small rotating columns of mist spinning above open water on brutally cold mornings — a whirlwind made of steam fog, typically a few feet to a few tens of feet tall. They need a large temperature contrast: very cold air moving across much warmer water. Water evaporates into the frigid air, immediately recondenses as steam fog, and where the rising air picks up rotation a visible vortex forms. The Great Lakes produce them during winter cold snaps, as do hot springs.
Two Everyday Clouds People Mistake for Rare Ones
These two turn up constantly in “what is this strange cloud?” posts. Both are among the commonest clouds on Earth — but knowing them makes the genuinely rare ones easier to spot, because several of the entries above form on them.
29. Cirrus
Thin white streaks high in the sky, often hooked at one end — “mares’ tails.” Cirrus sits above 20,000 feet and is made entirely of ice crystals. The swept appearance comes from the crystals falling while high-altitude wind shear drags the trails sideways: cirrus is essentially high-altitude virga.
It is common everywhere, but it is the necessary ingredient for several rare entries above — circumhorizontal arcs and most ice halos need exactly the right crystals in cirrus. Thickening cirrus is also a classic sign of an approaching warm front, usually 12 to 24 hours ahead of rain.
30. Cirrostratus
A thin milky veil so subtle that most people never register it as cloud at all — the sky just looks washed out and shadows go soft.
Its calling card is the halo. Because cirrostratus is a uniform sheet of ice crystals, it very often produces a 22° ring around the sun or moon — the most common of all ice halos, and the source of the old saying that a ring around the moon means rain is coming. That folklore is broadly sound: cirrostratus usually indicates an advancing warm front.
How to Actually See These
- Check near the sun, carefully. Iridescence, circumhorizontal arcs and halos all sit close to the sun and get lost in glare. Block it with a hand or a building edge — never look directly at it.
- Twilight is the productive hour. Noctilucent and nacreous clouds are only visible when the sun is below the horizon and lighting them from underneath.
- Look behind the storm, not just at it. Mammatus generally appear on the trailing anvil once the worst has passed — from shelter, not an open field.
- Take the window seat. Glories appear on the shaded side of the aircraft, and the view above a cloud deck is where several of these are easiest to catch.
- Photograph first, identify later. Kelvin-Helmholtz waves, pileus caps and horseshoe vortices frequently last less than a minute.
The sky runs this material continuously and free of charge, and almost nobody watches. If you want to catch it properly, our roundup of the best gifts for photographers covers the kit worth having when something like an asperitas display rolls through.
Image Credits
All photographs are used under Creative Commons licences via Wikimedia Commons. Images not listed below are public domain or CC0 and require no attribution.
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