Sky, coast & ground
What cloud is that?
The World Meteorological Organization recognises ten cloud genera and every cloud in the sky is one of them. Three questions get you there: how high is it, is it heaped or layered, and can you see the sun through it? Each entry also says what the cloud means, because a cloud is a reading of the atmosphere rather than a decoration.
Narrow it down
Showing all 29 clouds
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Cirrus
Ci
Delicate white streaks and hooks of ice crystal, drawn out by strong high-level wind into what people call mares' tails. Casts no shadow and never produces rain at the ground.
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Cirrostratus
Cs
A thin milky veil of ice crystals across the whole sky, so subtle you may only notice it because a bright 22-degree halo has appeared around the sun or moon. The halo is diagnostic.
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Cirrocumulus
Cc
A fine grain of tiny white ripples with no shading at all — hold your hand at arm's length and each cell is narrower than your little finger. The classic mackerel sky.
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Altostratus
As
A grey-blue sheet over the whole sky through which the sun shows as a pale watery disc with no halo — the textbook phrase is "as if seen through ground glass". Casts no shadows on the ground.
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Altocumulus
Ac
Rounded white and grey patches in regular rows or a broken sheet, each element clearly shaded underneath and about the width of one to three fingers at arm's length.
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Altocumulus castellanus
Ac cas
Mid-level cloud sprouting small turrets from a common base, like a row of battlements. It signals instability aloft, and on a summer morning it is the single most reliable sign of afternoon thunderstorms.
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Nimbostratus
Ns
A thick dark grey featureless layer that blots out the sun completely and rains or snows steadily for hours. Ragged shreds of cloud drift beneath it in the falling precipitation.
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Stratus
St
A uniform grey layer with no structure, low enough to hide hilltops, giving drizzle or nothing at all. The sun may show as a distinct outline when it is thin — unlike altostratus, where it is a blur.
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Stratocumulus
Sc
Lumpy grey and white rolls or patches covering most of the sky with narrow blue channels between them, each element wider than three fingers at arm's length. Rarely produces more than light drizzle.
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Cumulus humilis
Cu hum
Small detached white heaps with flat bases all at the same height and crisp cauliflower tops, wider than they are tall. The fair-weather cloud, and reliably so.
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Cumulus congestus
Cu con
A cumulus that has grown taller than it is wide, with hard sharply outlined cauliflower tops still made of water droplets. Also called towering cumulus, and it produces showers.
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Cumulonimbus
Can injure youCb
A cloud that spans the whole troposphere, with a dark base, hard towering sides and a fibrous top spreading downwind into a flat anvil. Produces lightning, hail and every severe weather phenomenon there is.
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Anvil (incus)
Cb inc
The flat fibrous top of a cumulonimbus, spread sideways where the updraught hit the stable tropopause and could rise no further. Its downwind edge tells you which way the storm is moving.
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Overshooting top
Can injure youA rounded bulge pushing up through the smooth anvil, meaning the updraught was strong enough to punch past the tropopause. Visible from tens of kilometres away and a reliable sign of a severe storm.
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Shelf cloud (arcus)
Can injure youA low wedge attached to the storm base, with a smooth rising front and a ragged turbulent underside, advancing ahead of the rain. Wind hits within minutes of it passing overhead.
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Wall cloud
Can injure youAn isolated lowering hanging beneath the rain-free base at the rear of a storm, often visibly rotating. It marks the main updraught and it is where tornadoes form.
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Mammatus
Rows of smooth hanging pouches on the underside of an anvil, lit dramatically at sunset. They form in sinking air and mean the storm is mature — usually on its departing side, not its leading edge.
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Altocumulus lenticularis
Ac len
A smooth almond or saucer, sometimes stacked in layers like a pile of plates, hanging motionless in a strong wind while the air streams through it. Standing waves over a ridge.
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Kelvin-Helmholtz waves
A row of curling breaking waves along the top of a cloud layer, exactly like surf. Forms where two layers move at different speeds; it lasts a couple of minutes.
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Asperitas
A chaotic wave-torn underside like the surface of a rough sea seen from below. Added to the WMO cloud atlas in 2017 — the first new formal cloud feature in over half a century.
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Virga
Trails of precipitation hanging beneath a cloud and stopping in mid-air, because the drops evaporate before they reach the ground. Curved streaks, often catching the light.
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Pileus
A smooth thin cap sitting briefly on top of a rising cumulus tower, formed as the tower lifts the moist layer above it. It lasts a minute or two and marks a vigorous updraught.
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Fractus and scud
Torn shapeless fragments of low cloud racing beneath a rain-producing deck, formed in the saturated air of falling precipitation. Detached from the main base, which is how you separate it from a wall cloud.
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Fog
Stratus cloud at ground level. Radiation fog forms in a valley on a clear calm night and burns off by mid-morning; advection fog rolls in off cold water and can last for days.
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Contrails
Straight white lines behind aircraft. Whether they persist is a real weather signal: contrails that vanish in seconds mean dry air aloft and settled weather, while contrails that spread into sheets mean the upper air is already near saturation and a front is on the way.
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Noctilucent clouds
Electric-blue filaments glowing against a deep twilight sky an hour or two after sunset, when everything else is dark. They are ice on meteoric dust in the mesosphere, lit by a sun that has already set for the ground.
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Nacreous clouds
Iridescent mother-of-pearl bands in pastel pink, green and gold, glowing well after sunset in a very cold winter twilight. Beautiful, rare, and involved in polar ozone destruction.
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Pyrocumulus
Can injure youA dirty grey-brown cumulus growing directly out of a smoke plume, driven by the heat of the fire below. In its most extreme form it generates its own lightning and its own wind.
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Roll cloud (volutus)
A long horizontal tube of cloud rolling about its own axis, completely detached from any other cloud. Unlike a shelf cloud it is not attached to a storm base.
Nothing in this key matches every filter at once. That usually means one observation is off — a colour read in shade, or a size guessed rather than measured. Clear the filter you are least sure about and look again.
Ten genera, two questions
The classification everyone still uses was proposed by Luke Howard in 1802 and codified by the World Meteorological Organization. It is built on two Latin roots and a modifier, and once you have those the whole scheme unpacks itself.
- Cumulus — heap. Convective, lumpy, with a flat base where rising air reached its condensation level.
- Stratus — layer. Formed by air lifted or cooled across a broad area, so it is flat and featureless.
- Cirrus — curl of hair. High and made of ice, so it is white, fibrous and casts no shadow.
- Nimbus — rain. Attached to the two genera that actually precipitate: nimbostratus and cumulonimbus.
- Alto- — the middle level, 2 to 6 km.
Combine them and the ten genera fall out: cirrus, cirrostratus and cirrocumulus high; altostratus and altocumulus in the middle; stratus, stratocumulus and nimbostratus low; cumulus and cumulonimbus vertical. There is no eleventh.
The height question is easier than it sounds because you do not have to measure anything. High clouds are white and cast no shadow on themselves, since ice crystals are sparse and translucent. Low clouds have grey undersides, because liquid water is dense enough to block light. Shading is a proxy for altitude and it is reliable.
The finger test, and why element size matters
Cirrocumulus, altocumulus and stratocumulus all look like patterned patches of cloud, and they sit at three different heights. Meteorologists separate them by apparent element size, which is a proxy for distance, and you measure it with your hand.
Hold your arm straight out. Your little finger covers about one degree of sky, three fingers about five degrees, and a clenched fist about ten.
| Element width at arm's length | Angular size | Cloud | Shading |
|---|---|---|---|
| Narrower than one little finger | Under 1° | Cirrocumulus | None — pure white |
| One to three fingers | 1–5° | Altocumulus | Clear grey shading below |
| Wider than three fingers | Over 5° | Stratocumulus | Heavy grey |
The shading column is the shortcut. Cirrocumulus is ice and never shades; if any element in the pattern has a grey underside, you are looking at altocumulus or lower. That one observation resolves the three faster than the finger measurement does.
Reading a warm front arriving
The most useful thing amateur cloud watching gives you is the ability to see a front coming twelve to twenty-four hours out, from the ground, with no equipment. A warm front approaches as a shallow wedge, so its cloud deck arrives high and thin and steadily lowers and thickens. The sequence is fixed:
- Cirrus. Wispy hooks appear from the direction the weather comes from. Rain is roughly a day away.
- Cirrostratus. The wisps merge into a milky veil and a halo appears around the sun or moon. This is the classic marker, and the old rhyme — a ring around the moon means rain soon — is meteorologically sound.
- Altostratus. The veil thickens and the sun blurs into a watery disc with no halo and no shadows on the ground. Rain is now within hours.
- Nimbostratus. The sun disappears, the base lowers, ragged scud races underneath, and steady rain begins and continues.
A cold front behaves completely differently: it is steep, so it produces a narrow line of towering cumulus and cumulonimbus with little warning, a short violent burst of rain and wind, and then a rapid clearance into cool air with fair-weather cumulus. If the sky went from blue to a black wall in twenty minutes, that is a cold front.
The clouds that mean take shelter
Four sky features are worth recognising for reasons that are not aesthetic.
Cumulonimbus. Any cloud with a hard towering side and a fibrous spreading top. Lightning strikes routinely up to 15 km from the parent cloud, into air that looks clear — the "bolt from the blue" is a real and documented phenomenon. The operative rule is simple and is the one the US National Weather Service publishes: if you can hear thunder, you are close enough to be struck. Go inside, and wait 30 minutes after the last thunder before going back out.
Shelf cloud. A wedge on the leading edge of a storm, smooth on top and ragged underneath, advancing ahead of the rain. It marks a gust front, and damaging straight-line winds arrive within minutes of it passing over. This is the one that catches people on lakes and beaches.
Wall cloud. An isolated lowering under the rain-free base at the back of a storm. If it is rotating, that is the mesocyclone, and tornadoes form there. It is not the same thing as a shelf cloud, and confusing the two costs people the time they need.
Overshooting top. A dome punching up through an otherwise flat anvil. It means the updraught beat the tropopause, which is a strong indicator of severe hail and wind. Visible from a long way off, which makes it useful for deciding whether to drive on.
None of this replaces a warning. Radar sees rotation you cannot, and official alerts should always take priority over what the sky looks like from where you are standing.
Optical effects, and what they tell you about the cloud
Halos and coronas are frequently mistaken for each other and they say opposite things about what the cloud is made of.
A 22-degree halo is a large ring around the sun or moon, about the span of a hand at arm's length, produced by refraction through hexagonal ice crystals. A halo means ice, which means high cloud, which usually means cirrostratus and an approaching front. Sundogs — bright spots on either side of the sun at the same height — come from the same crystals settling flat.
A corona is a small coloured disc tight around the sun or moon, produced by diffraction through water droplets. It means liquid, so a lower cloud, and the smaller and more colourful the corona the more uniform the droplet size. Iridescence at cloud edges is the same effect, and it is what makes nacreous clouds spectacular.
A glory is a set of coloured rings around the shadow of your own head, seen from an aircraft or a mountaintop looking into cloud below. A crepuscular ray is a shaft of sunlight through a gap, made visible by haze, and it says nothing about the cloud at all.
Photographing and reporting clouds
Clouds are one of the few subjects where a phone camera is genuinely adequate, provided you handle two problems.
Exposure. Auto exposure meters for the whole frame and blows out the cloud, which is the brightest thing in it. Tap the brightest part of the cloud to expose for it and let the foreground go dark, or drag the exposure slider down a stop.
Scale. A photograph of nothing but sky has no reference, and the same frame could be a small cumulus at two kilometres or a supercell at forty. Include a horizon, a tree line or a rooftop. That single decision is what makes a cloud photograph identifiable later.
If you want the observation to be useful to somebody, note the time, your location and which direction you were facing, and whether the cloud was moving. The Cloud Appreciation Society and the WMO International Cloud Atlas both accept submissions, and national weather services in most countries run spotter networks that take severe-weather reports from the public. In the United States that is SKYWARN, which trains volunteers and feeds their reports straight into warning decisions.
29 entries here cover the ten genera plus the supplementary features and special clouds people actually photograph. The full WMO atlas adds species, varieties and accessory clouds in combinations running to over a hundred names — cumulus radiatus, stratocumulus undulatus and the rest — which is more precision than the sky usually rewards from the ground.
Questions people ask about this
What are the ten types of cloud?
High: cirrus, cirrostratus, cirrocumulus. Middle: altostratus, altocumulus. Low: stratus, stratocumulus, nimbostratus. Vertical: cumulus and cumulonimbus. Every cloud in the sky belongs to one of these ten genera; everything else is a species, variety or supplementary feature added to one of them.
What does a ring around the sun or moon mean?
A 22-degree halo is refraction through hexagonal ice crystals in cirrostratus, a high thin cloud that typically runs ahead of a warm front. Rain often follows within 12 to 24 hours, so the folk rhyme about a ring around the moon has a real mechanism behind it. Do not confuse it with a corona, which is a small tight coloured disc caused by water droplets in lower cloud.
Which clouds mean a thunderstorm is coming?
Cumulonimbus is the storm itself — a cloud spanning the whole troposphere with a dark base and a fibrous anvil top. The early warning is cumulus congestus, a cumulus grown taller than it is wide with hard cauliflower tops; when those tops turn fibrous the cloud has glaciated and become a storm. On a summer morning, altocumulus castellanus — mid-level cloud sprouting little turrets — is the most reliable indicator of afternoon storms.
What is the difference between a shelf cloud and a wall cloud?
A shelf cloud is attached to the leading edge of a storm, wedge-shaped, smooth on top and ragged below, and it marks the gust front — damaging wind arrives within minutes. A wall cloud hangs below the rain-free base at the rear of the storm, is isolated rather than attached along a front, and often rotates; it marks the updraught where tornadoes form. Both mean take shelter, and they arrive at different moments.
What is that smooth lens-shaped cloud over the mountains?
Altocumulus lenticularis, formed in a standing wave downwind of a ridge. Air rises over the obstacle, condenses at the crest of the wave, and evaporates again as it descends, so the cloud stays in one place while a strong wind streams through it. That combination of smooth edges and apparent stillness is why it is the most commonly reported UFO in the sky.
Why do some contrails disappear and others spread across the sky?
Humidity at cruising altitude. When the upper air is dry, the ice crystals in an aircraft exhaust sublimate within seconds. When it is already near saturation, the crystals persist and spread into cirrus-like sheets. Persistent spreading contrails are therefore a genuine sign of moist upper air, which often means a front approaching in the next day.
Is fog a cloud?
Yes — it is stratus cloud that reaches the ground, and the only difference between them is where the base sits relative to you. Radiation fog forms in valleys on clear calm nights as the ground cools and burns off through the morning; advection fog forms when warm moist air moves over a cold surface, which is what rolls in off the sea and can persist for days.
Still not sure? Point a camera at it
A filter narrows clouds down to a shortlist. Orvik reads the photo itself and names the species, with the look-alikes ranked underneath so you can check the call yourself.