Every cloud that has ever crossed your sky belongs to one of ten Latin families. Learn them once, and the weather starts speaking in complete sentences.
Compiled & painted in CSS · after Luke Howard, 1803
The naming of clouds
In December 1802, a Quaker pharmacist named Luke Howard stood before a small London science club and did for the sky what Linnaeus had done for living things: he gave the clouds Latin names. Cirrus, a curl of hair; stratus, a spreading; cumulus, a heap; nimbus, the rain-bearer. Goethe wrote poems in the system’s honour. Two centuries on, the International Cloud Atlas still speaks Howard’s language, and every cloud you will ever see belongs to one of just ten genera built from his four roots.
The genera sort themselves by altitude into three étages, or storeys. The high clouds — cirrus, cirrocumulus, cirrostratus — are pure ice, five kilometres up or more, and can do nothing to you but foretell. The alto- pair occupies the middle storey. Stratus, stratocumulus and cumulus keep low company near the ground. Nimbostratus sprawls untidily across the middle and low storeys, and cumulonimbus declines to choose, using the whole column at once.
This atlas is built to be worked, not merely read. Filter the entries by étage, or by whether they can actually rain on you. Open the field notes folded into each card. Enlarge any plate. The plates themselves are painted entirely in CSS gradients — no photographs were taken, or indeed harmed.
The atlas
Ten genera, in the traditional order: from the ice streaks at the top of the troposphere down to the ground-hugging grey, ending with the one cloud that refuses the whole scheme.
Nothing up here rains. The high étage is ice dust — if precipitation reaches you, it started lower down. Loosen a filter to continue.
Cirrus
Ci · “a curl of hair”
Plate I — Cirrus uncinus, “mares’ tails”, toward evening
Étage
High · 5–13 km
Made of
Ice crystals
Brings
A fine evening; perhaps change tomorrow
The highest and most delicate of the genera: detached filaments of pure ice, combed into streaks by winds that can pass 200 km/h. Cirrus on its own promises nothing worse than a beautiful sunset — but it is worth watching whether it multiplies.
Field notes
Look for the hooked commas of cirrus uncinus: crystals falling from a small generating head into slower wind below. If the filaments thicken and begin to weld into a continuous veil over a few hours, a warm front is likely within a day or two.
Not to be confused with: spreading aircraft contrails, or cirrostratus — which arrives with a halo.
Cirrocumulus
Cc · “curl + heap”
Plate II — A mackerel sky, mid-morning
Étage
High · 5–13 km
Made of
Ice & supercooled water
Brings
Nothing; it is only passing through
A high granular sheet — thousands of tiny grains arranged in ripples: the classic “mackerel sky”. It is a fleeting cloud, usually a brief stage in cirrus becoming cirrostratus or the reverse, and it never brings weather of its own.
Field notes
The grains show no shading, and each appears smaller than one finger held at arm’s length. The sailors’ rhyme is fair: a mackerel sky is never long wet, never long dry — it means the upper air is changing its mind.
Not to be confused with: altocumulus, whose elements are larger and shaded underneath.
Cirrostratus
Cs · “curl + spreading”
Plate III — The 22° halo through an ice veil
Étage
High · 5–13 km
Made of
A veil of ice
Brings
Rain within a day, more often than not
A milky veil of ice so thin the sun still throws sharp shadows through it. Its signature is the 22° halo, a pale ring around sun or moon — the most reliable free forecast in the sky, since the veil usually rides a few hundred kilometres ahead of a warm front.
Field notes
Stretch out your arm and spread your hand, thumb over the sun: the halo sits just beyond your fingertips. Halo today, rain tomorrow proves right roughly two times in three — respectable odds for a proverb.
Not to be confused with: altostratus, which is thicker, lower, and kills the halo.
Altocumulus
Ac · “high heap”
Plate IV — A sheep-flock sky in rafts and rows
Étage
Middle · 2–7 km
Made of
Water droplets
Brings
A settled day — usually
The mid-level sheep-flock: rounded masses drawn up in rafts and rows, white above and grey below where each element shades itself. Mostly it decorates a settled day. The castle-turreted form is another matter entirely.
Field notes
Elements span one to three finger-widths at arm’s length and show true shading — the two things that separate them from cirrocumulus. Altocumulus castellanus before breakfast, turrets rising from a common base, is a classic advertisement for thunderstorms by mid-afternoon.
Not to be confused with: cirrocumulus (smaller, unshaded) or stratocumulus (fatter, lower).
Altostratus
As · “high spreading”
Plate V — The sun as through ground glass
Étage
Middle · 2–7 km
Made of
Water & ice, layered
Brings
The front the cirrus promised
A featureless grey-blue sheet at middling height, through which the sun shows — if at all — as though through ground glass. Nobody photographs altostratus; everybody stands under it. It usually means the warm front is nearly overhead.
Field notes
No halo, no shadows on the ground, no drama: those three negatives are the identification. Light rain or snow that reaches the ground reclassifies the sheet, by definition, into nimbostratus.
Not to be confused with: cirrostratus (shadows and a halo) or nimbostratus (darker, lower, wetter).
Nimbostratus
Ns · “rain spreading”
Plate VI — Steady rain, base dissolved by the falling of it
Étage
Middle, sagging low
Made of
Water & ice, deep
Brings
Hours of steady rain or snow
The rain-bearer proper: a thick, dark, formless layer that blots out the sun entirely, its base ragged and blurred by the precipitation falling from it. It is less a cloud you look at than weather you are inside.
Field notes
If you can still point to where the sun is, it is not nimbostratus yet. The base is impossible to fix by eye because rain dissolves it; ragged shreds of pannus drift beneath like torn paper.
Not to be confused with: thick altostratus (drier, brighter) or stratus (drizzle at most).
Stratus
St · “a spreading”
Plate VII — A grey lid, a few hundred metres up
Étage
Low · 0–600 m
Made of
Water droplets
Brings
Drizzle, at worst
Fog that has cleared its feet from the ground: a smooth grey lid, often only a few hundred metres up, swallowing hilltops and towers. At worst it drizzles. There is no violence in it at all.
Field notes
The layer is uniform enough that the sun, where visible, keeps a clean outline. Summer stratus burns off from the edges by late morning; a winter anticyclone can hold it in place for days on end.
Not to be confused with: nimbostratus (real rain, darker) — or hill fog, which is only stratus you happen to be standing in.
Stratocumulus
Sc · “spread heap”
Plate VIII — The commonest cloud on Earth
Étage
Low · 0.3–2 km
Made of
Water droplets
Brings
Gloom; rarely anything wetter
The commonest cloud on the planet: low grey-and-white rolls with lanes of sky between them, the compromise a stratus layer reaches with a convecting afternoon. It looks far worse than it behaves — rarely more than a spit of drizzle.
Field notes
Elements are fatter than a fist at arm’s length, dark beneath, bright in the gaps. Watch the lanes: widening blue means the layer is breaking up toward cumulus; closing grey means it is settling back toward stratus.
Not to be confused with: altocumulus (smaller elements, higher) or cumulus (detached, with cauliflower tops).
Cumulus
Cu · “a heap”
Plate IX — Cumulus humilis, the fair-weather signature
Étage
Low · 0.3–2 km
Made of
Water droplets
Brings
Fair weather — watch the towers
The child’s drawing of a cloud: detached, dense, flat-bottomed and cauliflower-topped, riding thermals over warm ground. Humilis, wider than it is tall, is the very signature of a fine day; congestus, its towering cousin, is the first act of a storm.
Field notes
All the cumulus in view float their bases at the same altitude — the condensation level — like ships on one waterline. Fair while flat; start paying attention when the towers grow taller than their bases are wide.
Not to be confused with: stratocumulus (merged into rolls) or a young cumulonimbus — check whether the summit has gone fibrous.
Cumulonimbus
Cb · “heap of rain”
Plate X — The anvil, spreading downwind
Étage
All of them · 0.5–16 km
Made of
Water below, ice above
Brings
Squalls, hail, lightning
The whole troposphere used as a single engine: a tower reaching from near the ground to the tropopause, spreading an anvil of ice downwind. Hail, squall lines, lightning and, in the worst cases, tornadoes all live here. It is the only genus with a genuine capacity for harm.
Field notes
The promotion from congestus happens the moment the summit stops looking like cauliflower and starts looking like combed hair — glaciation. The anvil points where the storm is going. Admire it from a distance measured in kilometres.
Not to be confused with: cumulus congestus, which keeps a crisp cauliflower crown all the way up.
The sky in section
Altitude is the first identification key. Meteorology slices the troposphere into three étages, and every genus keeps to its storey — save the two famous ones that refuse. Select any abbreviation to jump to its entry.
The second key fits in your hand. Held at arm’s length, one finger covers about a degree of sky: elements smaller than a finger are cirrocumulus; one to three fingers, altocumulus; a fist or more, stratocumulus. Half of cloud identification is deciding how far away the lumps are, and your arm is the only instrument required.
Colophon
There is no JavaScript in this document — not a script tag, not a handler, not a javascript: URL. The filters are ordinary radio buttons read relationally with :has(); the plates enlarge through :target; the field notes are native <details>; the appearance menu is a native popover placed with anchor positioning, flipping the page through color-scheme and light-dark(); the reading meter is a single scroll-driven animation counting a registered custom property from 0 to 100; the étage diagram aligns its label column across banded rows with subgrid; and each entry card turns broadside via a container query whenever filtering leaves it room to spread.
Baseline: an evergreen browser of mid-2023 — grid, container queries, :has(), subgrid and color-mix() are assumed. Everything newer is fenced with @supports and fails by subtraction: without :has() the filter and theme controls remove themselves and the full atlas reads in order; without scroll-driven animations there is simply no meter; without anchor positioning the menu opens centred, as popovers natively do. Readers who prefer reduced motion get no drifting skies, no smooth scrolling and no animated disclosure — only pages that turn.
Set in Fraunces and Newsreader. Plates painted with CSS gradients, masks and blur; no photography, no canvas, no SVG. After the WMO International Cloud Atlas and Luke Howard’s Essay on the Modifications of Clouds, 1803.
Plate I — Cirrus uncinus, toward eveningClose ✕Plate II — Cirrocumulus, a mackerel skyClose ✕Plate III — Cirrostratus with 22° haloClose ✕Plate IV — Altocumulus in rafts and rowsClose ✕Plate V — Altostratus, sun as through ground glassClose ✕Plate VI — Nimbostratus in steady rainClose ✕Plate VII — Stratus, a grey lidClose ✕Plate VIII — Stratocumulus rollsClose ✕Plate IX — Cumulus humilisClose ✕Plate X — Cumulonimbus capillatus incusClose ✕