Every cloud in the sky belongs to one of ten genera, sorted by the height at which it forms. Every wind has a number. Every halo has an angle. This page sets out the whole system in the order a person standing outside would meet it.
Cloud classification does not begin with shape. It begins with altitude, because altitude decides temperature, and temperature decides whether a cloud is built from water droplets or from ice. The three levels are called étages, and every genus sits in one of them.
Cold enough that these clouds are made entirely of ice crystals. They are thin, they do not produce rain that reaches the ground, and they let sunlight through — which is why they turn colour so dramatically at dawn and dusk. Because ice refracts light at fixed angles, this is also the étage where halos and sun dogs appear. Three genera live here: cirrus, cirrocumulus and cirrostratus.
A mixture of water droplets and ice, and the level where an approaching weather system announces itself. Altostratus turns the sun into a pale disc behind ground glass; when that sheet thickens and lowers into nimbostratus, the rain has already started somewhere. Altocumulus, by contrast, often means the air aloft is unstable — worth remembering on a summer morning.
Water droplets, close enough to see texture and shadow in. Stratus is fog that has lifted off the ground; stratocumulus is the grey honeycomb that covers more of the planet than any other cloud. Cumulus and cumulonimbus have their bases down here too, but a well-fed cumulonimbus does not stay — it can climb through all three étages and flatten against the tropopause.
Listed in the conventional order: high étage first, then middle, then low, ending with the one cloud that occupies all three at once. The two-letter abbreviations are the standard ones used on synoptic reports.
Detached filaments with a silky, fibrous look, often hooked at one end where the crystals have fallen into slower air. Sailors called the hooked form mares' tails. On its own it means nothing much; cirrus that thickens and spreads across the sky through a morning is the first visible edge of a warm front.
A thin sheet broken into small white grains or ripples, none of them wider than a finger held at arm's length. The regular pattern gives the mackerel sky of the old rhyme. It is the rarest of the high clouds and rarely lasts long — the same instability that ripples it usually dissolves it within the hour.
A transparent whitish veil so even that many people never notice it — the sky simply looks milky. Its signature is the 22° halo it throws around the sun or moon. Cirrostratus following a morning of thickening cirrus is one of the most reliable amateur forecasts there is: rain, usually within twelve to twenty-four hours.
Grey and white patches or rolls, larger than cirrocumulus and shaded on one side — that shading is how you tell the two apart at a glance. A sky full of altocumulus early on a warm morning is a standing warning of instability aloft, and thunderstorms by late afternoon are a reasonable bet.
A greyish or bluish sheet covering most of the sky. The sun shows through as a dim, edgeless disc — the standard description is as if seen through ground glass — and crucially it casts no shadow and throws no halo. That absence of a halo is what separates it from cirrostratus.
Dark, thick and featureless enough to blot the sun out completely. It is the cloud responsible for steady, patient rain or snow that lasts for hours rather than minutes. Its base is often ragged with torn shreds of low cloud drifting underneath, which is a useful confirmation that the rain is set in.
Grey or whitish patches with distinctly darker honeycombing between them — the classic unremarkable overcast of a temperate winter. It covers more of the earth's surface than any other cloud type, which makes it quietly one of the most important objects in the climate system. It rarely produces more than a drizzle.
A uniform grey layer with a base low enough to hide hilltops and rooftops. It is fog that has lifted a little, or is about to settle — the two are the same phenomenon at different heights. Stratus gives drizzle and ice prisms rather than proper rain, and it tends to burn off from below as the ground warms.
Detached, brilliant white, with sharp cauliflower tops and a flat base — the flatness marks the exact height at which rising air cooled to its dew point. Small, scattered, unchanging cumulus is the definition of fair weather. The same cloud growing taller than it is wide through an afternoon is the definition of a coming shower.
The thunderstorm cloud, and the only genus that routinely spans all three étages at once. A mature one flattens against the tropopause into the anvil that gives it its silhouette — the point at which it has stopped rising because there is nowhere warmer left to rise into. Hail, squalls, lightning and, at the extreme, tornadoes all come from this one cloud.
Francis Beaufort's scale of 1805 described what the wind does rather than how fast it moves, because there was no way to measure speed at sea. That is exactly why it still works: anyone can read it out of a window without an instrument.
| Force | Name | Speed | What you can see on land | |
|---|---|---|---|---|
| 0 | Calm | < 1 km/h | Smoke rises straight up. The sea is a mirror. | |
| 1 | Light air | 1 – 5 | Smoke drifts, but a weather vane will not turn. | |
| 2 | Light breeze | 6 – 11 | Leaves rustle. You feel the wind on your face. | |
| 3 | Gentle breeze | 12 – 19 | Small twigs move constantly; a light flag stands out. | |
| 4 | Moderate | 20 – 28 | Dust and loose paper lift. Small branches move. | |
| 5 | Fresh breeze | 29 – 38 | Small leafy trees begin to sway. Whitecaps everywhere. | |
| 6 | Strong breeze | 39 – 49 | Large branches in motion. Umbrellas become useless. | |
| 7 | Near gale | 50 – 61 | Whole trees move. Walking into it takes effort. | |
| 8 | Gale | 62 – 74 | Twigs break off. Progress on foot is seriously impeded. | |
| 9 | Strong gale | 75 – 88 | Chimney pots and slates come off. Fences give way. | |
| 10 | Storm | 89 – 102 | Trees uprooted. Considerable structural damage. Rare inland. | |
| 11 | Violent storm | 103 – 117 | Widespread damage. Very rarely experienced on land. | |
| 12 | Hurricane | 118 + | Devastation. The air is filled with foam and driven spray. |
Speeds are sustained, measured ten metres above open ground. Gusts routinely exceed them by half again.
Ice crystals are hexagonal, and hexagons refract light at angles that do not vary. That is why every one of these appears at exactly the same place in the sky every time, anywhere on earth — you can point to where it will be before you see it.
A pale ring around the sun or moon, cast by pencil-shaped ice crystals drifting in cirrostratus. Hold your hand at arm's length with fingers spread — the span from thumb to little finger is roughly 20°, which puts the ring just past your fingertip.
Two bright patches level with the sun and the same distance out as the halo, often with a faint red edge on the inner side. They need flat plate-shaped crystals hanging horizontally, which is why they show best when the sun is low — winter mornings are the season for them.
Water rather than ice, and the geometry is fixed: the bow always sits 42° from the point directly opposite the sun. That is why you never see one with the sun high overhead, and why the arc rises as the sun sets. A second, fainter bow at 51° has its colours reversed.
The shafts of light that fan out from behind a cloud bank. They are in fact parallel — the fanning is pure perspective, the same illusion that makes railway tracks converge. They are visible only when there is enough dust or haze in the air to scatter light back to your eye.
A single cloud tells you very little. The order in which clouds arrive tells you almost everything, because a weather front is a shape moving through the sky in a fixed sequence.
The classic warm front announces itself a full day ahead. First cirrus appears in the west, thin and hooked. Through the morning it thickens and joins up into a milky cirrostratus, and a halo rings the sun. By afternoon the veil has lowered and greyed into altostratus — the sun still visible but flat and shadowless, the halo gone. By evening the sheet is nimbostratus, the base is ragged, and the rain has set in. That whole sequence is one sloping surface of warm air sliding up over colder air, seen from underneath as it passes.
Cold fronts behave differently and much faster. They arrive as a line rather than a slope — often a visible wall of cumulonimbus with a sharp gust front running ahead of it. The wind swings round abruptly, the temperature drops within minutes, there is a short violent burst of rain or hail, and then it is over and the sky clears to scattered cumulus behind. If a summer afternoon goes suddenly still and green-tinged, that is the wall arriving.
On quiet days the useful signal is vertical growth. Cumulus that stays flat and scattered all afternoon means stable air and no surprises. Cumulus whose tops keep climbing, going from cotton-wool bumps to hard cauliflower heads taller than they are wide, means the atmosphere is unstable and a shower is being assembled overhead. Once a top goes fibrous and starts to spread sideways into an anvil, the cloud has become cumulonimbus and the shower has become a storm.
Two smaller signs are worth carrying. Virga — streaks of rain that hang below a cloud and stop in mid-air — means the air beneath is dry enough to evaporate the drops before they land, and often marks the edge of a system rather than its centre. And lenticular clouds, the smooth lens shapes that sit motionless downwind of hills, mark a standing wave in a strong steady airflow: the cloud is not moving even though the air through it is, which is why pilots treat them with respect.
None of this replaces a forecast. It does something a forecast cannot, which is to tell you what is happening at your position, now, in the twenty minutes before a model would notice. For a walk, a crossing or an afternoon outdoors, that is usually the interval that matters.
The terms that turn up in forecasts, shipping bulletins and mountain reports, defined in the shortest honest form.