The Nubivagant Archive · Field Series № 7
A field guide to the cloud genera — how the sky got its Latin, how to read it by height, and an index you can filter, pin and compare. No instruments required.
Until 1802 the sky was a country without names. Clouds were vapours, exhalations, omens — things too fugitive for the classifier’s net, and every naturalist who tried to hold them invented words that died with their author. What was wanted was a Linnaeus for the air.
He arrived in the person of Luke Howard, a thirty-year-old Quaker chemist who, one December evening, read a paper called On the Modifications of Clouds to the Askesian Society in London. Howard’s insight was modesty: three root shapes, named in plain Latin — cirrus, a curl of hair; cumulus, a heap; stratus, something spread flat — with compounds between them, and nimbus for the cloud that carries rain.
Latin did the quiet work: a dead language, stable and portable, so a cloud over Göttingen could be argued about in London. Goethe was so taken with the system that he wrote Howard a fan letter, and then four poems. The names survived because they describe appearance, not cause — you can classify a cloud from a hilltop with nothing but patience. The World Meteorological Organization’s atlas now lists ten genera, and this guide follows its order.
The genera tell you a cloud’s shape; the forecast lives in its height. In middle latitudes the troposphere is shelved into three storeys — the étages. High clouds, all ice, keep their bases above six kilometres and carry the prefix cirro-. Middle clouds, prefixed alto-, float between two and seven. Low clouds live below two, where the weather is made.
Two genera refuse the shelving. Cumulus and cumulonimbus are vertical builders, and the second of them is the exception that contains the whole sky: a cumulonimbus keeps its base in the low étage and presses its anvil against the tropopause, holding vapour, droplets and ice at once — every étage in a single body.
Read the sky from the top down. Mare’s-tail cirrus that thickens into a haloed sheet of cirrostratus is a warm front writing its name a day in advance; the veil will grey into altostratus, then lower into nimbostratus, and the rain will arrive on schedule. The diagram below is the whole method in one column.
Ten genera, in the atlas’s order. Narrow the shelf by étage, or by whether the cloud gives up its water; the count keeps up. Tick Pin for comparison on any card to send it to the table below, and open any plate at full size.
High étage · ice · “a lock of hair”
Mare’s tails and silky streaks drawn out by high wind; the hooked fall streaks are ice crystals evaporating on the way down. When cirrus thickens and lowers into cirrostratus, a warm front is usually a day behind it.
High étage · ice · “a curled heap”
A mackerel sky: rows of white grains too high to cast shadows. Sailors paired it with mare’s tails — “mackerel scales and mare’s tails make lofty ships carry low sails.” It is the least seen of the ten.
High étage · ice · “a curled layer”
A hair-thin veil of ice that can whiten the whole sky while shadows stay sharp. Its signature is the 22° halo around sun or moon, refracted through hexagonal crystals. Rain often follows within a day.
Middle étage · droplets · “a high heap”
Sheep-back dapples, larger and shaded where cirrocumulus is grainy: hold out a hand — elements wider than three fingers are altocumulus. Turreted morning castellanus warns of afternoon thunder.
Middle étage · droplets & ice · “a high layer”
A featureless grey sheet through which the sun shows as if behind ground glass, without a halo. Thickening and lowering, it becomes nimbostratus; the sequence is a front writing its name.
Middle étage, base sinking low · “a rain layer”
The anonymous rain sky — dark, ragged, low-scudded, its base dissolving into the rain or snow it produces. It does not threaten; it simply persists, sometimes for days.
Low étage · droplets · “something spread flat”
Fog that has lifted a few hundred feet: a flat grey blanket torn by hills, giving drizzle rather than rain. On coasts it arrives with the onshore wind and leaves when the land warms.
Low étage · droplets · “a spread-out heap”
The commonest sky on Earth: grey-white rolls and rounded masses with clear gaps between. Over oceans it covers a quarter of the globe; its rare volutus form is a single roll cloud along the horizon.
Low étage, building up · “a heap”
Fair-weather heaps with flat bases — the level where rising air condenses — and crisp cauliflower tops. Watch them after noon: bases that darken and tops that harden mean showers by evening.
Vertical, base to tropopause · “a heaped storm-cloud”
The king of clouds: a tower from a low base to the tropopause, capped by an anvil of ice. Thunder, hail, gust fronts and sometimes tornadoes — the only genus aviators are taught to avoid by name.
No genus matches that pairing — loosen a facet, or reset the board.
Nothing here is fixed. The table fills with whatever you pin from the index — two genera or ten, always in the atlas’s order.
No genera pinned yet. Each card in the index carries a Pin for comparison box; tick one and it appears here, side by side with whatever else you choose.
| Genus | Étage | Typical base | Composition | Precipitation | Name means |
|---|---|---|---|---|---|
| Cirrus | High | 6,000–12,000 m | Ice crystals | None | a lock of hair |
| Cirrocumulus | High | 5,000–12,000 m | Ice crystals | None | a curled heap |
| Cirrostratus | High | 5,500–13,000 m | Ice crystals | None | a curled layer |
| Altocumulus | Middle | 2,000–6,000 m | Droplets, often supercooled | Rare virga | a high heap |
| Altostratus | Middle | 2,000–7,000 m | Droplets and ice | Light, often virga | a high layer |
| Nimbostratus | Middle, sinking | 500–3,000 m | Droplets and ice | Continuous rain or snow | a rain layer |
| Stratus | Low | 0–600 m | Droplets | Drizzle | something spread flat |
| Stratocumulus | Low | 400–2,000 m | Droplets | Occasional light | a spread-out heap |
| Cumulus | Low, building | 600–2,000 m | Droplets | None, unless towering | a heap |
| Cumulonimbus | Vertical | 600–1,500 m | Droplets and ice | Heavy showers, hail, thunder | a heaped storm-cloud |
Every plate in this archive was obsolete before it dried. A cloud is an event, not an object — most of the specimens indexed here lived for less than an hour, and none of them will be seen again. The archive keeps them anyway, the way one keeps seats for guests who will not come. Ten names for the sky is not many. It is enough. Look up, name what you find, and let it go.
— The Keeper of Plates