Clouds

Seven specimens collected between the mesosphere and sea level — every plate computed in the browser, none of them photographed. Read from the top down. The descent is eighty‑five kilometres.

Seven plates One shader Zero photographs Begin descent ↓

How to read this atlas

In 1802 a London chemist named Luke Howard gave the clouds their Latin names, sorting the whole restless sky into a handful of genera the way Linnaeus had sorted plants. The scheme held. Every weather report since owes him a debt.

This atlas follows his method but reads in the wrong direction: it begins where the atmosphere runs out and descends, plate by plate, to the one cloud you can stand inside. Every specimen is computed by your machine as you watch — fractal noise, shaped and lit — and no photograph appears anywhere in this document. The instrument in the left margin tracks the descent and quotes the standard atmosphere as it goes.

Contents, by altitude
I	Noctilucent	82 000 m
II	Nacreous	21 000 m
III	Cumulonimbus	11 000 m
IV	Cirrus	9 400 m
V	Altocumulus	3 500 m
VI	Stratus	600 m
VII	Fog	1.7 m

Plate I Collected at 82 000 m

Noctilucent

Latin nox, night, + lucere, to shine.

The highest clouds ever observed, and the least reachable. They condense at the mesopause — colder than −120 °C, a millionth of sea-level pressure — as ice gathers on the smoke of burnt-up meteors. Sunlight is what betrays them: they shine only on summer nights at high latitudes, when the sun has set for the observer but still catches them from below the horizon. Nobody recorded one before 1885, two years after Krakatoa filled the upper sky with dust. Whether they are a new cloud or a newly noticed one is still an open argument.

Altitude: 76–85 km, mesopause
Constitution: water ice on meteoric smoke
Season: polar summer, twilight only
First record: 1885

Fig. 1Noctilucent billows an hour after sunset. Ridged fractal noise confined to a thin gaussian band; the stars are a hash function.

Plate II Collected at 21 000 m

Nacreous

From nacre — mother-of-pearl.

Mother-of-pearl clouds of the polar stratosphere, forming only in the deep cold of winter, below −78 °C. The iridescence is structural: ice crystals so uniform in size that they diffract sunlight coherently, the same trick a soap film performs. They show best when the sun sits just under the horizon and lights them against a darkening sky. Their beauty keeps poor company — the surfaces of polar stratospheric clouds host the chemistry that converts chlorine into its ozone-destroying form. The loveliest plate in this atlas has done the most damage.

Altitude: 15–25 km, stratosphere
Forms below: −78 °C
Class: polar stratospheric, type II
Seen: winter, sun below horizon

Fig. 2A nacreous stack in winter twilight. Warped low-frequency noise; the iridescence is a cosine palette keyed to optical depth.

Plate III Collected at 11 000 m

Cumulonimbus

Latin cumulus, a heap, + nimbus, a rain-cloud.

Met out of order, and unavoidably. Descending, its anvil arrives first — a sheet of ice pressed flat against the tropopause, the ceiling weather cannot pass — while its base waits ten kilometres of trouble below. The column between them runs on updrafts strong enough to loft hail above the cruising altitude of an airliner. Rain, hail, lightning, downbursts, tornadoes: the sky's entire criminal record belongs to this one genus. It is the only specimen here with its own aviation rule, which is to go around.

Base · top: 600 m · 12 000 m and higher
Contents: rain, hail, lightning
Updraft: to 50 m/s in severe cells
Aviation: avoided by rule

Fig. 3One cell at full height, evening light from the west. A width profile with an anvil term, filled with domain-warped noise and shaded by finite difference.

Plate IV Collected at 9 400 m

Cirrus

Latin — a lock of hair.

Ice this high never falls as weather; it falls as handwriting. Wind shear draws the crystals into filaments, and the hooks and commas — mare's tails, to sailors — are fall streaks: crystals dropping into slower air below, bending the line as they go. Cirrus says almost nothing about the sky you are in and a great deal about the sky that is coming. Thickening sheets of it run a day or two ahead of a warm front, which is why the proverb pairs it with shortened sail.

Altitude: 6–12 km
Constitution: ice crystals, wind-sheared
Reads as: mare's tails, fall streaks
Meaning: warm front, 24–48 h out

Fig. 4Cirrus fibratus with fall streaks. Fractal noise sheared along the wind axis; the hooks are a quadratic term.

Plate V Collected at 3 500 m

Altocumulus

Latin altum, height, + cumulus, a heap.

A mid-level layer broken into cellules: each cloudlet is a column of rising air fenced by sinking air, convection organised into a grid the way simmering milk organises itself. The pattern reads as fish scales — a mackerel sky — and sailors treated it as notice, since it often runs ahead of unsettled weather. At dusk the geometry lights from below and the layer becomes a ceiling of embers, which is how this specimen was collected. Look for coronas when the moon stands behind it; the droplets are small and uniform enough to diffract.

Altitude: 2–7 km
Structure: cellular convection
Lighting: from below, at dusk
Optics: coronas, iridescence

Fig. 5Altocumulus at last light. Cellular noise under perspective, patched by fractal noise; the undersides are shaded by offset sampling.

Plate VI Collected at 600 m

Stratus

Latin sternere — to spread out.

The plainest page in the catalogue, included on principle. Stratus is a featureless grey sheet lying low over everything: no cells, no towers, no filaments, no edges to speak of. It forms when a damp layer is lifted just barely, or cooled just barely, and it can hold for days. Nothing about it rewards a longer caption.

Altitude: 0–600 m
Structure: none
Persistence: days
Forecast: unchanged

Fig. 6Stratus. Two octaves seemed sufficient.

Plate VII Collected at 1.7 m — eye level

Fog

Origin unknown, fittingly.

Stratus with its base on the ground, and the only specimen that can be collected from inside. The formal definition is visibility below one kilometre; anything gentler is mist. Inside it, distances go first, then colours, then the idea that there was ever anything to see.

It is catalogued here at eye level, 1.7 metres, where the descent ends. Move your pointer through the plate; the fog will make room for you, briefly.

Altitude: 0 m
Visibility: below 1 000 m, by definition
Handling: pointer parts it

Fig. 7Valley fog at eye level, three ridgelines deep. Exponential fog over noise silhouettes; the cursor lowers the local density.