Storm Station 247

The Field Book/Vol. III, The Sky/FB-SKY-001

Cloud classification

Clouds are named in Latin ranks set by the WMO's International Cloud Atlas: ten genera, sorted into low, middle and high levels by the height of their bases, then species, varieties, supplementary features and accessory clouds. The name says what shape the cloud has, how high it is, and so how the air around it is moving.

Plate FB-SKY-001The generaRevision 1, 2026-09-24Status draftAlso called cloud genera, cloud types, étage, cirriform, cumuliform, stratiform, International Cloud Atlas, Luke Howard

Ten painted clouds side by side over a low landscape, each with its abbreviation: cirrus, cirrocumulus and cirrostratus in the high étage; altocumulus, altostratus and the top of nimbostratus in the middle étage; stratocumulus and stratus low; cumulus rising from a low base; and cumulonimbus towering through all three étages to an anvil of ice, with rain beneath it and beneath the nimbostratus. Arranged by height, not drawn to a height scale.
Fig. A The ten genera. The ten kinds of cloud in their three étages, high, middle and low, as the WMO Cloud Atlas names them.MaximizeThe drawing, to scale

The plates under it

The genera

The ranks

Beyond the genera

Storm clouds

What it is

Clouds are named the way plants are, in Latin, in ranks. The ranks are set by the International Cloud Atlas of the World Meteorological Organization. The first rank is the genus, and there are ten. Their names are built from four roots Luke Howard proposed in 1803: cirrus, a curl of hair; cumulus, a heap; stratus, a layer; and nimbus, a rain cloud. Alto marks the middle clouds. Each genus has a plate of its own: cirrus FB-SKY-010, cirrocumulus FB-SKY-011, cirrostratus FB-SKY-012, altocumulus FB-SKY-013, altostratus FB-SKY-014, nimbostratus FB-SKY-015, stratocumulus FB-SKY-016, stratus FB-SKY-017, cumulus FB-SKY-018 and cumulonimbus FB-SKY-019.

Fig. A sorts the ten by height into three levels, the étages. In the middle latitudes the low étage runs from the ground to 6,600 ft (2 km), the middle from 6,600 to 23,000 ft (2 to 7 km), and the high from 16,400 to 43,000 ft (5 to 13 km). The middle and high overlap. Cumulus and cumulonimbus have their bases in the low étage but can grow through all three, and nimbostratus, counted as a middle cloud, spreads above and below it.

What it is not

It is not a forecast. The name records the shape of a cloud and its height. A towering cumulus and a thunderstorm are different genera, but how dangerous a storm is comes from radar and the sounding, not the name.

Fog is not one of the genera. The Atlas treats it on its own, though fog that lifts off the ground is stratus.

Lookalikes

Fog
Cloud at the ground. The Atlas does not count it among the genera; it treats fog as a hydrometeor of its own. Fog lifting off the ground becomes stratus.
Contrails
Cloud made by aircraft exhaust. The Atlas names it by the genus it resembles and the word homogenitus, as cirrus homogenitus.
Smoke and haze
Dry particles, not water or ice. They are lithometeors, not cloud, though smoke from a large fire can build a real cumulus, a flammagenitus.

The machine

Each genus says how the air around it is moving. The layered clouds, stratus, altostratus, nimbostratus and cirrostratus, form where a broad sheet of air is lifted slowly or cooled from below; the air is stable. The heaped clouds, cumulus and cumulonimbus, form where parcels of air rise on their own; the air is unstable. The cirriform clouds are ice, high and cold, and fibrous because ice crystals fall and are drawn out by the wind.

Below the genus come the species, the varieties, the supplementary features and the accessory clouds, Fig. B. A full name reads across them: cumulonimbus capillatus incus is a thunderstorm whose top has turned to fibrous ice and spread into an anvil.

A painted table of how a cloud is named, rank by rank, beside three painted skies: cirrus fibratus, fine straight filaments of ice high over a mountain, 26,000 to 39,000 feet (8 to 12 kilometres); altocumulus lenticularis, smooth stacked lens shaped cloud over a ridge, 10,000 to 20,000 feet (3 to 6 kilometres); and cumulonimbus capillatus incus, a thunderstorm whose fibrous icy top spreads into a flat anvil, with rain reaching the ground; beneath, the counts of each rank: 15 species, 9 varieties, 11 supplementary features and 4 accessory clouds.
Fig. B How a cloud is named. Genus, species, varieties, then features and accessory clouds: three names read across the ranks.MaximizeThe drawing, to scale

The drawings

Each figure drawn as an engineering sheet, its parts numbered, to print at 11 by 17.

Classification of the ten cloud genera on one height scale from the ground to 43,000 feet (13 kilometres): cirrus, cirrocumulus and cirrostratus of ice crystals in the high étage, 16,400 to 43,000 feet (5 to 13 kilometres); altocumulus of water droplets and altostratus of droplets and ice in the middle étage, 6,600 to 23,000 feet (2 to 7 kilometres); stratocumulus and stratus of water droplets in the low étage, below 6,600 feet (2 kilometres); nimbostratus spanning the low and middle étages with rain reaching the ground; cumulus with a flat base near 3,300 feet (1 kilometre); and cumulonimbus rising from near 3,300 feet (1 kilometre) through all three étages to an anvil of ice near 39,000 feet (12 kilometres), with rain beneath.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-001-ATHE TEN GENERA ON ONE HEIGHT AXIS, WITH THE THREE ÉTAGES OF THE MIDDLE LATITUDES AND WHAT EACH CLOUD IS MADE OFFIG. A THE TEN GENERA BY HEIGHT010203040012345678910111213kft(km)LOWMIDDLEHIGHCirrusCiCirrocumulusCcCirrostratusCsAltocumulusAcAltostratusAsNimbostratusNsStratocumulusScStratusStCumulusCuCumulonimbusCbKEYIce crystalsWater dropletsRain or snow reaching the groundHEIGHTS TYPICAL OF THE MIDDLE LATITUDES.ÉTAGES AS THE INTERNATIONAL CLOUD ATLAS SETS THEM.142356TITLECloud classification, the ten genera by heightVOL. III THE SKY · THE GENERATYPECLASSIFICATIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-001-ADRAWN AS linework on paperSOURCES WMO, AMS, Office of the Federal Coordinator for Meteorology
Fig. A, the drawing The ten genera by height. The ten genera on one height axis, with the three étages of the middle latitudes and what each cloud is made of HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. The high étage
  2. The middle étage
  3. The low étage
  4. Where the middle and high étages overlap
  5. The anvil, spread against the tropopause
  6. Rain reaching the ground
Table of the ranks of the cloud classification: the 10 genera with their abbreviations, the 15 species, the 9 varieties, the 11 supplementary features and the 4 accessory clouds, each listed in full; beneath, three names read across the ranks: cumulonimbus capillatus incus, altocumulus stratiformis undulatus and cumulus congestus pileus.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-001-BTHE RANKS BELOW THE GENUS, AS THE INTERNATIONAL CLOUD ATLAS LISTS THEM, AND THREE NAMES READ ACROSS THE RANKSFIG. B HOW A CLOUD IS NAMEDGENUS10, ONE TO A CLOUDCirrus CiCirrocumulus CcCirrostratus CsAltocumulus AcAltostratus AsNimbostratus NsStratocumulus ScStratus StCumulus CuCumulonimbus CbSPECIES15, AT MOST ONEfibratusuncinusspissatuscastellanusfloccusstratiformisnebulosuslenticularisfractushumilismediocriscongestuscalvuscapillatusvolutusVARIETY9, ANY THAT APPLYintortusvertebratusundulatusradiatuslacunosusduplicatustranslucidusperlucidusopacusSUPPLEMENTARY FEATURE11, ANY THAT APPLYincusmammavirgapraecipitatioarcustubaasperitasfluctuscavummuruscaudaACCESSORY CLOUD4, ANY THAT APPLYpileusvelumpannusflumenTHREE NAMES, READ ACROSS THE RANKSCumulonimbuscapillatus-incus-A thunderstorm whose top has turned to fibrous ice and spread into an anvilAltocumulusstratiformisundulatus--A wide sheet of cloudlets in wavesCumuluscongestus--pileusA towering cumulus wearing a cap of cloud over its top123445TITLECloud classification, how a cloud is namedVOL. III THE SKY · THE GENERATYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-001-BDRAWN AS linework on paperSOURCES WMO, AMS, Office of the Federal Coordinator for Meteorology
Fig. B, the drawing How a cloud is named. The ranks below the genus, as the International Cloud Atlas lists them, and three names read across the ranks NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. Genus, one to a cloud
  2. Species, at most one
  3. Varieties, any that apply
  4. Supplementary features and accessory clouds, any that apply
  5. A name read across the ranks

Ingredients

  • Air cooled to its dew point, mostly by rising and expanding, so its water vapour condenses on particles
  • The stability of the air, which decides the shape: a broad slow lift in stable air makes a layer, parcels rising on their own in unstable air make heaps
  • The temperature at the cloud's height, which decides whether it is made of water droplets, ice crystals or both

Scales

time
minutes for a small cumulus; hours to days for a sheet of altostratus or cirrostratus
horizontal
under 0.6 mi (1 km) for a cumulus; hundreds of kilometres for a stratiform sheet
vertical
from the ground to about 43,000 ft (13 km) in the middle latitudes, and higher at the tops of the tallest cumulonimbus
orlanski
meso and synoptic

Signatures

surface
the station's sky condition; each layer's cover and base height; CB or TCU added when a cumulonimbus or towering cumulus is seen
satellite
cold bright tops for high clouds and cumulonimbus in the infrared; low stratus dark in the infrared and bright in the visible
radar
precipitation falling from nimbostratus and cumulonimbus; most other genera show no echo

The numbers

QuantityValue, and the kind of number it is
Genera10: cirrus, cirrocumulus, cirrostratus, altocumulus, altostratus, nimbostratus, stratocumulus, stratus, cumulus and cumulonimbusStandard, International Cloud Atlas
Low étageFrom the ground to 6,600 ft (2 km)Standard, International Cloud Atlas
Middle étage6,600 to 23,000 ft (2 to 7 km) in the middle latitudesStandard, International Cloud Atlas
High étage16,400 to 43,000 ft (5 to 13 km) in the middle latitudesStandard, International Cloud Atlas
Species15, by shape and inner structure, such as castellanus, lenticularis and congestusStandard, International Cloud Atlas
Varieties9, by arrangement and transparency, such as undulatus and translucidusStandard, International Cloud Atlas
Supplementary features and accessory clouds11 features, such as incus, mamma, virga and arcus; 4 accessory clouds: pileus, velum, pannus and flumenStandard, International Cloud Atlas
The four rootsCirrus, a curl of hair; cumulus, a heap; stratus, a layer; nimbus, a rain cloud. Proposed by Luke Howard in 1803Textbook, International Cloud Atlas
Sky cover reportedIn eighths of the sky: FEW 1 to 2, SCT 3 to 4, BKN 5 to 7, OVC 8Standard, Federal Meteorological Handbook No. 1
Airport ceilometerCloud bases to 12,000 ft (3.7 km) overheadStandard, Automated Surface Observing System (ASOS) User's Guide

How the station sees it

An airport station measures cloud bases directly overhead with a laser ceilometer, up to 12,000 ft, and reports each layer's cover in eighths of the sky: FEW, SCT, BKN or OVC. It does not name the genus. An observer adds CB or TCU when a cumulonimbus or a towering cumulus is in sight. The lowest layer reported as broken or overcast is the ceiling.

How it is warned

Clouds are not warned. One genus matters most: a cloud that makes thunder is by definition a cumulonimbus, so every severe thunderstorm warning, and nearly every tornado warning, is a warning about one, Thunderstorm FB-EVT-001.

See also

Sources

  1. World Meteorological Organization. International Cloud Atlas.
  2. American Meteorological Society. Glossary of Meteorology.
  3. Office of the Federal Coordinator for Meteorology. Federal Meteorological Handbook No. 1, Surface Weather Observations and Reports.
  4. National Weather Service. Automated Surface Observing System (ASOS) User's Guide.

Definition after the International Cloud Atlas. Plate FB-SKY-001, revision 1, 2026-09-24. The number is permanent; cite it.