Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-030

Species atlas

A species is the second rank of a cloud name. It says what shape the cloud has or how it is built inside. The International Cloud Atlas lists fifteen, and a cloud takes at most one; some species belong to one genus, others are shared by several.

Plate FB-SKY-030The ranksRevision 1, 2026-09-25Status draftAlso called cloud species, species, fibratus, castellanus, lenticularis, congestus, capillatus

Small paintings of the 15 species of the International Cloud Atlas, each painted in one genus and labelled with its full name, with every genus the Atlas gives it to beneath: cirrus fibratus, in cirrus and cirrostratus; cirrus uncinus, in cirrus; cirrus spissatus, in cirrus; altocumulus castellanus, in cirrus, cirrocumulus, altocumulus and stratocumulus; altocumulus floccus, in cirrus, cirrocumulus, altocumulus and stratocumulus; altocumulus stratiformis, in cirrocumulus, altocumulus and stratocumulus; cirrostratus nebulosus, in cirrostratus and stratus; altocumulus lenticularis, in cirrocumulus, altocumulus and stratocumulus; cumulus fractus, in stratus and cumulus; cumulus humilis, in cumulus; cumulus mediocris, in cumulus; cumulus congestus, in cumulus; cumulonimbus calvus, in cumulonimbus; cumulonimbus capillatus, in cumulonimbus; stratocumulus volutus, in altocumulus and stratocumulus.
Fig. A The fifteen species. Each species painted small, with the genera it belongs to.MaximizeThe drawing, to scale

What it is

A cloud's species is the second word of its name, after the genus. It says what shape the cloud has, or how it is built inside: in filaments or hooks, in turrets or tufts, in a sheet, a lens, a roll, ragged shreds, or a heap of a given size. The International Cloud Atlas lists fifteen, drawn in Fig. A with the genera each belongs to, and a cloud takes at most one.

Some species belong to a single genus. Uncinus and spissatus are cirrus alone; humilis, mediocris and congestus are the three sizes of cumulus FB-SKY-018; calvus and capillatus are the two stages of a cumulonimbus FB-SKY-019 top. Others are shared: castellanus and floccus appear in four genera, from cirrus down to stratocumulus. Altostratus and nimbostratus have no species at all.

What it is not

It is not the variety, which says how a cloud's parts are arranged and how much light it lets through, Varieties atlas FB-SKY-031. A cloud has one species but may carry several varieties. Nor is it a supplementary feature, a part attached to the cloud such as an anvil, Supplementary features atlas FB-SKY-032.

Lookalikes

Varieties
Arrangement and transparency, not shape; a cloud can have several at once
Supplementary features
Parts attached to a cloud, like an anvil or mamma; see FB-SKY-032
Genus
The first rank, one of ten; the species narrows it

The machine

Each species records how the air moves in the cloud. Turrets and tufts, castellanus and floccus, mean the air at that height is unstable and overturning, often a sign that showers and storms may follow later in the day. A lens, lenticularis, stands still in a wave of air downwind of mountains. A sheet, stratiformis, is a layer lifted broadly. The heaps of cumulus grow from humilis through mediocris to congestus as the updraft deepens, and a cumulonimbus goes from calvus to capillatus as its top turns to ice.

Fig. B puts nine of the fifteen on one sky over the plains, as an observer might see them over a day.

A painted sky over the plains, not to scale, with nine clouds numbered and named: high up, the hooked heads of cirrus uncinus, the straight filaments of cirrus fibratus and the rows of grains of cirrocumulus stratiformis; in the middle, two smooth lenses of altocumulus lenticularis and the turrets of altocumulus castellanus on a common base; low, two small flat cumulus humilis, a tall cumulus congestus, dark ragged shreds of stratus fractus, and a tall cumulonimbus capillatus with a fibrous streaked top.
Fig. B Nine species on one sky. As an observer on the plains might see them in a day.MaximizeThe drawing, to scale

The drawings

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

The 15 species of the International Cloud Atlas, each with a small drafted sketch, one line on what it is, and the genera the Atlas gives it to: fibratus in cirrus and cirrostratus; uncinus in cirrus; spissatus in cirrus; castellanus in cirrus, cirrocumulus, altocumulus and stratocumulus; floccus in cirrus, cirrocumulus, altocumulus and stratocumulus; stratiformis in cirrocumulus, altocumulus and stratocumulus; nebulosus in cirrostratus and stratus; lenticularis in cirrocumulus, altocumulus and stratocumulus; fractus in stratus and cumulus; humilis in cumulus; mediocris in cumulus; congestus in cumulus; calvus in cumulonimbus; capillatus in cumulonimbus; volutus in altocumulus and stratocumulus.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-030-AEVERY SPECIES THE INTERNATIONAL CLOUD ATLAS LISTS, AND THE GENERA IT BELONGS TO; SKETCHES NOT TO SCALEFIG. A THE FIFTEEN SPECIESfibratusDetached filaments, straight or gentlycurved, with no hooks or tuftsIn Ci, CsuncinusFilaments hooked at the top like a comma,without a rounded tuftIn CispissatusPatches dense enough to look grey towardthe sunIn CicastellanusTurrets rising from a common base, likebattlementsIn Ci, Cc, Ac, ScfloccusSmall tufts with ragged bases, oftentrailing fall streaksIn Ci, Cc, Ac, ScstratiformisSpread in a wide sheet or layerIn Cc, Ac, ScnebulosusA featureless veil or layer with no detailIn Cs, StlenticularisLens or almond shapes with sharp outlines,over and downwind of hillsIn Cc, Ac, ScfractusRagged shreds, their outlines changingfastIn St, CuhumilisSmall and flat, wider than it is tallIn CumediocrisAs tall as it is wide, the top with smallbulgesIn CucongestusTall and growing, its top like acauliflowerIn CucalvusThe top softening and smooth; thecauliflower is going, and no fibres yetIn CbcapillatusThe top fibrous and streaked with ice,often spread as an anvilIn CbvolutusA long horizontal tube that seems to rollabout its axisIn Ac, ScEACH SKETCH SMALL AND NOT TO SCALE; UNDER EACH NAME, THE GENERA THE ATLAS GIVES IT TO, BY THEIR ABBREVIATIONS.TITLESpecies atlas, the fifteen speciesVOL. III THE SKY · THE RANKSTYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-25IDFB-SKY-030-ADRAWN AS linework on paperSOURCES WMO, AMS
Fig. A, the drawing The fifteen species. Every species the International Cloud Atlas lists, and the genera it belongs to; sketches not to scale NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. fibratus
  2. uncinus
  3. spissatus
  4. castellanus
  5. floccus
  6. stratiformis
  7. nebulosus
  8. lenticularis
  9. fractus
  10. humilis
  11. mediocris
  12. congestus
  13. calvus
  14. capillatus
  15. volutus
Elevation of one sky over the plains, not to scale: high up, cirrus uncinus with hooked heads, cirrus fibratus in straight filaments and cirrocumulus stratiformis in rows of grains; in the middle, an altocumulus lenticularis lens over the hills and an altocumulus castellanus with turrets on a common base; low, a small flat cumulus humilis, a tall cumulus congestus, ragged stratus fractus, and a cumulonimbus capillatus with its fibrous anvil and rain beneath.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-030-BNINE SPECIES AS AN OBSERVER ON THE PLAINS MIGHT SEE THEM IN A DAY; NOT TO SCALE AND NOT ALL AT ONE HOURFIG. B NINE SPECIES ON ONE SKY123456789SEEN FROM THE GROUND ACROSS THE PLAINS; NOT TO SCALE, AND NOT ALL AT ONE HOUR.TITLESpecies atlas, nine species on one skyVOL. III THE SKY · THE RANKSTYPEELEVATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-25IDFB-SKY-030-BDRAWN AS linework on paperSOURCES WMO, AMS
Fig. B, the drawing Nine species on one sky. Nine species as an observer on the plains might see them in a day; not to scale and not all at one hour NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. Cirrus uncinus
  2. Cirrus fibratus
  3. Cirrocumulus stratiformis
  4. Altocumulus lenticularis
  5. Altocumulus castellanus
  6. Cumulus humilis
  7. Cumulus congestus
  8. Stratus fractus
  9. Cumulonimbus capillatus

Ingredients

  • The shape of a cloud, which the species names
  • How the air moves inside it: turrets and tufts where it overturns, lenses where it waves, sheets where it is lifted broadly
  • Growth over time, which moves a cloud from one species to the next

Scales

time
a species can change within minutes, as a mediocris grows into a congestus
horizontal
from a single turret to a sheet hundreds of miles across
vertical
from a flat humilis a few hundred feet deep to a capillatus through all three étages
orlanski
micro to synoptic

Signatures

surface
named by an observer from sight; an airport station reports cover and height, not species
satellite
lenticularis stands still downwind of mountains; congestus and capillatus show as growing, then cold, tops
radar
congestus may show a first echo; capillatus usually rains

The numbers

QuantityValue, and the kind of number it is
Species15: fibratus, uncinus, spissatus, castellanus, floccus, stratiformis, nebulosus, lenticularis, fractus, humilis, mediocris, congestus, calvus, capillatus, volutusStandard, International Cloud Atlas
How many to a cloudAt most oneStandard, International Cloud Atlas
Shared most widelycastellanus and floccus, each in four genera: cirrus, cirrocumulus, altocumulus and stratocumulusStandard, International Cloud Atlas
Belonging to one genusuncinus and spissatus (cirrus); humilis, mediocris and congestus (cumulus); calvus and capillatus (cumulonimbus)Standard, International Cloud Atlas
Genera with no speciesaltostratus and nimbostratusStandard, International Cloud Atlas
Where the name goesAfter the genus: altocumulus lenticularisStandard, International Cloud Atlas
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 the height and cover of each layer overhead with a ceilometer, up to 12,000 ft (3.7 km). It does not name the species. An observer adds TCU for towering cumulus, a congestus, and CB for a cumulonimbus.

How it is warned

Species are not warned. Two of them are worth watching: altocumulus castellanus on a summer morning, which says the middle air is unstable, and cumulus congestus, the step before a thunderstorm.

See also

Sources

  1. World Meteorological Organization. International Cloud Atlas.
  2. American Meteorological Society. Glossary of Meteorology.

Definition after the International Cloud Atlas. Plate FB-SKY-030, revision 1, 2026-09-25. The number is permanent; cite it.