Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-031

Varieties atlas

A variety is the third rank of a cloud name. It says how the parts of a cloud are arranged and how much light passes through it. The International Cloud Atlas lists nine; a cloud may carry more than one, except that translucidus and opacus exclude each other.

Plate FB-SKY-031The ranksRevision 1, 2026-09-25Status draftAlso called cloud varieties, variety, undulatus, radiatus, translucidus, opacus, duplicatus

Small paintings of the 9 varieties 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 intortus, in cirrus; cirrus vertebratus, in cirrus; altocumulus undulatus, in cirrocumulus, cirrostratus, altocumulus, altostratus, stratocumulus and stratus; cumulus radiatus, in cirrus, altocumulus, altostratus, stratocumulus and cumulus; altocumulus lacunosus, in cirrocumulus, altocumulus and stratocumulus; altocumulus duplicatus, in cirrus, cirrostratus, altocumulus, altostratus and stratocumulus; altostratus translucidus, in altocumulus, altostratus, stratocumulus and stratus; stratocumulus perlucidus, in altocumulus and stratocumulus; stratocumulus opacus, in altocumulus, altostratus, stratocumulus and stratus.
Fig. A The nine varieties. Each variety painted small, with the genera that may carry it.MaximizeThe drawing, to scale

What it is

A cloud's variety is the third word of its name, after the genus and species. It says how the parts of the cloud are arranged, in waves, in bands, in a net of holes or in two layers, and how much light passes through it. The International Cloud Atlas lists nine, drawn in Fig. A with the genera that may carry each.

A cloud may carry more than one variety, with one exception: translucidus, thin enough to show where the sun is, and opacus, thick enough to hide it, cannot both apply. Intortus and vertebratus belong to cirrus alone. Undulatus is the most widely shared, found in six genera. Nimbostratus and cumulonimbus have no varieties.

What it is not

It is not the species, which says what shape the cloud is and allows only one, Species atlas FB-SKY-030. Nor is it a supplementary feature, a part attached to the cloud such as virga or mamma, Supplementary features atlas FB-SKY-032. Straight bands along flight paths are contrails, Special clouds FB-SKY-034, not radiatus.

Lookalikes

Species
The shape of the cloud; one to a cloud. A variety adds arrangement and transparency
Supplementary features
Parts attached to the cloud, such as virga or mamma; see FB-SKY-032
Contrails
Straight bands along flight paths; radiatus bands are cloud drawn out by the wind

The machine

The varieties are mostly the work of the wind across a layer. Where the wind changes speed or direction with height, it draws a layer into regular waves, undulatus, or long parallel bands, radiatus, which seem to meet at the horizon only because they run away from the observer. Where air sinks or mixes through a thin sheet, it opens round holes, lacunosus. Two layers a short height apart, each formed at its own level, make duplicatus. Translucidus, perlucidus and opacus describe the light: whether the sun shows through, whether sky shows between the parts, and whether the layer hides the sun.

Fig. B puts eight of the nine on one sky.

A painted sky, not to scale, with eight clouds numbered and named: high up, pale bands of cirrus radiatus spreading from low in the sky and tangled filaments of cirrus intortus; in the middle, rows of altocumulus undulatus in waves, altocumulus lacunosus as a net of cloudlets around holes of blue sky, altocumulus duplicatus in two rows at two heights, and a grey sheet of altostratus translucidus with the sun showing through it; low, stratocumulus perlucidus with sky between its rolls and a thick dark stratocumulus opacus.
Fig. B Eight varieties on one sky. As an observer might see them; a cloud may carry more than one.MaximizeThe drawing, to scale

The drawings

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

The 9 varieties 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: intortus in cirrus; vertebratus in cirrus; undulatus in cirrocumulus, cirrostratus, altocumulus, altostratus, stratocumulus and stratus; radiatus in cirrus, altocumulus, altostratus, stratocumulus and cumulus; lacunosus in cirrocumulus, altocumulus and stratocumulus; duplicatus in cirrus, cirrostratus, altocumulus, altostratus and stratocumulus; translucidus in altocumulus, altostratus, stratocumulus and stratus; perlucidus in altocumulus and stratocumulus; opacus in altocumulus, altostratus, stratocumulus and stratus.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-031-AEVERY VARIETY THE INTERNATIONAL CLOUD ATLAS LISTS, AND THE GENERA THAT MAY CARRY IT; SKETCHES NOT TO SCALEFIG. A THE NINE VARIETIESintortusFilaments curved and tangled irregularlyIn CivertebratusFilaments set like the ribs and spine of afishIn CiundulatusWaves in the sheet or its elementsIn Cc, Cs, Ac, As, Sc, StradiatusParallel bands that seem to meet at thehorizonIn Ci, Ac, As, Sc, CulacunosusA net of round holes, often with fringededgesIn Cc, Ac, ScduplicatusTwo or more layers at slightly differentheightsIn Ci, Cs, Ac, As, SctranslucidusThin enough to show where the sun or moonisIn Ac, As, Sc, StSKY BETWEENperlucidusGaps between the elements show sky or ahigher cloudIn Ac, ScopacusThick enough to hide the sun or moonIn Ac, As, Sc, StEACH SKETCH SMALL AND NOT TO SCALE; UNDER EACH NAME, THE GENERA THE ATLAS GIVES IT TO, BY THEIR ABBREVIATIONS.TITLEVarieties atlas, the nine varietiesVOL. III THE SKY · THE RANKSTYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-25IDFB-SKY-031-ADRAWN AS linework on paperSOURCES WMO, AMS
Fig. A, the drawing The nine varieties. Every variety the International Cloud Atlas lists, and the genera that may carry it; sketches not to scale NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. intortus
  2. vertebratus
  3. undulatus
  4. radiatus
  5. lacunosus
  6. duplicatus
  7. translucidus
  8. perlucidus
  9. opacus
Elevation of one sky, not to scale: high up, cirrus radiatus in bands that seem to meet toward the horizon and cirrus intortus in tangled filaments; in the middle, altocumulus undulatus in waves, altocumulus lacunosus as a sheet with round holes, altocumulus duplicatus in two layers, and altostratus translucidus with the sun showing through; low, stratocumulus perlucidus with sky between its elements and a dark stratocumulus opacus.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-031-BEIGHT VARIETIES AS AN OBSERVER MIGHT SEE THEM; NOT TO SCALE AND NOT ALL AT ONE HOURFIG. B EIGHT VARIETIES ON ONE SKY12345678SEEN FROM THE GROUND; NOT TO SCALE, AND NOT ALL AT ONE HOUR. A CLOUD MAY CARRY MORE THAN ONE VARIETY.TITLEVarieties atlas, eight varieties on one skyVOL. III THE SKY · THE RANKSTYPEELEVATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-25IDFB-SKY-031-BDRAWN AS linework on paperSOURCES WMO, AMS
Fig. B, the drawing Eight varieties on one sky. Eight varieties as an observer might see them; not to scale and not all at one hour NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. Cirrus radiatus
  2. Cirrus intortus
  3. Altocumulus undulatus
  4. Altocumulus lacunosus
  5. Altocumulus duplicatus
  6. Altostratus translucidus
  7. Stratocumulus perlucidus
  8. Stratocumulus opacus

Ingredients

  • Wind that changes with height across a layer, which draws it into waves and bands
  • Air sinking or mixing through a thin layer, which opens holes in it
  • The depth of the layer, which decides whether the sun shows through

Scales

time
minutes to hours; waves and holes come and go as the layer moves
horizontal
from the spacing of waves, hundreds of feet to a mile or two, to bands crossing the whole sky
vertical
the depth of one layer, or two layers a short height apart for duplicatus
orlanski
micro to meso

Signatures

surface
named by an observer from sight; the station reports the layer's cover and height
satellite
undulatus and radiatus show as regular bands; lacunosus as a net of holes
radar
no echo from the varieties themselves

The numbers

QuantityValue, and the kind of number it is
Varieties9: intortus, vertebratus, undulatus, radiatus, lacunosus, duplicatus, translucidus, perlucidus, opacusStandard, International Cloud Atlas
How many to a cloudAny that apply, except translucidus and opacus togetherStandard, International Cloud Atlas
Shared most widelyundulatus, in six generaStandard, International Cloud Atlas
Belonging to one genusintortus and vertebratus (cirrus)Standard, International Cloud Atlas
Genera with no varietiesnimbostratus and cumulonimbusStandard, International Cloud Atlas
Where the name goesAfter the species, if there is one: altocumulus stratiformis undulatusStandard, 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 reports each layer's cover in eighths of the sky and its height, up to 12,000 ft (3.7 km), from its ceilometer. It does not name varieties, though a layer reported as broken or overcast is usually opacus.

How it is warned

Varieties are not warned. Undulatus in a thin layer is a sign of wind shear at that height, which pilots feel as turbulence.

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-031, revision 1, 2026-09-25. The number is permanent; cite it.