WMO_Cloud_Classification

WORLD METEOROLOGICAL ORGANIZATION CLOUD CLASSIFICATION

The classification of clouds is structured based on:

• Families

• Genera

• Species

• Varieties

Families (Etages)

• High Etage: 3 - 8 km (polar) to 6 - 18 km (tropical)

• Middle Etage: 2 - 4 km (polar) to 2 - 8 km (tropical)

• Low Etage: Surface - 2 km

• Clouds of Vertical Growth: Bases in low etage, tops in middle/high etages

Genera

Classification is also based on ten mutually exclusive groupings called genera:

• Cirrus (from L. a lock of hair)

  • Characteristics: High, detached, transparent clouds; white delicate filaments or patches; may produce halo phenomena.

  • Composition: Homogeneous, ice crystals.

  • Species: Cirrus fibratus, Cirrus uncinus, Cirrus spissatus, Cirrus castellanus, Cirrus floccus.

  • Varieties: Cirrus intortus, Cirrus radiatus, Cirrus vertebratus, Cirrus duplicatus.

  • Exhibits: Fallstreaks (virga), basal pouches (mamma), cloud top waves (fluctus).

• Cirrocumulus (from L. "cirrus" and "cumulus")

  • Characteristics: High, thin, transparent clouds; patches or layers with no shading.

  • Composition: Homogeneous, ice crystals.

  • Species: Cirrocumulus stratiformis, Cirrocumulus lenticularis, Cirrocumulus castellanus, Cirrocumulus floccus.

  • Varieties: Cirrocumulus undulatus, Cirrocumulus lacunosus.

  • Exhibits: Fallstreaks (virga) and basal pouches (mamma).

• Cirrostratus (from L. "cirrus" and "stratus")

  • Characteristics: High transparent veil; produces halo phenomena.

  • Composition: Homogeneous, ice crystals.

  • Species: Cirrostratus fibratus, Cirrostratus nebulosus.

  • Varieties: Cirrostratus undulatus, Cirrostratus duplicatus.

• Altocumulus (from L. "altum" and "cumulus")

  • Characteristics: White or grey patch/layer; may consist of rolls or cloudlets.

  • Composition: Homogeneous, primarily water droplets.

  • Species: Altocumulus stratiformis, Altocumulus lenticularis, Altocumulus castellanus, Altocumulus floccus, Altocumulus volutus.

  • Varieties: Altocumulus translucidus, Altocumulus perlucidus, Altocumulus opacus, Altocumulus duplicatus, Altocumulus undulatis, Altocumulus radiatus, Altocumulus lacunosus.

  • Exhibits: Virga, mamma, fluctus, tortuous skies (asperitas).

• Altostratus (from L. "altum" and "stratus")

  • Characteristics: Greyish or bluish homogeneous layer; may partially cover the sky.

  • Composition: Heterogeneous, contains water droplets, ice crystals, may include raindrops and snowflakes.

  • Species: No subdivision into species available.

  • Varieties: Altostratus translucidus, Altostratus opacus, Altostratus duplicatus, Altostratus undulatus, Altostratus radiatus.

  • Exhibits: Virga, mamma, rainfall (praecipitatio), pannus scudding clouds.

• Nimbostratus (from L. "nimbus" meaning rainy and "stratus")

  • Characteristics: Extensive low dark grey cloud layer with a diffuse base.

  • Composition: Heterogeneous, contains water droplets, ice crystals, raindrops, and/oder snowflakes.

  • Species: No subdivisions.

  • Varieties: No varieties present.

  • Exhibits: Virga, pannus, praecipitatio.

• Stratocumulus (from L. "stratus" and "cumulus")

  • Characteristics: Grey/white patch or sheet with dark parts; orderly groupings of cloudlets.

  • Composition: Homogeneous with water droplets; sometimes contains ice crystals.

  • Species: Stratocumulus stratiformis, Stratocumulus lenticularis, Stratocumulus castellanus, Stratocumulus volutus.

  • Varieties: Stratocumulus translucidus, Stratocumulus perlucidus, Stratocumulus opacus, Stratocumulus duplicatus, Stratocumulus undulatus, Stratocumulus radiatus, Stratocumulus lacunosus.

  • Exhibits: Mamma, virga, rarely praecipitatio.

• Stratus (from L. "sternere" meaning to cover or spread out)

  • Characteristics: Grey cloud layer with a distinct base; may contain raindrops.

  • Composition: Homogeneous with small water droplets; may contain ice crystals at low temperatures.

  • Species: Stratus nebulosus, Stratus fractus.

  • Varieties: Stratus translucidus, Stratus opacus, Stratus undulatus.

  • Exhibits: Praecipitatio (light drizzle), virga, fluctus.

Cumulus

• Cumulus (from L. for accumulation, heap)

  • Characteristics: Detached clouds, generally dense, vertically developing mounds, domes, or towers.

  • Composition: Water droplets near the base; ice crystals in upper parts.

  • Species: Cumulus humilis, Cumulus mediocris, Cumulus congestus, Cumulus fractus.

  • Varieties: Cumulus radiatus.

  • Exhibits: Pileus, vellum, virga, rarely praecipitatio, arcus, pannus, and tuba.

Cumulonimbus

• Cumulonimbus (from L. "cumulus" and "nimbus")

  • Characteristics: Dense cloud with considerable vertical extent; often has a flattened top.

  • Composition: Dark base with water droplets; supercooled droplets; cold top consists of ice crystals.

  • Species: Cumulonimbus calvus, Cumulonimbus capillatus.

  • Varieties: No varieties available.

  • Exhibits: Virga, mamma, praecipitatio, fluctus, pileus cap, velum sail, incus anvil, flumen, arcus or roll clouds, occasionally cavan holes, muras wall clouds, cauda tails, and/or tuba funnel clouds.

Species

Observations of peculiar shapes and internal structures allow genera to be subdivided into species:

• Fibratus: thin filaments without hooks (Cirrus, Cirrocumulus).

• Uncinus: comma-shaped, with hooks (Cirrus).

• Spissatus: thick, greyish Cirrus clouds.

• Castellanus: clouds with turret-like protuberances (Cirrus, Cirrocumulus, Altocumulus, Stratocumulus).

• Floccus: tuft-like formations (Cirrus, Cirrocumulus, Altocumulus).

• Stratiformis: spread out extensively (Altocumulus, Stratocumulus).

• Nebulosus: no distinct features (Cirrostratus, Stratus).

• Lenticularis: almond-shaped distinct outlines (Cirrocumulus, Altocumulus, Stratocumulus).

• Fractus: ragged irregular shapes (Stratus, Cumulus).

• Volutus: rolled or tumbled shapes (Stratocumulus, Altocumulus).

• Humilis: low, slight vertical extent (Cumulus).

• Mediocris: medium vertical extent (Cumulus).

• Congestus: markedly sprouting clouds with significant verticality (Cumulus).

• Calvus: early striated Cumulonimbus.

• Capillatus: distinct cirriform structure (Cumulonimbus).

Varieties

Varieties depend on the arrangement of cloud elements and transparency factors:

• Intortus: irregularly twisted filaments (Cirrus).

• Vertebratus: arranged like vertebrae (Cirrus).

• Duplicatus: superposed layers (Cirrus, Cirrostratus, Altocumulus, Altostratus, Stratocumulus).

• Undulatus: wavy undulations (Cirrocumulus, Cirrostratus, Altocumulus, Altostratus, Stratocumulus, Stratus).

• Radiatus: parallel bands converging to radiation points (Cirrus, Altocumulus, Altostratus, Stratocumulus, Stratus).

• Lacunosus: patches suggesting a net structure (Cirrocumulus, Altocumulus, rarely Stratocumulus).

• Translucidus: patches revealing sun/moon positions (Altocumulus, Altostratus, Stratocumulus, Stratus).

• Perlucidus: allowing light through (Altocumulus, Stratocumulus).

• Opacus: completely masking sunlight (Altocumulus, Altostratus, Stratocumulus, Stratus).

Supplementary Features

• Incus: anvil formation on Cumulonimbus tops.

• Mamma: hanging protuberances underneath various clouds.

• Virga: trails of precipitation not reaching the ground.

• Praecipitatio: precipitation reaching the surface.

• Arcus: dense dark roll on Cumulonimbus front.

• Tuba: inverted cone from Cumulonimbus base.

• Asperitas: wavelike base structures under unstable conditions.

• Cavum: circular fallstreak hole in specific cloud types.

• Murus: persistent lowering of cloud base in supercells.

• Cauda: tail-shaped clouds attached to wall clouds.

• Fluctas: wave structures on cloud tops.