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6.4: Cloud Classification

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    9564
  • Clouds are classified by their shapes (i.e., their morphology). The classification method introduced in 1803 by Luke Howard is still used today, as approved by the World Meteorology Organization (WMO). 

    WMO publishes an International Cloud Atlas with photos to help you identify clouds. As mentioned, you can easily find similar photos for free by pointing your web search engine at “cloud classification”, “cloud identification”, “cloud types”, or “International Cloud Atlas”.

    The categories and subcategories of clouds are broken into:

    • genera - main characteristics of clouds
    • species - peculiarities in shape & structure
    • varieties - arrangement and transparency
    • supplementary features and accessory clouds - attached to other (mother) clouds
    • mother clouds - clouds with attachments
    • meteors - precipitation (ice, water, or mixed) Most important are the genera.

    6.4.1. Genera

    The ten cloud genera are listed in Table 6-2, along with their official abbreviations and symbols as drawn on weather maps. These genera are mutually exclusive; namely, one cloud cannot have two genera. However, often the sky can have different genera of clouds adjacent to, or stacked above, each other.

    Table 6-2. Cloud genera      
    Genus Abbreviation WMO Symbol USA Symbol
    cirrus Ci Screen Shot 2020-02-16 at 1.41.29 AM.png Screen Shot 2020-02-16 at 1.41.40 AM.png
    cirrostratus Cs Screen Shot 2020-02-16 at 1.43.57 AM.png Screen Shot 2020-02-16 at 1.44.05 AM.png
    cirrocumulus Cc Screen Shot 2020-02-16 at 1.45.08 AM.png Screen Shot 2020-02-16 at 1.45.23 AM.png
    altostratus As Screen Shot 2020-02-16 at 1.46.29 AM.png Screen Shot 2020-02-16 at 1.46.39 AM.png
    altocumulus Ac Screen Shot 2020-02-16 at 1.47.27 AM.png Screen Shot 2020-02-16 at 1.47.44 AM.png
    nimbostratus Ns Screen Shot 2020-02-16 at 1.48.47 AM.png Screen Shot 2020-02-16 at 1.48.58 AM.png
    stratus St Screen Shot 2020-02-16 at 1.49.42 AM.png Screen Shot 2020-02-16 at 1.49.51 AM.png
    stratocumulus Sc Screen Shot 2020-02-16 at 1.50.44 AM.png Screen Shot 2020-02-16 at 1.50.52 AM.png
    cumulus Cu Screen Shot 2020-02-16 at 1.51.51 AM.png Screen Shot 2020-02-16 at 1.52.02 AM.png
    cumulonimbus Cb Screen Shot 2020-02-16 at 1.52.53 AM.png Screen Shot 2020-02-16 at 1.53.09 AM.png

    6.4.2. Species

    Subdividing the genera are cloud species. Table 6-3 lists the official WMO species. Species can account for:

    • forms (clouds in banks, veils, sheets, layers, etc.)
    • dimensions (horizontal or vertical extent)
    • internal structure (ice crystals, water droplets, etc.)
    • likely formation process (orographic lift, etc.)

    Species are also mutually exclusive. An example of a cloud genus with specie is “Altocumulus castellanus (Ac cas)”, which is a layer of mid-level lumpy clouds that look like castle turrets. 

    Table 6-3. WMO cloud species. (Ab. = abbreviation)
    Specie Ab Description
    calvus cal the top of a deep Cb that is starting to become fuzzy, but no cirrus (ice) anvil yet
    capillatus cap Cb having well defined streaky cirrus (ice) anvil
    castellanus cas small turrets looking like a crenellated castle. Turret height > diameter. Can apply to Ci, Cc, Ac, Sc. 
    congestus con very deep Cu filling most of troposphere, but still having crisp cauliflower top (i.e., no ice anvil). Often called Towering Cu (TCU).
    fibratus fib nearly straight filaments of Ci or Cs.
    floccus flo small tufts (lumps) of clouds, often with virga (evaporating precip.) falling from each tuft. Applies to Ci, Cc, Ac, Sc.
    fractus fra shredded, ragged, irregular, torn by winds. Can apply to Cu and St. 
    humilis hum Cu of small vertical extent. Small flat lumps.
    lenticularis len having lens or almond cross section. Called mountain wave clouds. Applies to Cc, Ac, Sc.
    mediocris med medium size Cu
    nebulosus neb Cs or St with veil or layer showing no distinct details
    spissatus spi thick Ci that looks grey.
    stratiformis str spreading out into sheets or horizontal layers. Applies to Cc, Ac, Sc.
    uncinus unc hook or comma shaped Ci
    volutus vol low horizontal tube-shaped roll cloud caused by a soliton wave or undular bore. For Ac, Sc.

    6.4.3. Varieties

    Genera and species are further subdivided into varieties (Table 6-4), based on:

    • transparency (sun or moon visible through cloud)
    • arrangement of visible elements

    These varieties are NOT mutually exclusive (except for translucidus and opacus), so you can append as many varieties to a cloud identification that apply. For example, “cumulonimbus capillatus translucidus undulatus” (Cb cap tr un). 

    Table 6-4. WMO cloud varieties. Ab. = Abbreviation.
    Variety Ab. Description
    duplicatus du superimposed cloud patches at slightly different levels. Applies to Ci, Cs, Ac, As, Sc.
    intortus in Ci with tangled, woven, or irregularly curved filaments
    lacunosus la honeycomb, chessboard, or regular arrangement of clouds and holes. Applies to Cc, Ac, and Sc.
    opacus op too thick for the sun or moon to shine through the cloud layer or patch. Applies to Ac, As, Sc, St.
    perlucidus pe a layer of clouds with small holes between elements. Applies to Ac, Sc.
    radiatus ra very long parallel bands of clouds that, due to perspective, appear to converge at a point on the horizon. Applies to Ci, Ac, As, Sc, and Cu. 
    translucidus tr layer or patch of clouds through which sun or moon is somewhat visible. Applies to Ac, As, Sc, St.
    undulatus un cloud layers or patches showing waves or undulations. Applies to Cc, Cs, Ac, As, Sc, St.
    vertebratus ve Ci streaks arranged like a skeleton with vertebrae and ribs, or fish bones. 

    6.4.4. Supplementary Features

    Table 6-5. WMO cloud supplementary features, and the mother clouds to which they are attached. 
    Feature Ab. Description
    arcus arc a dense horizontal roll cloud with tattered bottom edges, close to the ground, along and above the leading edge of Cb outflow gust-fronts. Usually attached to the Cb, but can separate from it as the gust front spreads. Often has an arch shape when viewed from above or below.
    asperitas asp well defined chaotic waves on the underside of Ac or Sc.
    cauda cau tail cloud at low altitude from a precipitation region toward (& attached to) a Cb wall cloud.
    cavum cav a large circular or linear hole in a Cc or Ac layer, caused by ice-nuclei seeding of supercooled cloud drops
    fluctus flu Kelvin-Helmholtz curls or breaking wave clouds with Ci, Ac, Sc, St, Cu.
    flumen flm inflow cloud bands at height of Cb base. Can look like a beaver tail.
    incus inc the upper portion of a Cb, spread out into an (ice) anvil with smooth, fibrous or striated appearance.
    mamma mam hanging protuberances like udders on the underside of Ci, Cc, Ac, As, Sc, and Cb anvil clouds.
    murus mur wall cloud. An isolated lowering of a Cb cloud base.
    praecipitatio pra precipitation falling from a cloud and reaching the ground. Mother clouds can be As, Ns, Sc, St, Cu, Cb.
    tuba tub a cloud column or tornado funnel cloud protruding from a cloud base, and indicating an intense vortex. Usually attached to Cb, but sometimes to Cu.
    virga vir visible precipitation trails that evaporate before reaching the ground. They can hang from Cc, Ac, As, Cu, Ns, Sc, Cu, or Cb clouds.

    6.4.5. Accessory Clouds

    Smaller features attached to other clouds.

    Table 6-6. WMO accessory clouds, and the mother clouds to which they are attached. (Ab.=abbreviation.)
    Name Ab. Description
    pannus pan ragged shreds, sometimes in a continuous layer, below a mother cloud and sometimes attached to it. Mother clouds can be As, Ns, Cu, Cb.
    pileus pil a smooth thin cap, hood, or scarf above, or attached to, the top of rapidly rising cumulus towers. Very transient, because the mother cloud quickly rises through and engulfs it. Mother clouds are Cu con or Cb.
    velum vel a thin cloud veil or sheet of large horizontal extent above or attached to Cu or Cb, which often penetrate it.