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  1. What are clouds?
  2. Who are the pioneers in classifying cloud types?
  3. What are the characteristics of clouds?
  4. Classification of Clouds by characteristic forms
  5. Clouds categorization according to their altitude.

    Written by: CHIU Hung-yu    June 2010


  1. What are clouds?

    Clouds consist of an aggregation of minute particles of water droplets or ice crystals suspended in the air and usually not touching the ground. The mass of small water droplets or ice crystals suspended in the atmosphere is visible during daytime, or under moonlight or city light at night. Clouds may also include larger particles of liquid water and ice, as well as particles of fumes, smoke or dust.

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  2. Who are the pioneers in classifying clouds?

    The work of the first published classification of clouds was done by the French naturalist Jean Lamarck in 1802. Lamarck defined and named a certain number of interesting cloud forms. In 1803, the English scientist Luke Howard published a more completed cloud classification covering nearly all possible cases. Subsequently, a number of meteorologists enriched the content of cloud classification in further details by adding new species or varieties.

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  3. What are the characteristics of clouds?

    The skies are changing eternally. Clouds are continuously evolving and can appear in numerous or infinite variety of forms including cumuliform, stratiform and cirriform. Clouds can be identified and categorized by their dimension, appearance, shape, structure and texture, luminance and transparency, colour, precipitation, cloud base height, altitude and vertical extent. Different types of clouds are related to weather. The World Meteorological Organization (WMO) has drawn up schemes of cloud classification for distinguishing and grouping clouds according to their appearance, altitudes, and, where possible, to their process of formation.

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  4. Classification of Clouds by characteristic forms

    The characteristic forms of clouds are classified in terms of genera (plural of genus). There are 10 main groups of genera or principal cloud types, which are mutually exclusive (see Table 1).

    Table 1.  The 10 genera 
    Cirrus
    Cirrocumulus
    Cirrostratus
    Altocumulus
    Altostratus
    Nimbostratus
    Stratocumulus
    Stratus
    Cumulus
    Cumulo
    -nimbus


    1
    Fig.1   A chart showing the 10 cloud genera or principal cloud types and the 3 possible etages. (not to scale)

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  5. Clouds categorization according to their altitude.

    By convention, the part of the atmosphere in which clouds are usually present is vertically divided into 3 etages which are high, middle and low. So, clouds are further categorized according to their height above the ground (etages) into high, middle and low level clouds. The vertical limits of etages vary with latitude, which increases from polar regions to tropical regions. The approximate cloud base altitude and vertical limits of etages are listed in the following table (see Table 2).

    Table 2.  The categorization of 3 Etages
    Etages Genera Polar regions Temperate regions Tropical regions
    High Cirrus,
    Cirrocumulus
    and Cirrostratus
    3-8 km 5-13 km 6-18 km
    Middle Altocumulus,
    (Altostratus and Nimbostratus)*
    2-4 km 2-7 km 2-8 km
    Low Stratocumulus,
    Stratus,
    (Cumulus and Cumulonimbus)*
    From
    Earth's
    surface
    to 2 km
    From
    Earth's
    surface
    to 2 km 
    From
    Earth's
    surface
    to 2 km

    *  If the height of some clouds in one etage extends into another etage, the etages could overlap.  Examples are the Altostratus, Nimbostratus, Cumulus and Cumulonimbus. In particular, the cloud bases of Cumulus and Cumulonimbus are usually in the low etage but often extend upward into the middle and high etages.


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   References: 
  1. "Manual on the Observation of Clouds and other Meteors", WMO No. 407, World Meteorological Organization, 1975.
  2. "WMO International Cloud Atlas", Volume I and II, World Meteorological Organization.
  3. "WMO Guide to Meteorological Instruments and Methods of Observation", WMO-No. 8 (7th edition), World Meteorological Organization, 2008.
  4. "Luke Howard, the Namer of Clouds", BBC, UK..

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Last revision date: <21 Dec 2012>