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Classification of Clouds

  • Cumulus clouds - form in rising air currents and which relate to unstable air.
  • Stratus clouds - form in horizontal layers as moist air is cooled below its saturation point.
  • Nimbus clouds - clouds which precipitation falls from.

High Clouds

Cloud bases from 16 500' to 45 000' mostly composed of ice crystals.

  • Cirrus (Ci) - very high, with thin wavy sprays of white cloud.
  • Cirrocumulus (Cc) - Thin, cotton or flake like.
  • Cirrostratus (Cs) - Very thin, high clouds that the sun or moon are visable through sometimes though of as a halo effect. Usually associated with an approaching warm front or occlusion meaning bad weather coming.

Middle Clouds

Cloud bases from 6 500' to 23 000' mostly composed of ice crystals or water droplets.

  • Altocumulus (Ac) - Layer of patches of rounded masses of cloud.
  • Altocumulus Castellanus (Acc) - Indicates instability, turbulence and shower activity. Can develop into cumulonimbus.
  • Altostratus (As) - Thick veil of grey cloud covering the whole sky indicates the approach of a warm front and some light rain or snow. Icing is also a danger in this cloud.

Low Clouds

Cloud bases from surface to 6 500' and are composed of water droplets which may be supercooled.

  • Stratus (St) - Cloud resembling fog but not resting on the ground. Drizzle is often present.
  • Stratocumulus (Sc) - layer of patches of rounded masses. Found in high pressure areas in the winter with some precipitation.
  • Nimbostratus (Ns) - Low layer of uniform dark grey cloud which can be more than 15 000' thick. Precipitation falls in the form of continuous rain or snow.

Clouds of Vertical Development

Cloud bases from as low as 1 500' composed of water droplets or ice crystals.

  • Cumulus (Cu) - Thick rounded and lumpy with flat bases and rounded tops. Flight under these clouds is usually full of turbulence.
  • Towering Cumulus (TCu) - Cumulus clouds formed into towering masses. Turbulence will be found under this cloud as well as heavy icing inside.
  • Cumulonimbus (Cb) - Heavy masses of cumulus clouds with an anvil shaped summit meaning thunderstorms. Stay out of these clouds as violent vertical currents and hail exist inside. A line of cumulonimbus means cold front.