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Clouds are described according to whether they are heaped, fibrous, layered or rain bearing and are further classified depending on the altitude of the base of the cloud.
The four groups of clouds are:
- Cirriform (fibrous)
- Cumuliform (heaped)
- Stratiform (layered)
- Nimbus (rain-bearing)
Clouds are further divided depending on the base of the cloud above sea level, giving us ten basic cloud types:
High level cloud
High level cloud refers to cloud with a base above 20,000 feet.
- Cirrus (Ci) - thin fibrous cloud
- Cirrocumulus (Cc) - thin granular layer of small lumps of cloud
- Cirrostratus (Cs) - thin uniform cover of cloud
Mid level cloud
Mid level cloud is cloud with a base above about 6,500 feet. (alto means medium)
- Altocumulus (Ac) - thin layer of lumps or heaps of cloud
- Altostratus (As) - thin uniform layer of cloud
Low level cloud
Low level cloud, described as having a base below about 6,500 feet, is of most interest to pilots and controllers because of the impact on the safe conduct of flight particularly with regard to poor visibility, turbulence and structural damage, icing, and runway contamination.
- Nimbostratus (Ns) - thick layer of rain bearing cloud.
- Stratocumulus (Sc) - layer of relatively small lumps and heaps (cumuliform) of cloud
- Stratus (St) - layer of uniform cloud
- Cumulus (Cu) - cumuliform clouds
- Cumulonimbus (Cb) - Thunderstorms
Other Cloud Types
Other cloud types of especial significance to aviation are:
- Stratus Fractus and Cumulus Fractus - fragments of stratus or cumulus below the base of Ns or As.
- Altocumulus Castellanus - a mass of small cumuliform clouds sharing the same base indicating growth of mid level cloud in an unstable atmosphere.
- Lenticularis - lens shaped clouds formed in standing waves over mountains often associated with severe turbulence
- Noctilucent Clouds - thin clouds of ice crystals which occur at great height in the Mesosphere.