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Clouds are formed by adequate water vapor and a method for air to cool. As air-cools, it reaches its saturation point. This process is in the form of sublimation or condensation and attaches to a particle in the air such as dust, salt, or smoke in the air.

Cloud bases in aviation are called ceilings.

Types of Clouds

Clouds are determined and classified according to the shape, characteristics, and height of the cloud base.

  • Low Clouds

    • Low to the Earth’s surface up to 6,500 feet AGL

    • Fog is classified as a Low Clouds

    • Made of water droplets and super-cooled droplets (icing), VFR flight not recommended

  • Middle Clouds

    • 6,500 feet up to 20,000 feet AGL

    • Made of water droplets, super-cooled

    • droplets, and ice crystals

    • May contain moderate icing

    • Can be turbulence on cross country flights

  • High Clouds

    • Wispy looking, which are 20,000 feet AGL and higher.

    • Formed in stable air

    • Made of ice crystals

    • No threat of icing or turbulence

Extensive Vertical Development Clouds

Towering Cumulus or Cumulonimbus Clouds indicate instability in atmosphere and contain turbulence.

Cumulonimbus clouds contain a lot of moisture (nimbus meaning rain) and unstable air and produce lightning, hail, tornadoes, gusty winds, wind shear. These clouds can form in a continues line, are nonfrontal bands of thunderstorms or a squall line.

Other Types of Clouds


Ceilings are used in aviation to give the lowest cloud layer reported, in either broken or overcast. Ceiling information can also be found on a METAR.

  • Broken means the sky is 5/8ths to 7/8ths covered with clouds.

  • Overcast means the sky is completely covered with clouds.

  • All other cloud covers such as few or scattered are not ceilings.


Is the horizontal distance an object can be seen by the naked eye. A METAR also gives current visibility, along with other weather reports.


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