Meteorologists classify clouds by their structure. The first classification of clouds is due to Luke Howard, who in 1803, introduced the Latin names Cirrus, Cumulus, Nimabus and Stratus for different clouds. The International Cloud Atlas published by the World Meteorological Organisation recognises ten main types. They are:
High Clouds Cloud base at 6 km (20,000 feet) or higher
- Cirrus : Detached fibrous clouds in the form of white feathers or narrow bands.
- Cirrocumulus : Thin white layers of high clouds, without shading.
- Cirrostratus : Transparent white clouds through which halos are often seen.
Medium Clouds Cloud base at or above 2 km (7,000 feet)
- Altocumulus : White or grey layer of cloud; sometimes seen in the form of rolls or round globules.
- Altostratus : Greyish cloud sheet. Halos cannot be seen through Altostratus clouds,
Low Clouds Clouds extending from the surface to 2 Km.
- Stratus : Generally a grey cloud layer of uniform base.
- Stratoscumulus : Grey or white patches; often appear as rolls or rounded masses of clouds.
- Cumulus : Detached clouds with sharp outlines. Rising towers or domes are often seen within a Cumulus cloud.
- Cumulonimbus : Heavy and dense shower clouds with top spread out in the form of an anvil.
- Nimbostratus : Grey or dark cloud layers from which we observe continuous rain.
Clouds at Ground Level: Mist and Fog
Mist is just a cloud that has formed at ground level. This phenomenon occurs especially after sunset, when earth cools down more rapidly than the surrounding air. Wet air condenses at ground level without even having to rise any higher. Fog is a dense mist in which visibility is less than a kilometer. Water vapour may also condense in small drops on the ground and on plants, forming dew. And when the temperature falls below 0º C, dew becomes ice, and is called hoarfrost or rime.