Cloud

In meteorology, a cloud is a visible mass of liquid droplets or frozen crystals made of water or various chemicals suspended in the atmosphere above the surface of a planetary body. These suspended particles are also known as aerosols. Clouds in earth's atmosphere are studied in the cloud physics branch of meteorology. Two processes, possibly acting together, can lead to air becoming saturated; cooling the air or adding water vapor to the air. In general, precipitation will fall to the surface; an exception is virga, which evaporates before reaching the surface.

The international cloud classification system is based on the fact clouds can show free-convective upward growth like cumulus, appear in non-convective layered sheets such as stratus, or take the form of thin fibrous wisps, as in the case of cirrus. Prefixes are used in connection with clouds: strato- for low clouds with limited convection that form mostly in layers, nimbo- for thick layered clouds that can produce moderate to heavy precipitation, alto- for middle clouds, and cirro- for high clouds. Whether or not a cloud is low, middle, or high level depends on how far above the ground its base forms. Cloud types with significant vertical extent can form in the low or middle altitude ranges depending on the moisture content of the air. Clouds in the troposphere have Latin names due to the popular adaptation of Luke Howard's cloud categorization system, which began to spread in popularity during December 1802. Synoptic surface weather observations use code numbers to record and report the types of tropospheric cloud visible at each scheduled observation time based on the height and physical appearance of the clouds.

While a majority of clouds form in Earth's troposphere, there are occasions when clouds in the stratosphere and mesosphere can be observed. These three main layers of the atmosphere where clouds may be seen are collectively known as the homosphere. Above this lies the thermosphere and exosphere, which together make up the heterosphere that marks the transition to outer space. Clouds have been observed on other planets and moons within the Solar System, but, due to their different temperature characteristics, they are composed of other substances such as methane, ammonia, and sulfuric acid.

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Entrainment (meteorology)
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Famous quotes containing the word cloud:

    Follow your saint, follow with accents sweet;
    Haste you, sad notes, fall at her flying feet.
    There, wrapped in cloud of sorrow, pity move,
    And tell the ravisher of my soul I perish for her love.
    Thomas Campion (1567–1620)

    Genius detects through the fly, through the caterpillar, through the grub, through the egg, the constant individual; through countless individuals the fixed species; through many species the genus; through all genera the steadfast type; through all the kingdoms of organized life the eternal unity. Nature is a mutable cloud which is always and never the same.
    Ralph Waldo Emerson (1803–1882)

    It cannot be denied that for a society which has to create scarcity to save its members from starvation, to whom abundance spells disaster, and to whom unlimited energy means unlimited power for war and destruction, there is an ominous cloud in the distance though at present it be no bigger than a man’s hand.
    Arthur Stanley Eddington (1882–1944)