In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may compose the droplets and crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture (usually in the form of water vapor) from an adjacent source to raise the dew point to the ambient temperature.

They are seen in the Earth's homosphere (which includes the troposphere, stratosphere, and mesosphere). Nephology is the science of clouds, which is undertaken in the cloud physics branch of meteorology.

The two methods of naming clouds in their respective layers of the atmosphere are Latin and common. Cloud types in the troposphere, the atmospheric layer closest to Earth's surface, have Latin names due to the universal adoption of Luke Howard's nomenclature. Formally proposed in 1802, it became the basis of a modern international system that divides clouds into five physical ''forms'' that appear in any or all of three altitude ''levels'' (formerly known as ''étages''). These physical types, in approximate ascending order of convective activity, include ''stratiform'' sheets, ''cirriform'' wisps and patches, ''stratocumuliform'' layers (mainly structured as rolls, ripples, and patches), ''cumuliform'' heaps, and very large ''cumulonimbiform'' heaps that often show complex structures. The physical forms are divided by altitude level into 10 basic genus-types.

The Latin names for applicable ''high-level'' genera in the troposphere carry a ''cirro''- prefix, and an ''alto''- prefix is added to the names of the ''mid-level'' genus-types. Clouds with sufficient vertical extent to occupy more than one altitude level are officially classified as ''low-'' or ''mid-level'' according to the altitude range at which each initially forms. However they are also more informally classified as ''multi-level'' or ''vertical'', which along with low level clouds, do not carry any altitude related prefixes. Most of the genera can be subdivided into ''species'' and further subdivided into ''varieties''. Very low stratiform clouds that extend down to the Earth's surface are given the common names fog and mist, but have no Latin names.

Several cloud types that form higher up in the stratosphere and mesosphere have common names for their main types, which may have the appearance of stratiform sheets, cirriform wisps, or stratocumuliform bands or ripples. They are seen infrequently, mostly in the polar regions of Earth. Clouds have been observed in the atmospheres of other planets and moons in the Solar System and beyond. However, due to their different temperature characteristics, they are often composed of other substances such as methane, ammonia, and sulfuric acid, as well as water.

The tabular overview that follows is very broad in scope. It draws from several methods of cloud classification, both formal and informal, used in different levels of the Earth's homosphere by a number of cited authorities. Despite some differences in methodologies and terminologies, the classification schemes seen in this article can be harmonized by using a cross-classification of form and level to derive the 10 tropospheric genera, the fog and mist that forms at surface level, and several additional major types above the troposphere. The cumulus genus includes four species that indicate vertical size and structure. This table should therefore not be seen as a strict or singular classification, but as an illustration of how various major cloud types are related to each other and defined through a full range of altitude levels from Earth's surface to the "edge of space".

{| class="wikitable" !Forms and levels !! Stratiform
non-convective !! Cirriform
mostly nonconvective !! Stratocumuliform
limited-convective !! Cumuliform
free-convective !! Cumulonimbiform
strong convective |- !Extreme-level | PMC: Noctilucent veils || Noctilucent billows or whirls|| Noctilucent bands || || |- !Very high-level | Nitric acid and water PSC ||Cirriform nacreous PSC ||Lenticular nacreous PSC || || |- !High-level | Cirrostratus ||Cirrus || Cirrocumulus || || |- !Mid-level | Altostratus || || Altocumulus || || |- !Low-level | Stratus || || Stratocumulus || Cumulus humilis or fractus || |- !Multi-level or moderate vertical | Nimbostratus || || || Cumulus mediocris || |- !Towering vertical | || || || Cumulus congestus || Cumulonimbus |- !Surface-level | Fog or mist || || || || |}

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