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. There are two methods of naming clouds in their respective layers of the homosphere, Latin and common.
Genus 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 that was formally proposed in 1802. It became the basis of a modern international system that divides clouds into five physical forms which can be further divided or classified into altitude levels to derive ten basic genera. The main representative cloud types for each of these forms are stratus, cirrus, stratocumulus, cumulus, and cumulonimbus. Low-level clouds do not have any altitude-related prefixes. However mid-level stratiform and stratocumuliform types are given the prefix alto- while high-level variants of these same two forms carry the prefix cirro-. Genus types with sufficient vertical extent to occupy more than one level do not carry any altitude related prefixes. They are classified formally as low- or mid-level depending on the altitude at which each initially forms, and are also more informally characterized as multi-level or vertical. Most of the ten genera derived by this method of classification 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.
In the stratosphere and mesosphere, clouds have common names for their main types. They may have the appearance of stratiform veils or 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.
Tropospheric clouds can have a direct effect on climate change on Earth. They may reflect incoming rays from the sun which can contribute to a cooling effect where and when these clouds occur, or trap longer wave radiation that reflects back up from the Earth's surface which can cause a warming effect. The altitude, form, and thickness of the clouds are the main factors that affect the local heating or cooling of Earth and the atmosphere. Clouds that form above the troposphere are too scarce and too thin to have any influence on climate change. Clouds are the main uncertainty in climate sensitivity.
The table 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 nomenclature, the classification schemes seen in this article can be combined by using an informal cross-classification of physical forms and altitude levels 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 which can affect both forms and levels. This table should 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."
The origin of the term "cloud" can be found in the Old English words clud or clod, meaning a hill or a mass of rock. Around the beginning of the 13th century, the word came to be used as a metaphor for rain clouds, because of the similarity in appearance between a mass of rock and cumulus heap cloud. Over time, the metaphoric usage of the word supplanted the Old English weolcan, which had been the literal term for clouds in general.
Ancient cloud studies were not made in isolation, but were observed in combination with other weather elements and even other natural sciences. Around 340 BC, Greek philosopher Aristotle wrote Meteorologica, a work which represented the sum of knowledge of the time about natural science, including weather and climate. For the first time, precipitation and the clouds from which precipitation fell were called meteors, which originate from the Greek word meteoros, meaning 'high in the sky'. From that word came the modern term meteorology, the study of clouds and weather. Meteorologica was based on intuition and simple observation, but not on what is now considered the scientific method. Nevertheless, it was the first known work that attempted to treat a broad range of meteorological topics in a systematic way, especially the hydrological cycle.
First comprehensive classification
Tropospheric cloud classification by altitude of occurrence: Multi-level and vertical genus-types not limited to a single altitude level include nimbostratus, cumulonimbus, and some of the larger cumulus species.
After centuries of speculative theories about the formation and behavior of clouds, the first truly scientific studies were undertaken by Luke Howard in England and Jean-Baptiste Lamarck in France. Howard was a methodical observer with a strong grounding in the Latin language, and used his background to classify the various tropospheric cloud types during 1802. He believed that the changing cloud forms in the sky could unlock the key to weather forecasting. Lamarck had worked independently on cloud classification the same year and had come up with a different naming scheme that failed to make an impression even in his home country of France because it used unusual French names for cloud types. His system of nomenclature included 12 categories of clouds, with such names as (translated from French) hazy clouds, dappled clouds, and broom-like clouds. By contrast, Howard used universally accepted Latin, which caught on quickly after it was published in 1803. As a sign of the popularity of the naming scheme, German dramatist and poet Johann Wolfgang von Goethe composed four poems about clouds, dedicating them to Howard. An elaboration of Howard's system was eventually formally adopted by the International Meteorological Conference in 1891. This system covered only the tropospheric cloud types, but the discovery of clouds above the troposphere during the late 19th century eventually led to the creation of separate classification schemes using common names for these very high clouds, which were still broadly similar to some cloud forms identified in the troposphere.
Formation in the homosphere: How air becomes saturated
Main article: Cloud physics
Cumulus clouds in May
Terrestrial clouds can be found throughout most of the homosphere, which includes the troposphere, stratosphere, and mesosphere. Within these layers of the atmosphere, air can become saturated as a result of being cooled to its dew point or by having moisture added from an adjacent source. In the latter case, saturation occurs when the dew point is raised to the ambient air temperature.
Adiabatic cooling occurs when one or more of three possible lifting agents – convective, cyclonic/frontal, or orographic – cause a parcel of air containing invisible water vapor to rise and cool to its dew point, the temperature at which the air becomes saturated. The main mechanism behind this process is adiabatic cooling. As the air is cooled to its dew point and becomes saturated, water vapor normally condenses to form cloud drops. This condensation normally occurs on cloud condensation nuclei such as salt or dust particles that are small enough to be held aloft by normal circulation of the air.
File:Cloud evolution in under a minute.ogv
Animation of cloud evolution from cumulus humilis to cumulonimbus capillatus incus
One agent is the convective upward motion of air caused by daytime solar heating at surface level. Airmass instability allows for the formation of cumuliform clouds that can produce showers if the air is sufficiently moist. On moderately rare occasions, convective lift can be powerful enough to penetrate the tropopause and push the cloud top into the stratosphere.
Frontal and cyclonic lift occur when stable air is forced aloft at weather fronts and around centers of low pressure by a process called convergence. Warm fronts associated with extratropical cyclones tend to generate mostly cirriform and stratiform clouds over a wide area unless the approaching warm airmass is unstable, in which case cumulus congestus or cumulonimbus clouds are usually embedded in the main precipitating cloud layer. Cold fronts are usually faster moving and generate a narrower line of clouds, which are mostly stratocumuliform, cumuliform, or cumulonimbiform depending on the stability of the warm airmass just ahead of the front.
Windy evening twilight enhanced by the Sun's angle, can visually mimic a tornado resulting from orographic lift
A third source of lift is wind circulation forcing air over a physical barrier such as a mountain (orographic lift). If the air is generally stable, nothing more than lenticular cap clouds form. However, if the air becomes sufficiently moist and unstable, orographic showers or thunderstorms may appear.
Along with adiabatic cooling that requires a lifting agent, three major nonadiabatic mechanisms exist for lowering the temperature of the air to its dew point. Conductive, radiational, and evaporative cooling require no lifting mechanism and can cause condensation at surface level resulting in the formation of fog.
Adding moisture to the air
Several main sources of water vapor can be added to the air as a way of achieving saturation without any cooling process: water or moist ground, precipitation or virga, and transpiration from plants
Classification: How clouds are identified in the troposphere
For a comprehensive listing of over 90 combinations of genera divided into species and subdivided into varieties with Latin etymologies, see List of cloud types.
Low stratus fractus partly obscuring nimbostratus over Deccan Plateau, India
Tropospheric classification is based on a hierarchy of categories with physical forms and altitude levels at the top. These are cross-classified into a total of ten genus types, most of which can be divided into species and further subdivided into varieties which are at the bottom of the hierarchy.
Cirrus fibratus clouds in March
Clouds in the troposphere assume five physical forms based on structure and process of formation. These forms are commonly used for the purpose of satellite analysis. They are given below in approximate ascending order of instability or convective activity.
Nonconvective stratiform clouds appear in stable airmass conditions and, in general, have flat, sheet-like structures that can form at any altitude in the troposphere. The stratiform group is divided by altitude range into the genera cirrostratus (high-level), altostratus (mid-level), stratus (low-level), and nimbostratus (multi-level). Fog is commonly considered a surface-based cloud layer. The fog may f
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