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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 and are studied in the cloud physics branch of meteorology.
Terrestrial cloud formation is the result of air in Earth's atmosphere becoming saturated due to either or both of two processes; cooling of the air and adding water vapor. With sufficient saturation, precipitation will fall to the surface; an exception is virga, which evaporates before reaching the surface.
Clouds in the troposphere, the atmospheric layer closest to Earth's surface, have Latin names due to the universal adaptation of Luke Howard's nomenclature. It was introduced in December 1802 and became the basis of a modern international system that classifies these tropospheric aerosols into several physical forms or categories, then cross-classifies them into families of low, middle and high according to cloud-base altitude range above Earth's surface. Clouds with significant vertical extent are often considered a separate family. One physical form shows free-convective upward growth into low or vertical heaps of cumulus. Other forms appear as non-convective layered sheets like low stratus, and as limited-convective rolls or ripples as with stratocumulus. Both of these layered forms have middle- and high-family variants identified respectively by the prefixes alto- and cirro-. Thin fibrous wisps of cirrus are a physical form found only at high altitudes. In the case of clouds with vertical extent, prefixes are used whenever necessary to express variations or complexities in their physical structures. These include cumulo- for complex highly convective vertical nimbus storm clouds, and nimbo- for thick stratiform layers with sufficient vertical depth to produce moderate to heavy precipitation. This process of cross-classification produces ten basic genus-types or genera, most of which can be subdivided into species and varieties. Synoptic surface weather observations use code numbers to record and report any type of tropospheric cloud visible at scheduled observation times based on its height and physical appearance.
While a majority of clouds form in Earth's troposphere, there are occasions when they can be observed at much higher altitudes in the stratosphere and mesosphere. Clouds that form above the troposphere have common names for their main types, but are sub-classified alpha-numerically rather than with the elaborate system of Latin names given to cloud types in the troposphere. These three main atmospheric layers that can produce clouds, along with the lowest part of the cloudless thermosphere, are collectively known as the homosphere. Above this lies the heterosphere (which includes the rest of the thermosphere and the exosphere) 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.
All weather-related clouds form in the troposphere, the lowest layer of Earth's atmosphere. This generally happens when one or more lifting agents causes 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. Atmospheric pressure decreases with altitude, so the rising air expands in a process that expends energy and causes the air to cool, which reduces its capacity to hold water vapor. If the air is cooled to its dew point and becomes saturated, it normally sheds vapor it can no longer retain which condenses into cloud.
The altitude at which this begins to happen is called the lifted condensation level, which roughly determines the height of the cloud base. Water vapor in saturated air is normally attracted to condensation nuclei such as salt particles that are small enough to be held aloft by normal circulation of the air. If the condensation process occurs below the freezing level in the troposphere, the nuclei help transform the vapor into very small water droplets. Clouds that form just above the freezing level are composed mostly of supercooled liquid droplets, while those that condense out at higher altitudes where the air is much colder generally take the form of ice crystals. An absence of sufficient condensation particles at and above the condensation level causes the rising air to become supersaturated and the formation of cloud tends to be inhibited.
There are three main agents of vertical lift. One comprises two closely related processes which work together. Frontal lift and cyclonic lift occur when stable or slightly unstable air, which has been subjected to little or no surface heating, is forced aloft at weather fronts and around centers of low pressure. Cloud droplets form when the air is lifted beyond the condensation level where water vapor condenses on so-called nuclei; (small particles) that grow to a size of typically 0.002 mm (.00008 in). In a cloud the droplets collide to form larger droplets. These larger droplets remain aloft as long as the drag force of the air dominates over the gravitational force for small particles. If the cloud droplets continue to grow past this size, they become too heavy to be held aloft as the gravitational force overcomes the atmospheric drag, and they fall from the cloud as rain. When this process takes place just above the freezing level, the vapor tends to condense into supercooled water droplets, which with additional lifting and growth in size, can eventually turn into freezing rain. At temperatures well below freezing, the vapor desublimates into ice crystals that average about 0.25 mm in length. Continuing lift and desublimation will tend to increase the number of ice crystals which may combine until they are too heavy to be supported by the vertical air currents and fall out as snow.
Another agent is the buoyant convective upward motion caused by significant daytime solar heating at surface level, or by relatively high absolute humidity. Air warmed in this way becomes increasingly unstable. This causes it to rise and cool until temperature equilibrium is achieved with the surrounding air aloft. If air near the surface becomes extremely warm and unstable, its upward motion can become quite explosive resulting in towering clouds that can break through the tropopause or cause severe weather. Strong convection upcurrents may allow the droplets to grow to nearly .08 mm (.003 in) before precipitating as heavy rain from an active thundercloud. More occasionally, very warm unstable air is present around fronts and low-pressure centers. As with non-frontal convective lift, increasing instability promotes upward vertical cloud growth and raises the potential for severe weather.
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 will 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, there are three other main mechanisms for lowering the temperature of the air to its dew point, all of which occur near surface level and do not require any lifting of the air. Conductive, radiational, and evaporative cooling can cause condensation at surface level resulting in the formation of fog. Conductive cooling takes place when air from a relatively mild source area comes into contact with a colder surface, as when mild marine air moves across a colder land area. Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath. This type of cooling is common during the night when the sky is clear. Evaporative cooling happens when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature, or sometimes to the point of saturation.
There are five main ways water vapor can be added to the air. Increased vapor content can result from wind convergence over water or moist ground into areas of upward motion. Precipitation or virga falling from above also enhances moisture content. Daytime heating causes water to evaporate from the surface of oceans, water bodies or wet land. Transpiration from plants is another typical source of water vapor. Lastly, cool or dry air moving over warmer water will become more humid. As with daytime heating, the addition of moisture to the air increases its heat content and instability and helps set into motion those processes that lead to the formation of cloud or fog.
There are forces in the atmosphere such as wind shear and downdrafts that can impact the structural integrity of a cloud. However, as long as the air remains saturated, the natural force of cohesion that hold the molecules of a substance together acts to keep the cloud from breaking up. Dissolution of the cloud can occur when the process of adiabatic cooling ceases after the passage of a weather disturbance or following the loss of daytime heating of the lower troposphere. Upward lift of the air is replaced by subsidence. This leads to at least some degree of adiabatic warming of the air which can result in the cloud droplets evaporating and turning back into invisible water vapor.
Atmospheric convergence is a process that involves the horizontal inflow and accumulation of air at a given location, as well as the rate at which this happens. This accumulation causes air to rise. If higher altitude divergence (horizontal outflow) of an equal amount occurs simultaneously above the same location, the surface atmospheric pressure is theoretically not affected. However, much of the surface convergence that occurs in the atmosphere is caused by the drawing in of air in the form of wind currents towards areas of low pressure that are the product of unequal heating of the Earth's surface.
Although the local distribution of clouds can be significantly influenced by topography, the global prevalence of cloud cover tends to vary more by latitude. This is the result of atmospheric motion driven by the uneven horizontal distribution of net incoming radiation from the sun. Cloudiness reaches maxima close to the equator and near the 50th parallels of latitude in the northern and southern hemispheres. These are zones of low pressure that encircle the Earth as part of a system of large latitudinal cells that influence atmospheric circulation. In both hemispheres working away from the equator, they are the tropical Hadley cells, the mid-latitude Ferrel, and the polar cells. The 50th parallels coincide roughly with bands of low pressure situated just below the polar highs. These extratropical convergence zones are occupied by the polar fronts where air masses of polar origin meet and clash with those of tropical or subtropical origin. This leads to the formation of weather-making extratropical cyclones composed of cloud systems that may be stable or unstable to varying degrees according to the stability characteristics of the various airmasses that are in conflict.
Near the equator, increased cloudiness is due to the presence of the low-pressure Intertropical Convergence Zone or monsoon trough. Monsoon troughing in the western Pacific reaches its latitudinal zenith in each hemisphere above and below the equator during the late summer when the wintertime surface high-pressure ridge in the opposite hemisphere is strongest. The trough can reach as far as the 40th parallel north in East Asia during August and the 20th parallel south in Australia during February. Its poleward progression is accelerated by the onset of the summer monsoon which is characterized by the development of lower air pressure of greater instability over the warmest parts of the various continents. Cloud cover formed in this way tends to be unstable and free-convective in nature. The resulting weather systems often produce heavy showers and thunderstorms These can result in the formation of tropical storms and hurrucanes composed mainly of towering thunderclouds. In the southern hemisphere, the trough associated with the Australian monsoon reaches its most southerly latitude in February, oriented along a west-northwest to east-southeast axis.
Divergence is the opposite of convergence. In the Earth's atmosphere, it involves the hoizontal outflow of air from the upper part of a rising column of air, or from the lower part of a subsiding column often associated with an area or ridge of high pressure.
Cloudiness reaches minima near the poles and in the subtropics close to the 20th parallels, north and south. The latter are sometimes referred to as the horse latitudes. The presence of a large-scale high-pressure subtropical ridge on each side of the equator reduces cloudiness at these low latitudes. Heating of the Earth near the equator leads to large amounts of upward motion and convection along the monsoon trough or intertropical convergence zone. These rising air currents diverge in the upper troposphere and move away from the equator at high altitude in both northerly and southerly directions. As it moves towards the mid-latitudes on both sides of the equator, the air cools and sinks. The resulting air mass subsidence creates a subtropical ridge near the 30th parallel of latitude in both hemispheres where the formation of cloud is minimal. At surface level, the sinking air diverges again with some moving back to the equator and completing the vertical cycle. This circulation on each side of the equator is known as the Hadley cell in the tropics. Many of the world's deserts are caused by these climatological high-pressure areas.
Similar patterns also occur at higher latitudes in both hemispheres. Upward currents of air along the polar fronts diverge at high tropospheric altitudes. Some of the diverging air moves to the poles where air mass subsidence inhibits cloud formation and leads to the creation of the polar areas of high pressure. Divergence occurs near surface level resulting in a return of the circulating air to the polar fronts where rising air currents can create extensive cloud cover and precipitation. This vertical cycle comprises the polar cell in each latitudinal hemisphere. Some of the air rising at the polar fronts diverges away from the poles and moves in the opposite direction to the high level zones of convergence and subsidence at the subtropical ridges on each side of the equator. These mid-latitude counter-circulations create the Ferrel cells that encircle the globe in the northern and southern hemispheres.
Luke Howard, a methodical observer with a strong grounding in the Latin language, used his background to categorize the various tropospheric cloud types and forms during December 1802. He believed that the changing cloud forms in the sky could unlock the key to weather forecasting. Jean-Baptiste Lamarck worked independently on cloud categorization and came 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 twelve categories of clouds, with such names as (translated from French) hazy clouds, dappled clouds and broom-like clouds. Howard used universally accepted Latin, which caught on quickly. As a sign of the popularity of the naming scheme, the German dramatist and poet Johann Wolfgang von Goethe composed four poems about clouds, dedicating them to Howard. Classification systems would be proposed by Heinrich Dove of Germany in 1828 and Elias Loomis of the United States in 1841, but neither became the international standard that Howard's system became. It was formally adopted by the International Meteorological Commission in 1929.
Howard's original system established three general cloud categories based on physical appearance and process of formation: cirriform (mainly detached and wispy), cumuliform or convective (mostly detached and heaped, rolled, or rippled), and non-convective stratiform (mainly continuous layers in sheets). These were cross-classified into lower and upper families. Cumuliform clouds forming in the lower level were given the genus name cumulus, and low stratiform clouds the genus name stratus. Physically similar clouds forming in the upper height range were given the genus names cirrocumulus (generally showing more limited convective activity than low level cumulus) and cirrostratus, respectively. Cirriform category clouds were identified as always upper level and given the genus name cirrus. To these, Howard added the genus nimbus for clouds of complex structure producing significant precipitation that came to be identified as a distinct nimbiform physical category.
Around 1840–41, German meteorologist Ludwig Kaemtz added stratocumulus as a mostly detached low-cloud genus of limited convection with both cumuliform- and stratiform characteristics similar to upper-level cirrocumulus. This had the effect of creating a stratocumuliform category that included rolled and rippled clouds separately from the more freely convective heap clouds. About fifteen years later, Emilien Renou, director of the Parc Saint-Maur and Montsouris observatories, began work on an elaboration of Howard's classifications that would lead to the introduction during the 1870s of altocumulus (physically more closely related to stratocumulus than to cumulus) and altostratus. These were respectively stratocumuliform and stratiform cloud genera of a newly defined middle height range above stratocumulus and stratus but below cirrocumulus and cirrostratus, with free convective cumulus and non-convective nimbus occupying more than one altitude range as clouds with vertical extent. In 1880, Philip Weilbach, secretary and librarian at the Art Academy in Copenhagen, and like Luke Howard, an amateur meteorologist, proposed and had accepted by the permanent committee of the International Meteorological Organization (IMO), a forerunner of the present-day World Meteorological Organization (WMO), the designation of a new free-convective vertical genus type, cumulonimbus, which would be distinct from cumulus and nimbus and identifiable by its often very complex structure (frequently including a cirriform top and what are now recognized as multiple accessory clouds), and its ability to produce thunder. With this addition, a canon of ten cloud genera was established that came to be officially and universally accepted. At about the same time, several cloud specialists proposed variations that came to be accepted as species subdivisions and varieties determined by more specific variable aspects of the structure of each genus. A further modification of the genus classification system came when an IMC commission for the study of clouds put forward a refined and more restricted definition of the genus nimbus which was effectively recategorized as a stratiform cloud type. It was then renamed nimbostratus and published with the new name in the 1932 edition of the International Atlas of Clouds and of States of the Sky. This left cumulonimbus as the only nimbiform physical category-type as indicated by its root-name. In 1976, the National Aeronautics and Space Administration (NASA) published a cloud classification that showed a change in name of the nimbiform category to cumulonimbiform, although some other agencies have continued to recognize the earlier category name.
As established by Howard and his successors, clouds are commonly grouped into physical categories that can be up to five in number: cirriform, cumuliform, cumulonimbiform, stratocumuliform, and stratiform. These designations distinguish a cloud's physical structure and process of formation.
Cirriform-category clouds generally have a wispy fibrous appearance and form at high tropospheric altitudes along the very leading edges of a frontal or low-pressure weather disturbance and often along the fringes of its other borders. In general, they are non-convective but occasionally acquire a tufted or turreted appearance caused by small-scale high-altitude convection. These high clouds do not produce precipitation as such but are often accompanied or followed by lower-based clouds that do.
Cumuliform clouds typically have flat bases and puffy domed tops. They are the product of localized but generally free-convective lift and can vary in vertical extent depending on the stability characteristics of the air mass where they are forming. The smallest fair weather cumuliform types occur with only minimal instability and can therefore be considered clouds of limited convection. Incoming short-wave radiation generated by the sun is re-emitted as long-wave radiation when it reaches Earth's surface. This process warms the air closest to ground and increases air mass instability by creating a steeper temperature gradient from warm or hot at surface level to cold aloft. Moderate instability allows for the formation of cumuliform clouds of moderate size that can produce light showers if the airmass is sufficiently moist. The more the air is heated from below, the more unstable it tends to become. This may cause large towering cumuliform clouds to form in the lower half of the troposphere with tops growing into the upper levels. These buildups can cause moderate to occasionally heavy showers. They tend to be more concentrated and intense when they are associated with fast-moving unstable cold fronts.
The largest free-convective cumuliform clouds occur in very unstable air and often have complex structures that include cirriform tops and multiple accessory clouds and are sometimes classified separately as cumulonimbiform. At maturity, they have very strong updrafts that can penetrate the tropopause. They can produce thunderstorms and a variety of types of lightning including cloud-to-ground that can cause wildfires. Other convective severe weather may or may not be associated with thunderstorms and include heavy rain or snow showers, hail, strong wind shear, downbursts, and tornadoes.
In general, stratiform-category clouds have a flat sheet-like structure and form at any altitude in the troposphere where there is sufficient condensation as the result of non-convective lift of relatively stable air, especially along warm fronts, around areas of low pressure, and sometimes along stable slow moving cold fronts. In general, precipitation falls from stratiform clouds in the lower half of the troposphere. If the weather system is well-organized, the precipitation is generally steady and widespread. The intensity varies from light to heavy according to the thickness of the stratiform layer as determined by moisture content of the air and the intensity of the weather system creating the clouds and weather. Unlike free convective cumuliform and cumulonimbiform clouds that tend to grow upward, stratiform clouds achieve their greatest thickness when precipitation that forms in the middle level of the troposphere triggers downward growth of the cloud base to near surface level. Stratiform clouds can also form in precipitation below the main frontal cloud deck where the colder air is trapped under the warmer airmass being forced above by the front. Non-frontal low stratiform cloud can form when advection fog is lifted above surface level during breezy conditions.
Clouds of this physical structure have both cumuliform and stratiform characteristics and generally form as a result of limited convection in slightly unstable air. They can form at any altitude in the troposphere wherever and whenever there is sufficient moisture and lift. High stratocumuliform clouds also tend show some cirriform characteristics or form in association with cirriform clouds. If a poorly organized low-pressure weather system is present, virga or weak intermittent precipitation may fall from those stratocumuliform clouds that form mostly in the low and lower-middle height ranges of the troposphere.
The individual genus types result from the physical categories being cross-classified by height range family within the troposphere. A general consensus exists as to the designation of high, middle, and low families, the makeup of the basic canon of ten cloud genera that results from this cross-classification, and the family affiliation of non-vertical genus types. Several but not all methods of altitude classification treat clouds with significant vertical extent as a separate family. The base-height range for each family varies depending on the latitudinal geographical zone. Moderate and towering vertical clouds can have low or middle-altitude bases depending on the moisture content of the air.
Clouds of the high family form at altitudes of 3,000 to 7,600 m (10,000 to 25,000 ft) in the polar regions, 5,000 to 12,200 m (16,500 to 40,000 ft) in the temperate regions and 6,100 to 18,300 m (20,000 to 60,000 ft) in the tropical region. All cirriform clouds are classified as high-range and thus constitute a single genus cirrus (Ci). Stratocumuliform and stratiform clouds in the high-altitude family carry the prefix cirro-, yielding the respective genus names cirrocumulus (Cc) and cirrostratus (Cs). Strato- is excluded from cirrocumulus to avoid double prefixing. Most high cloud forms as a result of natural atmospheric processes. However, contrails formed from the exhaust of high-flying aircraft can persist and spread into formations resembling cirrus, cirrocumulus, or cirrostratus. This variant has no special WMO designation, but is sometimes given the faux-Latin name Aviaticus.
The family of middle clouds typically comprises one stratocumuliform and one stratiform genus. They are prefixed by alto-, yielding the genus names altocumulus (Ac) and altostratus (As). Strato- is also excluded from altocumulus. These clouds can form as low as 2,000 m (6,500 ft) above surface at any latitude, but may be based as high as 4,000 m (13,000 ft) near the poles, 7,000 m (23,000 ft) at mid latitudes, and 7,600 m (25,000 ft) in the tropics.
Low clouds are found from near surface up to 2,000 m (6,500 ft). This family mainly includes one stratocumuliform and one stratiform genus whenever vertical clouds are classified separately. When a low stratiform cloud contacts the ground, it is called fog, although radiation and advection types of fog do not form from stratus layers. Genus types in this family either have no prefix or carry one that refers to a characteristic other than altitude. Of the two main cloud types in this family, the prefixed genus is stratocumulus (Sc), a low altitude cloud of limited convection, and the non-prefixed genus is non-convective stratus (St) that usually forms into a comparatively thin layer. Small fair weather cumulus (Cu) of limited convection is also often included with this family due to its lack of vertical development compared to moderate and towering cumulus types.
Upward-growing free-convective clouds have low to middle bases that form anywhere from near surface to about 2,400 m (8,000 ft) in temperate climates, and often much higher in arid regions, even to the very top of the middle altitude range of the troposphere. This family, when recognized as such, includes the singular cumuliform and cumulonimbiform genus types, and one stratiform genus. The first of these is free-convective cumulus (Cu) that carries no prefix. It usually forms in the low-altitude range except during conditions of very low relative humidity when the clouds bases can rise into the middle range. The other two types have non height-related prefixes. Cumulonimbus (Cb) is prefixed according to its free-convective characteristics. Nimbostratus (Ns) is a deep non-convective stratiform genus that normally forms from middle-altitude altostratus and achieves vertical extent as it thickens during precipitation with the base subsiding into the low altitude range. The nimbo- prefix refers to its ability to produce continuous rain or snow over a wide area, aspecially ahead of a warm front.
Some methods of height classification limit the term vertical to upward-growing free-convective cumuliform and cumulonimbiform genera whose vertical thickness exceeds their horizontal base-width. Downward-growing nimbostratus can be as thick as most upward-growing vertical cumulus, but its horizontal extent tends to be even greater. This sometimes leads to the exclusion of this genus type from the family of vertical clouds. Authorities who follow this approach usually classify nimbostratus either as low to denote its normal base height range, or as middle, based on the altitude range at which it normally forms. Sometimes the term multi-level is used for all very thick or tall cloud types including nimbostratus to avoid the connotation of 'vertical' with free-convective cumuliform only. Alternatively, some classifications do not recognize a vertical family designation and include all vertical free-convective cumuliform and cumulonimbiform types with the family of low clouds.
Nimbostratus and some cumulus in this family usually only achieve comparatively moderate vertical extent. However, with sufficient airmass instability, upward-growing cumuliform clouds can grow to towering proportions. Although genus types with vertical extent are often considered a single family, the International Civil Aviation Organization (ICAO) further distinguishes towering vertical clouds as a separate group or sub-group by specifying that these very large cumuliform and cumulonimbiform types must be identified by their standard names or abbreviations in all aviation observations (METARS) and forecasts (TAFS) to warn pilots of possible severe weather and turbulence. When towering vertical types are considered separately, they comprise the aforementioned cumulonimbus genus and one cumulus species, cumulus congestus (Cu con). The latter is a sub-type of the genus cumulus. This species is designated towering cumulus (Tcu) by ICAO. There is no stratiform type in this group because by definition, even very thick stratiform clouds cannot have towering vertical structure, although they may be accompanied by embedded towering cumuliform or cumulonimbiform types.
Genus types are divided into species that indicate specific structural details. However, because these latter types are not always restricted by height range, some species can be common to several genera that are differentiated mainly by altitude.
Good examples of species common to more than one genus are the stratiformis and lenticularis types, each of which is common to mostly stable stratocumuliform genera in the high-, middle-, and low-height ranges (cirrocumulus, altocumulus, and stratocumulus, respectively). Stratiformis species normally occur in extensive sheets or in smaller patches where there is only minimal convective activity. Lenticularis species tend to have lens-like shapes tapered at the ends. They are most commonly seen as orographic mountain-wave clouds, but can occur anywhere in the troposphere where there is strong wind shear combined with sufficient airmass stability to maintain a generally flat cloud structure.
Cirrus clouds have a couple of species that are unique to the wispy structures of this genus and an additional species which is also seen with high stratiform clouds. Uncinus filaments with upturned hooks and spissatus filaments that merge into dense patches are both considered cirriform species. However the species fibratus can be seen with cirrus and with cirrostratus that is transitional to or from cirrus. Cirrostratus at its most characteristic tends to be mostly of the stratiform species nebulosus, which creates a rather diffuse appearance lacking in structural detail. Altostratus and nimbostratus clouds always have this physical appearance without significant variation or deviation and, therefore, do not need to be subdivided into species. Low stratus is also of the species nebulosus except when broken up into ragged sheets of stratus fractus.
With increasing airmass instability, castellanus structures, which resemble the turrets of a castle when viewed from the side, can be found with any stratocumuliform genus. This species is also sometimes seen with convective patches of cirrus, as are the more detached tufted floccus species, which are common to cirrus, cirrocumulus, and altocumulus, but not stratocumulus.
With the exception of stratocumulus castellanus, local airmass instability in the lower levels tends to produce clouds of the more freely convective cumulus and cumulonimbus genera, whose species are mainly indicators of degrees of vertical development. A cumulus cloud initially forms as a cloudlet of the species fractus or humilis that shows only slight vertical development. If the air becomes more unstable, the cloud tends to grow vertically into the species mediocris, then congestus, the tallest cumulus species. With further instability, the cloud may continue to grow into cumulonimbus calvus (essentially a very tall congestus cloud that produces thunder), then ultimately capillatus when supercooled water droplets at the top turn into ice crystals giving it a cirriform appearance.
Genus and species types are further subdivided into varieties whose names can appear after the species name to provide a fuller description of a cloud. Some cloud varieties are not restricted to a specific altitude range or physical structure, and can therefore be common to more than one genus or species.
All cloud varieties fall into one of two main groups. One group identifies the opacities of particular low and middle cloud structures and comprises the varieties translucidus (translucent), perlucidus (opaque with translucent breaks), and opacus (opaque). These varieties are always identifiable for cloud genera and species with variable opacity. All three are associated with the stratiformis species of altocumulus and stratocumulus. However, only two are seen with altostratus and stratus nebulosus whose uniform structures prevent the formation of a perlucidus variety. Opacity-based varieties are not applied to high clouds because they are always translucent, or in the case of cirrus spissatus, always opaque. Similarly, these varieties are also not attached to moderate and towering vertical clouds because they are always opaque.
A second group describes the occasional arrangements of cloud structures into particular patterns that are discernable by a surface-based observer (cloud fields usually being visible only from a significant altitude above the formations). These varieties are not always present with the genera and species with which they are otherwise associated, but only appear when atmospheric conditions favor their formation. Intortus and vertebratus varieties occur on occasion with cirrus fibratus. They are respectively filaments twisted into irregular shapes, and those that are arranged in fishbone patterns, usually by uneven wind currents that favor the formation of these varieties. The variety radiatus is associated with cloud rows of a particular type that appear to converge at the horizon. It is sometimes seen with the fibratus and uncinus species of cirrus, the stratiformis species of altocumulus and stratocumulus, all species of cumulus, and with the genus altostratus. Another variety, duplicatus (closely spaced layers of the same type, one above the other), is sometimes found with cirrus of both the fibratus and uncinus species, and with altocumulus and stratocumulus of the species stratiformis and lenticularis. The variety undulatus (having a wavy undulating base) can occur with any clouds of the species stratiformis or lenticularis, and with altostratus. It is only rarely observed with stratus nebulosus. Under conditions of strong atmospheric wind-shear and instability, this wave-like formation may break into regularly spaced crests. This variant has no separate WMO Latin designation, but is sometimes known informally as Kelvin-Helmholtz. The variety lacunosus is caused by localized downdrafts that create circular holes in the form of a honeycomb or net. It is occasionally seen with cirrocumulus and altocumulus of the species stratiformis, castellanus, and floccus, and with stratocumulus of the species stratiformis and castellanus.
It is possible for some species to show combined varieties at one time, especially if one variety is opacity-based and the other is pattern-based. An example of this would be an opaque layer of altocumulus stratiformis arranged in seemingly converging rows. The full technical name of a cloud in this configuration would be altocumulus stratiformis opacus radiatus, which would identify respectively its genus, species, and two combined varieties.
Supplementary features are not further subdivisions of cloud types below the species and variety level. Rather, they are either hydrometeors or special cloud formations with their own Latin names that form in association with certain cloud genera, species, and varieties.
One group of supplementary features are not actual cloud formations but rather precipitation that falls when water droplets that make up visible clouds have grown too heavy to remain aloft. Virga is a feature seen with clouds producing precipitation that evaporates before reaching the ground, these being of the genera cirrocumulus, altocumulus, altostratus, nimbostratus, stratocumulus, cumulus, and cumulonimbus. When the precipitation reaches the ground without completely evaporating, it is designated as the feature praecipitatio. This normally occurs with altostratus opacus, which can produce widespread but usually light precipitation, and with thicker clouds that show significant vertical development. Of the latter, upward-growing cumulus mediocris produces only isolated light showers, while downward growing nimbostratus is capable of heavier, more extensive precipitation. Towering vertical clouds have the greatest ability to produce intense precipitation events, but these tend to be localized unless organized along fast-moving cold fronts. Showers of moderate to heavy intensity can fall from cumulus congestus clouds. Cumulonimbus, the largest of all cloud genera, has the capacity to produce very heavy showers. Low stratus clouds usually produce only light precipitation, but this always occurs as the feature praecipitatio due to the fact this cloud genus lies too close to the ground to allow for the formation of virga.
The heavier precipitating clouds, nimbostratus, towering cumulus (cumulus congestus), and cumulonimbus, also typically see the formation in precipitation of the pannus feature, low ragged clouds of the genera and species cumulus fractus or stratus fractus. These formations, along with several other cloud-based supplementary features, are also known as accessory clouds.
After the pannus types, the remaining supplementary features comprise cloud formations that are associated mainly with upward-growing cumuliform and cumulonimbiform clouds of free convection. Incus is the most type-specific supplementary feature, seen only with cumulonimbus of the species capillatus. A cumulonimbus incus cloud top is one that has spread out into a clear anvil shape as a result of rising air currents hitting the stability layer at the tropopause where the air no longer continues to get colder with increasing altitude. The mamma feature forms on the bases of clouds as downward-facing bubble-like protuberances caused by localized downdrafts within the cloud. It is also sometimes called mammatus, an earlier version of the term used before a standardization of Latin nomenclature brought about by the World Meterorological Organization during the 20th century. The best-known is cumulonimbus with mammatus, but the mamma feature is also seen occasionally with cirrus, cirrocumulus, altocumulus, altostratus, and stratocumulus. Pileus is a cap cloud that can form over a cumulonimbus or large cumulus cloud, whereas a velum feature is a thin horizontal sheet that sometime forms like an apron around the middle or in front of the parent cloud . An arcus feature is a roll or shelf cloud that forms along the leading edge of a squall line or thunderstorm outflow. Some arcus clouds form as a consequence of interactions with specific geographical features. Perhaps the strangest geographically specific arcus cloud in the world is the Morning Glory, a rolling cylindrical cloud that appears unpredictably over the Gulf of Carpentaria in Northern Australia. Associated with a powerful "ripple" in the atmosphere, the cloud may be "surfed" in glider aircraft. A tuba feature is a cloud column that may hang from the bottom of a cumulus or cumulonimbus. A newly formed or poorly organized column might be comparatively benign, but can quickly intensify into a funnel cloud or tornado.
Clouds initially form in clear air or become clouds when fog rises above surface level. The genus of a newly formed cloud is determined mainly by air mass characteristics such as stability and moisture content. If these characteristics change over a period of time, the genus tends to change accordingly. When this happens, the original genus is called a mother cloud. If the mother cloud retains much of its original form after the appearance of the new genus, it is termed a genitus cloud. One example of this is stratocumulus cumulogenitus, a stratocumulus cloud formed by the partial spreading of a cumulus type when there is a loss of convective lift. If the mother cloud undergoes a complete change in genus, it is considered to be a mutatus cloud. It is theoretically possible for some lengthy terminologies to emerge by combining the names of all applicable genera, species, varieties, and supplementary features to provide a complete description of an active and evolving genitus or mutatus cloud formation. As an extreme example, a flat opaque layer of altocumulus formed by the spreading of cumulus arranged in parallel bands accompanied by precipitation not reaching the ground could be termed altocumulus stratiformis opacus radiatus cumulogenitus virga.
Stratocumulus clouds can be organized into "fields" that take on certain specially classified shapes and characteristics. In general, these fields are more discernable from high altitudes than from ground level. They can often be found in the following forms:
Weather maps plotted and analyzed at weather forecasting centers employ special symbols to denote various cloud families, genera, species, varieties, mutations, and cloud movements that are considered important to identify conditions in the troposphere that will assist in preparing the forecasts. The cloud symbols are translated from numerical codes included with other meteorological data that make up the contents of international synoptic messages transmitted at regular intervals by professionally trained staff at major weather stations. In a couple of cases, an entire genus like cirrocumulus is represented by one cloud symbol, regardless of species, varieties, or any other considerations. In general though, the codes and their symbols are used to identify cloud types at the species level. A number of varieties and supplementary features are also deemed important enough to have their own weather map symbols. For the sake of economy, a particular genus, species, or variety may share a numerical reporting code and symbol with another similar cloud type. Sometimes, a separate symbol is used to indicate whether or not a particular genus has transformed or emerged from a mother cloud of another genus, or is increasing in amount or invading the sky (usually in the form of parallel bands in a radiatus configuration) ahead of an approaching weather disturbance.
The international synoptic code (or SYNOP) provides for reporting the three basic altitude ranges for tropospheric clouds, but makes no special provision for multi-level clouds that can occupy more than one altitude range at a particular time. Consequently, cloud genera with significant vertical development are coded as low when they form in the low or lower-middle altitude range of the troposphere and achieve vertical extent by growing upward into the middle or high altitude range, as is the case with cumulus and cumulonimbus. Conversely, nimbostratus is coded as middle because it usually initially forms at mid-altitudes of the troposphere and becomes vertically developed by growing downward into the low altitude range. Because of the structure of the SYNOP code, a maximum of three cloud symbols can be plotted for each reporting station that appears on the weather map; one symbol each for a low (or upward growing vertical) cloud type, a middle (or downward growing vertical) type, and one for a high cloud type.
The symbol used on the map for each of these levels at a particular observation time will be for the genus, species, variety, mutation, or cloud motion that is considered most important according to criteria set out by the World Meteorological Organization (WMO). If these elements for any synoptic cloud level at the time of observation are deemed to be of equal importance, then the type which is predominant in amount is coded by the observer and plotted on the weather map. Although the SYNOP code has no separate formal classification for vertical or multi-level clouds, the observer procedure for selecting numerical codes is designed to give high reporting priority to those genera or species that show significant vertical development.
The identification and reporting of clouds contribute to the process of weather forecasting. Satellite pictures used in conjunction with the cloud symbols plotted on weather maps provide the forecaster with important information about conditions within the troposphere and the weather systems that form as a result.
The presence of significant high cloud cover indicates an organized low-pressure disturbance or an associated warm front is about 300 km away from the point of observation. Clouds associated with warm fronts tend to be mostly high cirriform at first, changing to stratiform at progressively lower altitude levels as the front approaches. However, if cirrocumulus also appears, there is greater airmass instability arriving with the front which increases the risk that thunderstorms may accompany the system. When these high clouds progressively invade the sky and the barometric pressure begins to fall, precipitation associated with the disturbance is likely about 6 to 8 hours away. A thickening and lowering of cirrostratus into mid-level altostratus is a good sign the warm front or low has moved closer and precipitation may begin within less than six hours hours. A further thickening of the altostratus is often accompanied by virga and the arrival of precipitation is imminent. The cloud layer achieves significant vertical extent as it lowers and changes into nimbostratus. Rain or snow begins to reach surface level at the beginning of a precipitation event that can last up to 36 hours depending and the size of the weather system and its speed of movement. As the low and the warm front pass, the nimbostratus thins out into low stratus and the precipitation tapers off.
A cold front tends to give less warning of its approach because it usually moves faster than a warm front and has a narrower band of clouds and weather. If the cold front is active enough to produce thunderstorms, anvil cirrus clouds may spread ahead of the front as a warning of its approach. The other cloud types associated with a cold front depend on atmospheric conditions such as air mass stability and wind shear, but are mostly cumuliform or stratocumuliform, with mid-level altocumulus giving way to lower stratocumulus and intermittent light precipitation if there is only slight airmass instability. With significant instability, vertically developed cumulus or cumulonimbus with showers and thunderstorms will form along the front.
After the passage of the front, the sky usually clears as high pressure builds in behind the system, although significant amounts of cumulus or stratocumulus, often in the form of long bands called cloud streets may persist if the air mass behind the front remains humid. Small and unchanging amounts of cumulus or cirrus clouds in an otherwise clear sky are usually indications of continuing fair weather as long as the barometric pressure remains comparatively high.
There is evidence that clouds, especially in the weather-making troposphere, contain biological ice nuclei that may play a key role in the formation of precipitation. Bioprecipitation, the concept of rain-making bacteria, was proposed by David Sands from Bozeman Campus, Montana State University, USA. Such microbes – called ice nucleators – are found in rain, snow, and hail throughout the world. These bacteria may be part of a constant feedback between terrestrial ecosystems and tropospheric clouds and may even have evolved the ability to promote rainstorms as a means of dispersal. They may rely on the rainfall to spread to new habitats, much as some plants rely on windblown pollen grains.
Moderate vertical sub-group:
Towering vertical sub-group:
This class forms at altitudes of about 15,000–25,000 m (49,200–82,000 ft) during the winter when the stratosphere is coldest and has the best chance of triggering condensation caused by adiabatic cooling. It is typically very thin with an undulating cirriform appearance. Moisture is very scarce in the stratosphere, so cloud at this altitude range is rare and is usually restricted to polar regions where the air is coldest.
The formation of Polar stratospheric cloud is limited to a single very high range of altitude, so this class is not divided into height-related families. Polar stratospheric has a generally cirriform structure and appearance and does not have separate genus types, species, or varieties. Instead, the classification is alpha-numeric and is based on chemical makeup rather than variations in physical appearance.
Polar mesospheric clouds are the highest in the atmosphere and occur mostly at altitudes of 80 to 85 km (50 to 53 mi), which is about ten times the altitude of tropospheric high clouds. From ground level, they can occasionally be seen illuminated by the sun during deep twilight. Ongoing research indicates that convective lift in the mesophere is strong enough during the polar summer to cause adiabatic cooling of small amount of water vapour to the point of saturation. This tends to produce the coldest temperatures in the entire atmosphere just below the mesopause. These conditions result in the best environment for the formation of polar mesospheric clouds. There is also evidence that smoke particles from burnt-up meteors provide much of the condensation nuclei required for the formation of noctilucent cloud.
Because of the need for maximum cooling of the water vapor to produce these clouds, their distribution tends to be restricted to polar regions of Earth during the respective summer seasons in the northern and southern hemispheres. Sightings are rare more than 40 degrees south of the north pole or north of the south pole.
Polar mesospheric clouds all tend to form at an extreme altitude range and are consequently not classified into height-related families. They are given the Latin name Noctilucent because of their illumination well after sunset and before sunrise. An alpha-numeric classification is used to identify variations in physical appearance.
Striking cloud colorations can be seen at many altitudes in the homosphere, which includes the troposphere, stratosphere, and mesophere. The first recorded colored cloud was seen by Nathan Ingleton in 1651, he wrote the event in his diary but the records were destroyed in 1666, in the Great Fire of London. The color of a cloud, as seen from Earth, tells much about what is going on inside the cloud.
In the troposphere, dense, deep clouds exhibit a high reflectance (70% to 95%) throughout the visible spectrum. Tiny particles of water are densely packed and sunlight cannot penetrate far into the cloud before it is reflected out, giving a cloud its characteristic white color, especially when viewed from the top. Cloud droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with depth into the gases. As a result, the cloud base can vary from a very light to very-dark-grey depending on the cloud's thickness and how much light is being reflected or transmitted back to the observer. Thin clouds may look white or appear to have acquired the color of their environment or background. High tropospheric clouds appear mostly white if composed entirely of ice crystals or supercooled water droplets.
As a tropospheric cloud matures, the dense water droplets may combine to produce larger droplets. If the droplets become too large and heavy to be kept aloft by the air circulation, they will fall from the cloud as rain. By this process of accumulation, the space between droplets becomes increasingly larger, permitting light to penetrate farther into the cloud. If the cloud is sufficiently large and the droplets within are spaced far enough apart, a percentage of the light that enters the cloud is not reflected back out but is absorbed giving the cloud a darker look. A simple example of this is one's being able to see farther in heavy rain than in heavy fog. This process of reflection/absorption is what causes the range of cloud color from white to black.
Other colors occur naturally in tropospheric clouds. Bluish-grey is the result of light scattering within the cloud. In the visible spectrum, blue and green are at the short end of light's visible wavelengths, whereas red and yellow are at the long end. The short rays are more easily scattered by water droplets, and the long rays are more likely to be absorbed. The bluish color is evidence that such scattering is being produced by rain-size droplets in the cloud. A cumulonimbus cloud that appears to have a greenish/bluish tint is a sign that it contains extremely high amounts of water; hail or rain. Supercell type storms are more likely to be characterized by this but any storm can appear this way. Coloration such as this does not directly indicate that it is a severe thunderstorm, it only confirms its potential. Since a green/blue tint signifies copious amounts of water, a strong updraft to support it, high winds from the storm raining out, and wet hail; all elements that improve the chance for it to become severe, can all be inferred from this. In addition, the stronger the updraft is, the more likely the storm is to undergo tornadogenesis and to produce large hail and high winds. Yellowish clouds may occur in the late spring through early fall months during forest fire season. The yellow color is due to the presence of pollutants in the smoke. Yellowish clouds caused by the presence of nitrogen dioxide are sometimes seen in urban areas with high air pollution levels.
Within the troposphere, red, orange, and pink clouds occur almost entirely at sunrise/sunset and are the result of the scattering of sunlight by the atmosphere. When the angle between the sun and the horizon is less than 10 percent, as it is just after sunrise or just prior to sunset, sunlight becomes too red due to refraction for any colors other than those with a reddish hue to be seen. The clouds do not become that color; they are reflecting long and unscattered rays of sunlight, which are predominant at those hours. The effect is much like if one were to shine a red spotlight on a white sheet. In combination with large, mature thunderheads, this can produce blood-red clouds. Clouds look darker in the near-infrared because water absorbs solar radiation at those wavelengths.
In high latitude regions of the stratosphere, nacreous clouds occasionally found there during the polar winter tend to display quite striking displays of mother-of-pearl colorations. This is due to the refraction and diffusion of the sun's rays through thin clouds with supercooled droplets that often contain compounds other than water. At still higher altitudes up in the mesospere, noctilucent clouds made of ice crystals are sometimes seen in polar regions in the summer. They typically have a silvery white coloration that can resemble brightly illuminated cirrus.
The role of tropospheric clouds in regulating weather and climate remains a leading source of uncertainty in projections of global warming. This uncertainty arises because of the delicate balance of processes related to clouds, spanning scales from millimeters to planetary. Hence, interactions between the large-scale (synoptic meteorology) and clouds becomes difficult to represent in global models. The complexity and diversity of clouds, as outlined above, adds to the problem. On the one hand, white-colored cloud tops promote cooling of Earth's surface by reflecting short-wave radiation from the sun. Most of the sunlight that reaches the ground is absorbed, warming the surface, which emits radiation upward at longer, infrared, wavelengths. At these wavelengths, however, water in the clouds acts as an efficient absorber. The water reacts by radiating, also in the infrared, both upward and downward, and the downward long-wave radiation results in some warming at the surface. This is analogous to the greenhouse effect of greenhouse gases and water vapor.
High tropospheric genus-types, cirrus, cirrocumulus, and cirrostratus, particularly show this duality with both short-wave albedo cooling and long-wave greenhouse warming effects. On the whole though, ice-crystal clouds in the upper troposphere tend to favor net warming. However, the cooling effect is dominant with lower stratocumuliform and stratiform clouds made of very small water droplets that have an average diameter of about 0.002 mm (.00008 in)., especially when they form in extensive sheets that block out more of the sun. These include middle-altitude layers of altocumulus and altostratus as well as low stratocumulus, and stratus. Small-droplet aerosols are not good at absorbing long-wave radiation reflected back from Earth, so there is a net cooling with almost no long-wave effect. This effect is particularly pronounced with low clouds that form over water. Low and vertical heaps of cumulus, towering cumulus, and cumulonimbus are made of larger water droplets ranging in diameter from .005 to about 0.3mm. Nimbostratus cloud droplets can also be quite large, up to 0.3mm in diameter. These larger droplets associated with vertically developed clouds are better able to trap the long-wave radiation thus migitating the cooling effect to some degree. However, these large often precipitating clouds are variable or unpredictable in their overall effect because of variations in their concentration, distribution, and vertical extent. Measurements taken by NASA indicate that on the whole, the effects of low and middle clouds that tend to promote cooling are outweighing the warming effects of high clouds and the variable outcomes associated with vertically developed aerosols.
As difficult as it is to evaluate the effects of current cloud cover characteristics on climate change, it is even more problematic to predict the outcome of this change with respect to future cloud patterns and events. As a consequence, much research has focused on the response of low and vertical clouds to a changing climate. Leading global models can produce quite different results, however, with some showing increasing low-level clouds and others showing decreases.
Polar stratospheric and mesospheric clouds are not common or widespread enough to have a significant effect on climate. However an increasing frequency of occurrence of noctilucent clouds since the 19th century may be the result of climate change.
New research indicates a global brightening trend. The details are not fully understood, but much of the global dimming (and subsequent reversal) is thought to be a consequence of changes in aerosol loading in the atmosphere, especially sulfur-based aerosol associated with biomass burning and urban pollution. Changes in aerosol burden can have indirect effects on clouds by changing the droplet size distribution or the lifetime and precipitation characteristics of clouds.
Cloud cover has been seen on most other planets in the solar system. Venus's thick clouds are composed of sulfur dioxide and appear to be almost entirely stratiform. They are arranged in three main layers at altitudes of 45 to 65 km that obscure the planet's surface and can produce virga. No embedded cumuliform types have been identified, but broken stratocumuliform wave formations are sometimes seen in the top layer that reveal more continuous layer clouds underneath. On Mars, cirrus, cirrocumulus and stratocumulus composed of water-ice have been detected mostly near the poles. Water-ice fogs have also been detected on this planet.
Both Jupiter and Saturn have an outer cirriform cloud deck composed of ammonia, an intermediate stratiform haze-cloud layer made of ammonium hydrosulfide, and an inner deck of cumulus water clouds. Embedded cumulonimbus are known to exist near the Great Red Spot on Jupiter. The same category-types can be found covering Uranus, and Neptune, but are all composed of Methane. Saturn's moon Titan has cirrus clouds believed to be composed largely of methane. The Cassini–Huygens Saturn mission uncovered evidence of a fluid cycle on Titan, including lakes near the poles and fluvial channels on the surface of the moon.
In October 2013, the detection of high altitude optically thick clouds in the atmosphere of Kepler-7b was announced, and, in December 2013, also in the atmospheres of GJ 436 b and GJ 1214 b.
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