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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. 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.
All weather-related clouds form in the troposphere, the lowest major layer of the 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 dust, ice, and salt 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. The average size of a newly formed droplet is around .02 mm (.0008 in). High clouds that form above the freezing level are composed mostly 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. Newly formed cloud droplets that are lifted beyond the condensation level tend to increase in number and coalesce until they grow to a size of up to .04 mm (.002 in). They 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.
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.
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 over the warmest parts of the various continents. 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.
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 (usually classified separately as nimbiform) producing significant precipitation.
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 would be formalized by the early 20th. century. 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 of altocumulus (physically more closely related to stratocumulus than to cumulus) and altostratus during the 1870s. 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 International Meteorological Committee (IMC) the designation of a new free-convective vertical genus type, cumulonimbus, which would be distinct from cumulus and nimbus and identifiable by its appearance and 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. One 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. 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. In 1976, the National Aeronautics and Space Administration (NASA) published a classification that added a fifth physical category, cumulonimbiform, in recognition of the unique and often complex structures of cumulonimbus clouds that are often more readily descerned by satellite photography than by surface observations.
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 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.
These are the product of localized free-convective lift. Incoming shortwave radiation generated by the sun reflects back as longwave radiation when it reaches the 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 often have complex structures 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.
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. 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.
In general, stratiform-category clouds 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.
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 bases depending on the moisture content of the air.
Clouds of the high family form at altitudes of 10,000 to 25,000 ft (3,000 to 7,600 m) in the polar regions, 16,500 to 40,000 ft (5,000 to 12,200 m) in the temperate regions and 20,000 to 60,000 ft (6,100 to 18,300 m) 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.
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 6,500 ft (2,000 m) above surface at any latitude, but may be based as high as 13,000 ft (4,000 m) near the poles, 23,000 ft (7,000 m) at mid latitudes, and 25,000 ft (7,600 m) in the tropics.
Low clouds are found from near surface up to 6,500 ft (2,000 m). This family 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.
Upward-growing free convective clouds have low to middle bases that form anywhere from near surface to about 8,000 ft (2,400 m) 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 non-convective 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 significant rain or snow over a wide area.
Some methods of cloud-height classification reserve the term vertical for upward-growing free-convective cumuliform and cumulonimbiform genera. Downward-growing nimbostratus is then classified as low  to denote it's normal base height range, or as middle, based on the altitude range at which it normally forms. Some classifications do not use a vertical family designation and include all free-convective cumuliform and cumulonimbiform types with the family of low clouds.
Nimbostratus and some cumulus clouds 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 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 turbulance. When towering vertical cloud types are considered separately as a group, 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.
In the modern system of nomenclature, cumulonimbus is something of an anomaly when classified strictly as a cumuliform-category cloud. The cumuliform designation appears in the prefix rather than the root, which refers instead to the cloud's ability to produce storms and heavy precipitation. This apparent reversal of prefix and root is a carry-over from the mid 19th century, when nimbus was the root word for all precipitating clouds. Since nimbus was classified separately from the cumulus, stratus, and cirrus categories during that time, it was usual to refer to it as a nimbiform cloud that comprised its own physical category. Even with modern classification, a mature cumulonimbus with its flat base, heaped midsection, and feathery top, along with accessory clouds that can appear in a variety of shapes and forms, has a composite structure that has led to a partial revival, initially by NASA, of the old nimbiform designation that uses the more restricted cumulonimbiform category name.
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. 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 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 identitification 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 cirrus or cirrostratus 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 stratiform in structure at all altitude levels. 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 24 to 36 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 24 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 movememnt. 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 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 stratocumulus 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 Montana State University. 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.
ICAO towering vertical:
These clouds form 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. Also known as mother of pearl clouds, they are typically very thin with a cirriform appearance.
Moisture is very scarce in the stratosphere, so clouds at these altitudes are rare. When they occur, it is usually over polar regions where the air is coldest.
These clouds all tend to form at the same very high range of altitudes and so are not divided into height-related families. They are typically similar in cirriform structure and appearance and therefore do not have genus types, species, or varieties. Instead, polar stratospheric clouds are given the name Nacreous based on their mother of pearl colorations and are sub-classified alpha-numerically 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. Clouds in the mesosphere form by adiabatic cooling of water vapor to supersaturation which leads to nucleation or condensation of ice crystals onto small dust particles. Upward propagating gravity waves from below the mesosphere caused by jet streams, cumulonimbus thunder clouds, and very high mountains carry water vapor up from the stratosphere to the upper mesosphere. This process occurs most strongly in the summer, and takes the place of frontal, cyclonic, and convective lift which cause most adiabatic cooling in the lower atmosphere. 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.
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 the 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 due to the refraction and diffusion of the sun's rays through thin ice crystal clouds that often contain compounds other than water. At still higher altitudes up in the mesospere, noctilucent clouds sometimes seen in polar regions in the summer usually appear a silvery white 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 shortwave 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 radiation results in a net warming at the surface. This is analogous to the greenhouse effect of greenhouse gases and water vapor.
High tropospheric clouds, such as cirrus, particularly show this duality with both shortwave albedo cooling and longwave greenhouse warming effects that nearly cancel or slightly favor net warming with increasing cloud cover. The shortwave effect is dominant with middle and low clouds like altocumulus and stratocumulus, which results in a net cooling with almost no longwave effect. As a consequence, much research has focused on the response of low 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.
Within our Solar System, any planet or moon with an atmosphere also has clouds. Venus's thick clouds are composed of sulfur dioxide. Mars has high, thin clouds of water ice. Both Jupiter and Saturn have an outer cloud deck composed of ammonia clouds, an intermediate deck of ammonium hydrosulfide clouds and an inner deck of water clouds. Saturn's moon Titan has 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. Uranus and Neptune have cloudy atmospheres dominated by water vapor and methane gas.
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