A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They 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's 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.
Clouds can show convective development like cumulus, appear in 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 cumuliform-category clouds that show some stratiform characteristics, nimbo for thick stratiform 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 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 for 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 where clouds in the stratosphere and mesosphere are observed. Clouds have been observed on other planets and moons within the Solar System, but, due to their different temperature characteristics, they are composed of other substances such as methane, ammonia, and sulfuric acid.
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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. 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.[1]
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 all clouds producing significant precipitation.[2]
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. 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 cumuliform (of limited convection) and stratiform cloud genera, respectively, 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 renamed Nimbostratus.[2]
In general, clouds will form in the troposphere 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. Depending on the temperature at the altitude of saturation, the vapor condenses into tiny visible water droplets or sublimates into ice crystals that can be held aloft by normal wind currents. The main mechanism behind this process is Adiabatic cooling.[3] Atmospheric pressure decreases with altitude, so the rising air expands in a process that expends energy which causes the air to cool, which reduces its capacity to hold water vapor. If it is cooled to its dew point and becomes saturated, it sheds vapor it can no longer retain which condenses into cloud.
The altitude at which this happens is called the lifted condensation level which roughly determines the height of the cloud base.[4] Water vapor in saturated air is normally attracted to condensation nuclei such as dust, ice, and salt. If the condensation level is below the freezing level in the troposphere, the nuclei help transform the vapor into very small water droplets that appear as cloud. When this process takes place above the freezing level, the vapor will condense into supercooled water droplets; or at temperatures well below freezing, sublimate into ice crystals. An absence of sufficient condensation particles at the point of saturation will cause the rising air to become supersaturated and the formation of cloud will tend to be inhibited.
There are three main agents of vertical lift. One is the convective upward motion of locally unstable air caused by significant daytime solar heating at surface level. Air warmed in this way tends to become unstable and rises and cools until temperature equilibrium is achieved with the surroinding air aloft. If the air near surface becomes extremely warm and unstable, its upward motion can become quite explosive resulting in towering clouds that can break through the tropopause and/or cause severe weather. Another agent is the large-scale circulation of stable or slightly unstable air (which has been subjected to little or no surface heating) being lifted at weather fronts and around centres of low pressure, (frontal and cyclonic lift respectively in the northern hemisphere - frontal and anticyclonic lift in the southern hemisphere). More occasionally, very warm unstable air might be present around fronts and low pressure centres. Generally the more unstable the air, the higher the upward growing cloud tops will be, raising the potential for severe weather. A third lifting agent is wind circulation forcing stable or slightly unstable air over a physical barrier such as a mountain (orographic lift). If the air is generally stable, nothing more than lenticular or cap clouds will form. However, if the air becomes sufficiently moist and unstable, orographic showers and/or thunderstorms may appear.[5]
There are three other main mechanisms for cooling the air to its dew point: conductive, radiational, and evaporative. These can cause condensation at surface level resulting in the formation of fog. Conductive cooling occurs when the air comes into contact with a colder surface,[6] usually by being blown from one surface to another, for example from a liquid water surface to colder land. Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath.[7] Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature, or until it reaches saturation.[8]
The main ways water vapour is added to the air are: wind convergence over water or moist ground into areas of upward motion,[9] precipitation or virga falling from above,[10] daytime heating evaporating water from the surface of oceans, water bodies or wet land,[11] transpiration from plants,[12] and cool or dry air moving over warmer water. As with daytime heating, the addition of moisture to the air increases it's heat content and instability and helps set into motion those processes that lead to the formation of cloud and/or fog.[13]
As established by Howard, clouds are grouped into three physical categories: cirriform, cumuliform, and stratiform. These designations distinguish a cloud's physical structure and process of formation. All weather-related clouds form in the troposphere, the lowest major layer of the Earth's atmosphere.[14]
Cumuliform-category clouds are the product of localized convective or orographic lift. Incoming shortwave radiation generated by the Sun reflects back as longwave radiation when it reaches the Earth's surface, a process that warms the air closest to ground. The more the air is heated, the more unstable it will tend to become.[14] If the airmass is only slightly unstable, clouds of limited convection that show both cumuliform and stratiform characteristics will form. If a poorly organized weather system is present, weak intermittent precipitation may fall from these clouds. Greater airmass instability caused by more intense radiational surface heating by the Sun will create a steeper temperature gradient from warm or hot at surface level to cold aloft. This may cause larger clouds of free convection to form and rise to greater heights, especially if associated with fast-moving unstable cold fronts. Large free-convective types can produce light to moderate showers if the airmass is sufficiently moist. The largest free-convective towering clouds produce thunderstorms and a variety of types of lightning including cloud-to-ground that can cause wildfires.[15] Other convective severe weather may or may not be associated with thunderstorms and include heavy rain or snow showers, hail,[16] strong wind shear, downbursts,[17] and tornadoes.[18]
In general, stratiform-category clouds form as the result of non-convective lift of relatively stable air, especially along slow-moving warm fronts, around areas of low pressure, and sometimes along stable slow moving cold fronts.[14] In general, precipitation is steady and widespread, with intensity varying 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. Low 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.[19]
Cirriform-category clouds form mostly at high altitudes along the very leading edges of a frontal and/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 can merge and thicken into lower stratiform layers that do.[20]
The individual genus types result from the physical categories being cross-classified by height range family within the troposphere. These include family A (high), family B (middle), family C (low), family D1 (moderate vertical with low to middle bases), and family D2 (towering vertical with low to middle bases). The family designation for a particular genus is determined by the base height of the cloud and its vertical extent. The base height range for each family varies depending on the latitudinal geographical zone.[21]
Families A and B: All Cirriform-category clouds are classified as high range family A and thus constitute a single genus cirrus (Ci). Cumuliform and stratiform-category clouds in the high-altitude family carry the prefix cirro, yielding the respective genus names cirrocumulus (Cc) and cirrostratus (Cs). Similar genera in the middle-range family B are prefixed by alto, yielding the genus names altocumulus (Ac) and altostratus (As).[14]
Families C and D1: Any cumuliform or stratiform genus in these two families either has no prefix or carries one that refers to a characteristic other than altitude. The two non-prefixed genera are non-convective low stratus (St: family C),and free-convective moderate vertical cumulus (Cu: family D1). One prefixed cloud in this group is stratocumulus (Sc), a limited convection genus of the low-altitude family C that has some stratiform characteristics (as do the middle- and high-based genera altocumulus and cirrocumulus, the genus names of which exclude strato to avoid double-prefixing). The other prefixed cloud is nimbostratus (Ns), a non-convective family D1 genus that has moderate vertical extent and whose prefix refers to its ability to produce significant precipitation.[22]
Family D2: This family comprises large towering free-convective clouds that typically occupy all altitude ranges and, therefore, also carry no height related prefixes. They comprise the genus cumulonimbus (Cb) and the cumulus species cumulus congestus (Cu con). The latter type is designated towering cumulus (Tcu) by the International Civil Aviation Organization. Under conditions of very low humidity, free-convective clouds may form above the low-altitude range and, therefore, be found only at middle- and high-tropospheric altitudes. In the modern system of cloud nomenclature, cumulonimbus is something of an anomaly. The cumuliform-category 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 nineteenth century, when nimbus was the root word for all precipitating clouds.[20]
Major precipitation clouds: Although they do not comprise a family as such, cloud genera with nimbo or nimbus in their names are the principal bearers of precipitation. Although nimbostratus initially forms in the middle height range, it can be classified as moderate vertical because it achieves considerable thickness despite not being a convective cloud like cumulonimbus, the other main precipitating cloud genus. Frontal lift can push the top of a nimbostratus deck into the high-altitude range while precipitation drags the base down to low altitudes.[23] The World Meteorological Organization (WMO) classifies nimbostratus as a middle cloud whose base typically thickens down into the low altitude range during precipitation.[24]
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. The best examples of these are the species stratiformis, lenticularis, and castellanus, which are common to cumuliform genera of limited convection in the high-, middle-, and low-height ranges (cirrocumulus, altocumulus, and stratocumulus, respectively). Stratiformis species normally occur in extensive sheets or in smaller patches with 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. Castellanus structures, which resemble the turrets of a castle when viewed from the side, can also be found in convective patches of cirrus, as can the more detached tufted floccus species, which are common to cirrus, cirrocumulus, and altocumulus. However, floccus is not associated with stratocumulus in the lower levels where local airmass instability 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 will initially form as a cloudlet of the species humilis showing only slight vertical development. With increasing airmass instability, it will tend 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. [24]
Cirrus clouds have several additional species unique to the wispy structures of this genus, which include uncinus, filaments with upturned hooks, and spissatus, filaments that merge into dense patches. One exception is the species fibratus, which can be seen with cirrus and also with cirrostratus that is transitional to or from cirrus. Cirrostratus at its most characteristic tends to be mostly of the species nebulosus, which creates a rather diffuse appearance lacking in structural detail. All altostratus and nimbostratus clouds share this physical appearance without significant variation or deviation and, therefore, do not need to be subdivided into species. Low continuous stratus is also of the species nebulosus except when broken up into ragged sheets of stratus fractus. This latter fractus species also occurs with ragged cumulus.[24]
Genus and species types are further subdivided into varieties. Some varieties are determined by the opacities of particular low and middle cloud structures and comprise translucidus (translucent), opacus (opaque), and perlucidus (opaque with translucent breaks). These varieties are always identifiable for cloud genera and species with variable opacity, including family B altocumulus and altostratus, and family C stratocumulus and stratus. Opacity based varieties are not applied to family A high clouds that are always translucent, or conversely， to family D1 or D2 clouds with significant vertical extent that are always opaque. 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.[24]
Other varieties are determined by the arrangements of the 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 upper wind currents and air mass stability and/or humidity patterns favor their formation. The variety undulatus (having a wavy undulating base) can occur with high, middle, and low stratiform types and with limited convective cumuliform genera (usually of the stratiformis and/or lenticularis species) when there are uneven upper wind currents, but not with clouds of significant vertical extent. Another variety, duplicatus (closely spaced layers of the same type, one above the other), is sometimes found with any of the same genera and species except cirrocumulus, although not necessarily at the same time as the undulatus variety. The variety radiatus is seen when cloud rows of a particular type appear to converge at the horizon. It is sometimes seen with various species of cirrus, altocumulus, stratocumulus, and cumulus, and with the genus altostratus that has no species. Intortus and/or vertebratus varieties occur on occasion with cirrus types, and 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. Probably the most uncommonly seen is the variety lacunosus, caused by localized downdrafts that punch circular holes into high, middle, and/or low cumuliform cloud layers of limited convection, usually of the stratiformis species.[24]
It is possible for some species of low, middle, and high genera to show combined varieties at one time (but not the vertical clouds which are limited to the radiatus variety, and only with species of the genus cumulus), as with a translucent layer of altocumulus stratiformis arranged in seemingly converging rows. The full technical name of a cloud in this configuration would be altocumulus stratiformis translucidus radiatus, which would identify respectively its genus, species, and two combined varieties.[24]
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.
The hydrometeors are one group of supplementary features are not actual cloud formations but 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 family B altostratus opacus, which can produce widespread but usually light precipitation, and with clouds of the thicker moderate vertical family D1. Of the latter family, cumulus mediocris produces only isolated light showers, while nimbostratus is capable of heavier more extensive precipitation. Towering vertical family D2 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 and/or stratus fractus.[24]
Another group of supplementary features comprises cloud formations that are associated mainly with cumuliform clouds of free convection. 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. A tuba feature is a cloud column that may hang from the bottom of a cumulus or cumulonimbus. An arcus feature is a roll or shelf cloud that forms along the leading edge of a squall line or thunderstorm outflow.[24] 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. 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. 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.
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:
High clouds form between 10,000 and 25,000 ft (3,000 and 8,000 m) in the polar regions, 16,500 and 40,000 ft (5,000 and 12,000 m) in the temperate regions and 20,000 and 60,000 ft (6,000 and 18,000 m) in the tropical region. It is the only height range family that includes genera from all three physical categories.[21]
Family A includes:
Cirrus clouds are generally non-convective except castellanus and floccus species. They often form along a high altitude jetstream and at the very leading edge of a frontal or low pressure disturbance where they may merge into cirrostratus.
Middle clouds tend to form at 6,500 ft (2,000 m) but may form at heights up to 13,000 ft (4,000 m), 23,000 ft (7,000 m) or 25,000 ft (8,000 m) depending on the latitudinal region. In general, the warmer the climate the higher the cloud base. Family B usually comprises one cumuliform and one stratiform-category genus.[26]
Family B includes:
Low clouds are found from near surface up to 6,500 ft (2,000 m).[21] Family C also typically includes one cumuliform and one stratiform-category genus.[26] When low stratiform clouds contact the ground, they are called fog, although radiation and advection types of fog do not form from stratus layers.
Family C includes:
Family D1 clouds have low to middle bases anywhere from near surface to about 10,000 ft (3,000 m) and, therefore, do not fit very well into the conventional height ranges of low, middle, and high. This group continues the pattern of comprising one cumuliform and one stratiform-category genus. Cumulus usually forms in the low-altitude range, but bases may rise into the lower part of the middle range during conditions of very low relative humidity. Nimbostratus normally forms from altostratus in the middle-altitude range and achieves vertical extent when the base subsides into the low range during precipitation.[27] Some methods of cloud-height classification reserve the term vertical for upward-growing free-convective cumuliform clouds.[28] Downward-growing nimbostratus is then classified as low or middle, even when it becomes very thick as a result of this process - which is often augmented by frontal lift causing non-convective upward growth as well. Some authorities do not use a vertical family designation at all, and therefore also include free-convective cumulus types with the family of low clouds.[21]
Family D1 includes:
These clouds can have strong vertical currents and rise far above their bases, which form anywhere in the low to lower-middle altitude range from near surface to about 10,000 ft (3,000 m). Like smaller cumuliform clouds in family D1, these towering giants usually form in the low-altitude range at first, but the bases can rise into the middle range when the moisture content of the air is very low. Unlike families A through D1 that each include a cumuliform and stratiform-category genus, the family of towering clouds has instead one distinct cumuliform-category genus, cumulonimbus, and one species of cumulus, a genus otherwise considered a cloud of moderate vertical development. By definition, even very thick stratiform clouds cannot have towering vertical extent or structure, although they may be accompanied by embedded towering cumuliform types. As with the moderate vertical clouds, some authorities do not recognize a separate family of towering vertical types, and, instead, classify them as low family C. Others designate vertical clouds separately from low, middle, and high, but consider moderate and towering vertical types to be a single family. However, the International Civil Aviation Organization (ICAO) distinguishes towering vertical clouds by specifying that these very large family D2 cumuliform types must be identified by genus names or standard abbreviations in all aviation observations (METARS) and forecasts (TAFS) to warn pilots of possible severe weather and turbulance.[30]
Family D2 includes:
Non-WMO varient
A few relatively uncommon clouds can be found above the troposphere, where moisture is very scarce. These include polar mesospheric noctilucent clouds and nacreous polar stratospheric clouds. They are composed mostly of ice crystals and occur at high latitudes, mostly within 40 degrees of the poles[36] in the mesosphere and stratosphere, respectively. Most clouds above the troposphere have a wispy or fibrous appearance and can be mistakenly identified as high-tropospheric cirrus clouds.
Nacreous clouds occur in the stratosphere most typically at altitudes of 15,000–25,000 m (50,000–80,000 ft) during the winter when that part of the atmosphere is coldest and has the best chance of triggering condensation. Also known as mother of pearl clouds, they are typically very thin with a cirriform appearance. Nacreous clouds are sub-classified alpha-numerically based on chemical makeup rather than variations in physical appearance.
Subtypes
The polar air in the mesosphere is colder during the summer so it is mostly at this time of year that noctilucent clouds are seen.[37][38] They can occasionally be seen illuminated by the sun during deep twilight at ground level. Noctilucent clouds are the highest in the atmosphere and occur mostly at altitudes of 80 to 85 kilometers (50 to 53 mi),[39] in the mesosphere. An alpha-numeric sub-classification is used to identify variations in physical appearance.
Subtypes
Subtypes
Subtypes
The first recorded coloured cloud was seen by Nathan Ingleton in 1651, he wrote the event in his diary but the records were destroyed on 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. Dense, deep tropospheric 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.[40] 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 and non-tropospheric clouds appear mostly white if composed entirely of ice crystals and/or supercooled water droplets.
As a tropospheric cloud matures, the dense water droplets may combine to produce larger droplets, which may combine to form droplets large enough to fall 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, it may be that a percentage of the light that enters the cloud is not reflected back out before it is absorbed. 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.[41]
Other colors occur naturally in 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.[42] 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.[43]
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.[44] 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.
The role of clouds in regulating weather and climate remains a leading source of uncertainty in projections of global warming.[45] 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 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 other showing decreases.[46][47]
New research indicates a global brightening trend.[48] 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[49] or the lifetime and precipitation characteristics of clouds.[50]
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 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.[51][52]
Within our Solar System, any planet or moon with an atmosphere also has clouds. Venus's thick clouds are composed of sulfur dioxide.[53] 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.[54][55] Saturn's moon Titan has clouds believed to be composed largely of methane.[56] 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.[57][58]
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