Cloud

Cumulus cloudscape over Swifts Creek, Australia

For other uses, see Cloud (disambiguation).

A cloud is a visible mass of water droplets or frozen ice crystals suspended in the Earth's atmosphere above the surface of the Earth or other planetary body. Clouds in the Earth's atmosphere are studied in the nephology or 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. Generally, precipitation will fall to the surface; an exception is virga which evaporates before reaching the surface. Clouds can show convective development like cumulus, be in the form of layered sheets such as stratus, or appear in thin fibrous wisps as with cirrus. Prefixes are used in connection with clouds: strato for low cumulus-category clouds that show some stratiform characteristics, nimbo for low to middle 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. Some cloud types can form in the low or middle ranges depending on the moisture content of the air. Clouds 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 the 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, or sulfuric acid.

Part of the Nature series on Weather
Calendar seasons
Spring · Summer Autumn · Winter
Tropical seasons Dry season · Wet season
Storms
Thunderstorm · Supercell Downburst · Lightning Tornado · Waterspout Tropical cyclone (Hurricane) Extratropical cyclone Winter storm · Blizzard · Ice storm Dust storm · Firestorm  · Cloud
Precipitation
Drizzle · Rain  · Snow · Graupel Freezing rain · Ice pellets · Hail
Topics
Meteorology · Climate Weather forecasting Heat wave · Air pollution
Weather portal
v · d · e

Contents 1 Latin tropospheric nomenclature: historical background 2 Formation: How the air becomes saturated 2.1 Cooling air to its dew point 2.2 Adding moisture to the air 3 Tropospheric classification 3.1 Physical categories 3.2 Families and cross-classification into genera 3.3 Species 3.4 Varieties 3.5 Supplementary features 3.6 Stratocumulus fields 4 Summary of families, genera, species, and associated weather 4.1 High (Family A) 4.2 Middle (Family B) 4.3 Low (Family C1) 4.4 Low to middle with some vertical extent (Family C2) 4.5 Low to middle with considerable vertical extent (Family D) 5 Above the troposphere 6 Colouration 7 Effects on climate 7.1 Global brightening 7.2 Rainmaking bacteria 8 Other worlds

Latin tropospheric nomenclature: historical background

Altocumulus lenticularis forming over mountains in Wyoming with lower layer of cumulus mediocris and higher layer of cirrus spissatus.

Luke Howard, a methodical observer who had a strong grounding in the Latin language, used his background to categorize the various tropospheric cloud types and forms during December of 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. Even though their naming schemes were different, Howard's won out as Lamarck's did not even make an impression in his home country of France as it used unusual French names for cloud types, whereas Howard used universally accepted Latin. Howard's naming scheme 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. Later 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 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 respectively given the genus names cirrocumulus and cirrostratus. 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 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 respectively cumuliform and stratiform cloud genera of a newly defined middle height range above stratocumulus and stratus but below cirrocumulus and cirrostratus, with cumulus and nimbus occupying more than one altitude range. 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 genus cumulonimbus which would be distinct from cumulus. 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 which was renamed Nimbostratus.^[2]

Formation: How the air becomes saturated

Cooling air to its dew point

Late-summer rainstorm in Denmark

Clouds generally form when rising air is cooled to its dew point, the temperature at which the air becomes saturated.^[3] Water vapour normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. Condensation at surface level results in the formation of fog. If sufficient condensation particles are not present, the air will become supersaturated and the formation of cloud or fog will be inhibited.
There are four main mechanisms for cooling the air to its dew point: adiabatic cooling which tends to produce cloud, and conductive, radiational, and evaporative cooling that can result in the formation of fog. Adiabatic cooling occurs when air rises and expands.^[4] The air can rise due to convection, large-scale atmospheric lift along weather fronts and around centres of low pressure, or as a result of being forced over a physical barrier such as a mountain (orographic lift).^[5] 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]

Adding moisture to the air

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,^[13]

Tropospheric classification

• 

  Low cloud weather map symbols
  
• 

  Middle cloud weather map symbols
  
• 

  High cloud weather map symbols
  

Physical categories

As established by Howard, cloud types or genera are grouped into three physical categories: cirriform, cumuliform or convective, 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]

Vertical family D Cumulonimbus capillatus with anvil shaped incus supplementary feature. Higher layer of family A cirrus spissatus near top of image.

Cumuliform-category clouds are the product of localized convective or orographic lift.^[15] If the airmass is only slightly unstable, clouds of limited convection will form that show both cumuliform and stratiform characteristics. If a poorly organized weather system is present, weak intermittent precipitation may fall from these clouds. With greater airmass instability caused by a steeper temperature gradient from warm or hot at surface level to cold aloft, cumuliform clouds of free convection will form and rise to greater heights, especially if associated with fast moving unstable cold fronts. Large free-convective clouds can produce light to moderate showers if the airmass is sufficiently moist. The largest free-convective cumuliform types produce thunderstorms and a variety of types of lightning including cloud-to-ground that can cause wildfires,^[16]. Other convective severe weather may or may not be associated with thunderstorms and include heavy rain or snow showers, hail,^[17] strong wind shear, downbursts,^[18] and tornadoes.^[19]
Stratiform-category clouds generally 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.^[20] Precipitation is generally 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 force above by the front. Non-frontal low stratiform cloud can form when advection fog is lifted above surface level during breezy conditions.^[21]
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. They are generally 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 lowers stratiform layers that do.^[22]

Families and cross-classification into genera

Cloud classification by altitude of occurrence

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 C1 (low), family C2 (low to middle with some vertical extent), and family D (low to middle with considerable vertical extent). 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.^[23]
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).^[24]
Families C1 and C2: 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 C1 low stratus (St) and free-convective C2 low to middle cumulus (Cu). One prefixed cloud in this group is stratocumulus (Sc), a limited convection genus of the low altitude family C1 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 genus of the low to middle altitude family C2 that has some vertical extent and whose prefix refers to its ability to produce significant precipitation.^[25]
Family D: This family usually includes only large free-convective clouds of considerable vertical development 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). 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 anomoly. 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. ^[22]
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. As such, nimbostratus, although forming in the middle height range, is also sometimes classified as vertical because it can achieve 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.^[26]

Species

Natural beauty of cumulus fractus clouds in Nepali sky

Genus types are subdivided 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.^[27]
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 also occurs 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. Altostratus and nimbostratus share this physical appearance without significant variation or deviation and are therefore not formally 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.^[28]

Varieties

Genus and species types are further subdivided into varieties some of which are determined by the opacities of particular low and middle cloud structures (translucidus, opacus, and perlucidus; the last of which is opaque with translucent breaks). By implication rather than formal designation, all family A high clouds are 'translucidus'. Conversely, all clouds with at least some significant vertical extent, including low to middle family C2 nimbostratus and cumulus, are 'opacus', as are the tall vertical family D clouds, cumulonmbus and cumulus congestus.^[29]
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). The variety undulatus (having a wavy undulating base) is common to all high, middle, and low genera except those with significant vertical extent. Another common variety, duplicatus (closely spaced layers of the same genus, one above the other) is found with all the same genera except cirrocumulus. The variety radiatus is associated with cloud rows of a particular genus that appear to converge at the horizon and is seen mostly with cirrus, altocumulus, altostratus, stratocumulus, and cumulus.^[30]
Intortus and vertebratus varieties occur only with the genus cirrus and are respectively filaments twisted into irregular shapes and those that are arranged in fishbone patterns. Probably the most uncommonly seen is the variety lacunosus caused by localized downdrafts that punch circular holes into high, middle, and/or low cloud layers of limited convection.^[31]

Supplementary features

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 altostratus opacus which can produce widespread but usually light precipitation, and the thicker low to middle genera. Of the latter, cumulus mediocris produces only isolated light showers, while nimbostratus of the same family C2 is capable of heavier more extensive precipitation. Of the family D clouds, cumulus congestus can produce showers of moderately heavy intensity, with cumulonimbus having 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.^[32]
Another group of supplementary features are 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, while 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.^[33] 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 which 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. Mamma (sometimes known informally as mammatus) form on the bases of clouds as downward facing bubble-like protuberences caused by localized downdrafts within the cloud. 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 supplimentary 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 fields

Stratocumulus clouds can be organized into 'fields' that take on certain specially classified shapes and characteristics. They can often be found in the following forms:

• Actinoform, which resembles a leaf or a spoked wheel.
• Closed cell, which is cloudy in the center and clear on the edges, similar to a filled honeycomb.
• Open cell, which resembles a honeycomb, with clouds around the edges and clear, open space in the middle.^[34]

Summary of families, genera, species, and associated weather

High (Family A)

High family A cirrus uncinus and cirrus fibratus upper left merging into cirrostratus fibratus with some higher cirrocumulus floccus upper right.

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.^[23]
Family A includes:

• Genus Cirrus (Ci): Fibrous wisps of delicate white ice crystal cloud that show up clearly against the blue sky.

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.

• • Species Cirrus fibratus (Ci fib): Fibrous cirrus with no tufts or hooks.
  • Species Cirrus uncinus (Ci unc): Hooked cirrus filaments.
  • Species Cirrus spissatus (Ci spi): Patchy dense cirrus.
  • Species Cirrus castellanus (Ci cas): Partly turreted cirrus.
  • Species Cirrus floccus (Ci flo): Partly tufted cirrus.
• Genus Cirrocumulus (Cc): A cloud layer of limited convection composed of ice crystals and/or supercooled water droplets appearing as small white rounded masses or flakes in groups or lines with ripples like sand on a beach. They occasionally form alongside cirrus and/or cirrostratus clouds at the very leading edge of an active weather system.
  • Species Cirrocumulus stratiformis (Cc str): Sheets or relatively flat patches of cirrocumulus.
  • Species Cirrocumulus lenticularis (Cc len): Lens shaped cirrocumulus.
  • Species Cirrocumulus castellanus (Cc cas): Turreted cirrocumulus.
  • Species Cirrocumulus floccus (Cc flo): Tufted cirrocumulus.
• Genus Cirrostratus (Cs): A thin non-convective ice crystal veil that typically gives rise to halos caused by refraction of the sun's rays. The sun and moon are visible in clear outline. Cirrostratus typically thickens into altostratus ahead of a warm front or low pressure area.
  • Species Cirrostratus fibratus (Cs fib): Fibrous cirrostratus less detached than cirrus.
  • Species Cirrostratus nebulosus (Cs neb): A featureless veil of cirrostratus.^[35]

Middle (Family B)

Family B middle clouds over Santa Clarita, CA. Altocumulus floccus producing virga near top and middle of image merging into altostratus near horizon.

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 region. Generally the warmer the climate, the higher the cloud base. Family B usually comprises one cumuliform and one stratiform-category genus.^[36]
Family B includes:

• Genus Altocumulus (Ac): A cloud layer of limited convection usually in the form of irregular patches or rounded masses in groups, lines, or waves. High altocumulus may resemble cirrocumulus but is usually thicker and composed of water droplets so that the bases show at least some light grey shading. Opaque altocumulus associated with a weak frontal or low presssure disturbance can produce very light intermittent precipitation.
  • Species Altocumulus stratiformis (Ac str): Sheets or relatively flat patches of altocumulus.
  • Species Altocumulus lenticularis (Ac len): Lens shaped altocumulus.
  • Species Altocumulus castellanus (Ac cas): Turreted altocumulus.
  • Species Altocumulus floccus (Ac flo): Tufted altocumulus.
• Genus Altostratus (As): An opaque or translucent non-convective veil of grey/blue-grey cloud that often forms along warm fronts and around low pressure areas where it may thicken into nimbostratus. Altostratus is usually composed of water droplets but may be mixed with ice crystals at higher altitudes. Widespread opaque altostratus can produce light continuous or intermittent precipitation.
  • Altostratus is not subdivided into species.^[37]

Low (Family C1)

Low family C1 stratocumulus stratiformis clouds mainly in foreground with low to middle family C2 cumulus humilis and cumulus mediocris in the foreground and background

Low clouds are found from near surface up to 6,500 ft (2,000 m).^[23] Family C1 also typically includes one cumuliform and one stratiform-category genus.^[38] 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 C1 includes:

• Genus Stratocumulus (Sc): A cloud layer of limited convection usually in the form of irregular patches or rounded masses similar to altocumulus but having larger elements with deeper gray shading. Opaque stratocumulus associated with a weak frontal or low pressure distrubance can produce very light intermittent precipitation. This cloud often forms under a precipitating deck of altostratus or high based nimbostratus associated with a well developed warm front, slow moving cold front, or low pressure area. This can create the illusion of continuous precipitation of more than very light intensity falling from stratocumulus.
  • Species Stratocumulus stratiformis (Sc str): Sheets or relatively flat patches of stratocumulus.
  • Species Stratocumulus lenticularis (Sc len): Lens shaped stratocumulus.
  • Species Stratocumulus castellanus (Sc cas): Turreted stratocumulus.
• Genus Stratus (St): A uniform layer of non-convective cloud resembling fog but not resting on the ground.
  • Species Stratus nebulosus (St neb): A featureless veil of stratus sometimes producing light drizzle.
  • Species Stratus fractus (St fra): A ragged broken up sheet of stratus that often forms in precipitation falling from a higher cloud deck. This species may also result from a continuous sheet of stratus becoming broken up by the wind. ^[39]

Low to middle with some vertical extent (Family C2)

Low to middle family C2 nimbostratus cloud covering the sky with a scattered layer of low family C1 stratus fractus in the middle of the upper half of the image.

Family C2 clouds can be based 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 but the base typically subsides into the low range during precipitation.^[23] Both cloud types can achieve significant thickness and are sometimes classified as family D, especially in Europe.^[40] However, regular cumulus, by definition, does not match the vertical extent of towering cumulus (cumulus congestus) or cumulonimbus. Very thick nimbostratus can approximate towering cumulus, but falls well short the vertical extent of well developed cumulonimbus clouds.
Family C2 includes:

• Genus Cumulus^[41] (Cu): Clouds of free convection with clear-cut flat bases and domed tops. Towering cumulus (cumulus congestus) are usually classified as family D clouds of considerable vertical development.
  • Species Cumulus fractus (Cu fra): Cumulus clouds broken up into ragged and changing fragments.
  • Species Cumulus humilis (Cu hum): Small cumulus clouds usually with just a light grey shading underneath.
  • Species Cumulus mediocris (Cu med): Cumulus clouds of moderate size with medium grey shading underneath. Cumulus mediocris can produce scattered showers of light intensity.
• Genus Nimbostratus (Ns): A diffuse dark grey non-convective layer that looks feebly illuminated from the inside. It normally forms from altostratus along warm fronts and around low pressure areas and produces widespread steady precipitation that can reach moderate or heavy intensity.
  • Nimbostratus is not subdivided into species.^[42]

Low to middle with considerable vertical extent (Family D)

Stages of a cumulonimbus cloud's life.

These clouds can have strong vertical currents and rise far above their bases which form anywhere from near surface to about 10,000 ft (3,000 m) and consquently elude classification into the traditional three height ranges Like smaller cumuliform clouds in family C2, these towering giants usually form in the low altitude range but the bases can rise into the middle range when the moisture content of the air is very low. Unlike families A through C2 that each include a cumuliform and stratiform-category genus, family D instead has one cumuliform-category genus and one cumulus species, but no stratiform-category genus or species except for the occasional inclusion of very thick nimbostratus.^[43]
Family D includes:

• Genus Cumulonimbus (Cb): Heavy towering masses of free convective cloud with dark grey to nearly black bases that are associated with thunderstorms and showers. Thunderstorms can produce a range of severe weather that includes hail, tornadoes, a variety of other localized strong wind events, several types of lightning, and local very heavy downpours of rain that can cause flash floods, although lightning is the only one of these that requires a thunderstorm to be taking place. Generally, cumulonimbus require moisture, an unstable air mass, and a lifting force (heat) in order to form. Cumulonimbus typically go through three stages: the developing stage, the mature stage, and the dissipation stage.^[44] The average thunderstorm has a 24 km (15 mi) diameter. Depending on the conditions present in the atmosphere, these three stages take an average of 30 minutes to go through.^[45]
  • Species Cumulonimbus calvus (Cb cal): Cumulonimbus clouds with very high clear-cut domed tops similar to towering cumulus.
  • Species Cumulonimbus capillatus (Cb cap): Cumulonimbus clouds with very high tops that have become fibrous due to the presence of ice crystals.
• Genus Cumulus (Cu)^[46][47]
  • Species Cumulus congestus (WMO: Cu Con/ICAO: TCu): Towering cumulus clouds of great vertical size, usually with dark grey bases, and capable of producing showers of moderately heavy intensity.
    • Pyrocumulus (No official abbreviation): Cumulus clouds associated with volcanic eruptions and large scale fires. Pyrocumulus is not recognised by the WMO as a distinct genus or species.^[48]

Above the troposphere

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^[49] in the mesosphere and stratosphere respectively. Polar stratospheric clouds occur 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. The polar air in the mesosphere is, counter-intuitively, colder during the summer so it is mostly at this time of year that noctilucent clouds are seen.^[50][51]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),^[52] in the mesosphere. Most clouds above the troposphere have a wispy or fibrous appearance and can be mistakenly identified as high tropospheric cirrus clouds.

Colouration

An occurrence of altocumulus and cirrocumulus cloud iridescence

Sunset reflecting shades of pink onto grey stratocumulus clouds.

The color of a cloud, as seen from the 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.^[53] 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 which enters the cloud is not reflected back out before it is absorbed. A simple example of this is 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.^[54]
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, while 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-sized droplets in the cloud. A greenish tinge to a cloud is produced when sunlight is scattered by ice. A cumulonimbus cloud emitting green is a sign that it is a severe thunderstorm,^[55] capable of heavy rain, hail, strong winds and possible tornadoes. 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.^[56]
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.^[55] 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.

Effects on climate

Global cloud cover, averaged over the month of October, 2009. NASA composite satellite image; larger image available here.

See also: Cloud cover and Cloud feedback

The role of clouds in regulating weather and climate remains a leading source of uncertainty in projections of global warming.^[57] 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 the Earth's surface by reflecting shortwave radiation from the Sun. However radiation that makes it to the ground is reflected back in long wavelengths that are easily absorbed by water in the clouds resulting in a net warming at surface level.
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. Consequently, much research has focused on the response of low clouds to a changing climate. Leading global models can produce quite different results, though, with some showing increasing low-level clouds and other showing decreases.^[58][59]

Global brightening

New research indicates a global brightening trend.^[60] 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^[61] or the lifetime and precipitation characteristics of clouds.^[62]

Rainmaking bacteria

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 plants rely on windblown pollen grains.^[63][64]

Other worlds

Main article: Extraterrestrial atmospheres

Within our Solar System, any planet or moon with an atmosphere also has clouds. Venus's thick clouds are composed of sulfur dioxide.^[65] 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.^[66][67] Saturn's moon Titan has clouds believed to be composed largely of methane.^[68] 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.^[69][70]