Cloud

A cloud is a visible mass of droplets of water or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae. Clouds are studied in the nephology or cloud physics branch of meteorology.

On Earth the condensing substance is typically water vapor, which forms small droplets or ice crystals, typically 0.01 mm (0.00039 in) in diameter. When surrounded by billions of other droplets or crystals they become visible as clouds. Dense deep clouds exhibit a high reflectance (70% to 95%) throughout the visible range of wavelengths. They thus appear white, at least from the top. Cloud droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with depth into the gases, hence the gray or even sometimes dark appearance at the cloud base. Thin clouds may appear to have acquired the color of their environment or background and clouds illuminated by non-white light, such as during sunrise or sunset, may appear colored accordingly. Clouds look darker in the near-infrared because water absorbs solar radiation at those wavelengths.

Classification

A cumulus cloudscape over Swifts Creek, Victoria, Australia

Cloud types or genera are grouped into three general categories: cirriform (mainly detached and wispy), cumuliform or convective (mostly detached and heaped, rolled or rippled), and stratiform (mainly continuous layers in sheets). These designations distinguish a cloud's physical structure and process of formation. Cumulus-type clouds are the product of localized convective lift in slightly to extremely unstable air. Stratus-type clouds generally form as the result of non-convective lift of relatively stable air, especially along slow moving fronts and around areas of low pressure. Most cirrus clouds are non-convective but occasionally aquire a tufted or turreted apparance caused by small scale high altitude convection. All weather-related cloud types form in the troposphere, the lowest major layer of the earth's atmosphere. The individual genus types result from the categories being cross-classified by height range within the troposphere. This is determined by the base height of the cloud, not the cloud top, and base height ranges may vary depending on the geographical zone. All Cirrus clouds are classified as high and thus constitute a single genus. Cumulus and stratus clouds in the high altitude range carry the prefix 'cirro', while similar genera in the middle range are prefixed by 'alto'. Any cumuliform or stratiform genus in the low or low to middle range either has no prefix or carries one that refers to a characteristic other than altitude. A vertically developed cloud genus or species typically occupies all altitude ranges and therefore has no height related prefix.

An early partial version of this system was proposed in 1802, when it was presented to the Askesian Society by Luke Howard, a British chemist and amateur meteorologist. He began with the three general categories described above: Cirriform, cumuliform, and stratiform. He then cross-classified these into lower and upper levels by identifying the cirriform category as always high by giving it the genus name cirrus. Howard additionally gave the genus names cirrocumulus and cirrostratus respectively to cumuliform and stratiform clouds forming in the upper height range. Lower level cumuliform and stratiform clouds kept their physical category names without prefixes which became their respective genus names, cumulus and stratus. To these, Howard added the genus Nimbus for all clouds producing significant precipitation.

Around 1840-41, German meteorologist Ludwig Kaemtz added stratocumulus as a mostly detached low cloud genus with both cumuliform and stratiform characteristics. 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 and altostratus during the 1870's. These were respectively cumuliform and stratiform cloud genera of a newly defined middle height range above stratocumulus and stratus but below cirrocumulus and cirrostratus. 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.^[1]

A sky of cirrus clouds.

High clouds (Family A)

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.^[2]

Clouds in Family A include:

• Genus Cirrus (Ci): Fibrous wisps of delicate white ice crystal cloud that show up clearly against the blue sky. Generally non-convective except castellanus and floccus species.
  • Species Cirrus fibratus (Ci fib)
  • Species Cirrus uncinus (Ci unc)
  • Species Cirrus spissatus (Ci spi)
  • Species Cirrus castellanus (Ci cas)
  • Species Cirrus floccus (Ci flo)
• Genus Cirrocumulus (Cc): A cloud layer of limited convection appearing as small white rounded masses or flakes in groups or lines with ripples like sand on a beach.
  • Species Cirrocumulus stratiformis (Cc str)
  • Species Cirrocumulus lenticularis (Cc len)
  • Species Cirrocumulus castellanus (Cc cas)
  • Species Cirrocumulus floccus (Cc flo)
• Genus Cirrostratus (Cs): A thin non-convective veil that typically gives rise to halos. Sun and moon visible in clear outline. Typically thickens into altostratus ahead of a warm front or low pressure area.
  • Species Cirrostratus fibratus (Cs fib)
  • Species Cirrostratus nebulosus (Cs neb)

Middle clouds (Family B)

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. Nimbostratus clouds are sometimes included with the middle clouds.^[2] The World Meterological Organization classifies Nimbostratus as a middle cloud that can thicken down into the low height range during precipitation.^[3]

Clouds in Family B include:

• 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 bases show at least some light grey shading.
  • Species Altocumulus stratiformis (Ac str)
  • Species Altocumulus lenticularis (Ac len)
  • Species Altocumulus castellanus (Ac cas)
  • Species Altocumulus floccus (Ac flo)
• Genus Altostratus (As): 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.
  • Not subdivided into species

Low clouds (Family C1)

Stratocumulus clouds, between Cumulus in the foreground and background

These are found from near surface up to 6,500 ft (2,000 m)^[2] and include the genus stratus. When stratus clouds contact the ground, they are called fog, although not all fog forms from stratus.

Clouds in Family C1 include:

• 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.
  • Species Stratocumulus stratiformis (Sc str)
  • Species Stratocumulus lenticularis (Sc len)
  • Species Stratocumulus castellanus (Sc cas)
• Genus Stratus (St): A uniform layer of non-convective cloud resembling fog but not resting on the ground.
  • Species Stratus nebulosus (St neb)
  • Species Stratus fractus (St fra)

Low to middle clouds (Family C2)

These clouds can be based anywhere from near surface to about 10,000 ft (3,000 m). 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 may subside into the low range during precipitaion.

Clouds in Family C2 include:

• Genus Cumulus^[4] (Cu): Clouds of free convection with clear-cut flat bases and domed tops. Towering cumulus (cumulus congestus) are usually classified as clouds of vertical development (Family D).
  • 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.
• Genus Nimbostratus (Ns): A diffuse dark grey non-convective layer that looks feebly illuminated from the inside. It is a precipitation cloud that normally forms along warm fronts and around low pressure areas.
  • Not subdivided into species

Vertical clouds (Family D)

A typical anvil shaped Cumulonimbus incus

These clouds can have strong up-currents, rise far above their bases and form at many heights.

Clouds in Family D include:

• Genus Cumulonimbus (Cb): Heavy towering masses of free convective cloud that are associated with thunderstorms and showers. They form in very instable air masses, especially along fast moving cold fronts.
  • 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.
    • Supplimentary Feature Cumulonimbus capillatus incus (Cb cap inc): An incus cloud top is one that has spread out into a clear anvil shape as a result of hitting the inversion layer at the top of the troposphere.
    • Supplimentary Feature Cumulonimbus with mammatus (Cb mam): A mammatus cloud base is characterized by downward facing bubble-like protuberances caused by localized downdrafts within the cloud. Official WMO term Cumulonimbus mamma.
• Genus Cumulus (Cu)^[5][6]
  • Species Cumulus congestus (WMO: Cu Con/ICAO: TCu): Towering cumulus clouds of great vertical size, usually with dark grey bases.
    • Pyrocumulus (No official abbreviation): Cumulus clouds associated with volcanic eruptions and large scale fires. Not recognised by WMO as a distinct genus or species.

Other clouds

A few clouds can be found above the troposphere; these include noctilucent and polar stratospheric clouds (or nacreous clouds), which occur in the mesosphere and stratosphere respectively.

Some clouds form as a consequence of interactions with specific geographical features. Perhaps the strangest geographically specific cloud in the world is 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.

Cloud fields

A cloud field is simply a group of clouds but sometimes cloud fields can take on certain shapes that have their own characteristics and are specially classified. Stratocumulus clouds 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.

Colors

Cloud iridescence occurring in clouds

Sunset reflecting shades of grey and pink in clouds.

The color of a cloud, as seen from the Earth, tells much about what is going on inside the cloud. Clouds form because warm air tends to absorb water vapor, which is lighter than air, thus helping the mixture to rise. As it rises the air containing it cools and the vapor tends to condense out of the air as micro-droplets. These 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. As a 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. For the same reason, the undersides of large clouds and heavy overcasts can appear as various degrees of grey shades, depending on how much light is being reflected or transmitted back to the observer.

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 an imminent sign of heavy rain, hail, strong winds and possible tornadoes.

Yellowish clouds are rare but 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.

Red, orange and pink clouds occur almost entirely at sunrise/sunset and are the result of the scattering of sunlight by the atmosphere. 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 and climate

Global cloud cover, averaged over the month of October, 2009. The outlines of the continents can often be traced through observations of clouds alone, with the sharpest outlines where very dry land is surrounded by ocean. NASA composite satellite image; larger image available here.

The role of clouds in regulating weather and climate remains a leading source of uncertainty in projections of global warming.^[7] 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. High clouds, such as cirrus, tend to have both shortwave and longwave (i.e., albedo and greenhouse) effects that nearly cancel, while low clouds like stratocumulus have a strong shortwave effect but almost no longwave effect. As such, 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.^[8][9]

Global brightening

New research indicates a global brightening trend.^[10] 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^[11] or the lifetime and precipitation characteristics of clouds^[12].

Bacteria in clouds

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, according to Brent Christner, a microbiologist at Louisiana State University. 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, Christner said.^[13][14]

Other planets

Within our Solar System, any planet or moon with an atmosphere also has clouds. Venus's clouds are composed of sulfuric acid droplets. 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. Uranus and Neptune have cloudy atmospheres dominated by methane gas.

Saturn's moon Titan has clouds believed to be composed largely of droplets of liquid 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.

Gallery

Mixed clouds over Santa Clarita, CA.	In mountainous areas one often finds the peaks above the clouds as seen here with the Piz Bernina in the Swiss Alps.	Clouds and cloud bow above the Pacific Ocean.	Rain clouds over the North Sea taken from the coast of Herne Bay, Kent.
Rain clouds in Kerala, India.	Lenticular cloud over Wyoming.	The Space Shuttle Endeavour launches through some clouds.	Water clouds on Earth seen from space

See also

References

Bibliography

• Hamblyn, Richard The Invention of Clouds – How an Amateur Meteorologist Forged the Language of the Skies Picador; Reprint edition (August 3, 2002). ISBN 0312420013
• http://www.ldeo.columbia.edu/news/2006/04_14_06.htm Could Reducing Global Dimming Mean a Hotter, Dryer World?

External links

• BadMeteorology's explanation of why clouds form
• Chitambo Clouds – Clouds and other meteorological phenomena Photographs and info. on different types of clouds.
• Monthly maps of global cloud cover, from NASA's Earth Observatory
• Cloud Appreciation Society Aesthetics of clouds
• A Planetary Order (Terrestrial Cloud Globe)
• Shuttle Views the Earth: Clouds from Space
• Details of main cloud types and sub types
• USA Today Understanding clouds and Fog
• clouds that look as if they were sculpted out of the sky