Clouds

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, AustraliaCloud types or genera are grouped into three general categories: cirriform (wispy), stratiform (layered in sheets) and convective or cumuliform (heaped, rolled and/or rippled). These designations distinguish a cloud's physical structure and process of formation. 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. Each cloud genus is divided into species and/or varieties determined by more specific aspects of it structure and/or process of formation in any particular situation. 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. This system was proposed in 1802, when it was presented to the Askesian Society by Luke Howard.





A sky of cirrus clouds.[edit] High clouds (Family A)

High clouds will 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.[1]



Clouds in Family A include:



Genus Cirrus (Ci)

Genus Cirrocumulus (Cc)

Genus Cirrostratus (Cs)

[edit] 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.[1] The World Meterological Organization classifies Nimbostratus as a middle cloud that can thicken down into the low height range during precipitation.



Clouds in Family B include:



Genus Altocumulus (Ac)

Genus Altostratus (As)

[edit] Low clouds (Family C1)



Stratocumulus clouds, between Cumulus in the foreground and backgroundThese are found from near surface up to 6,500 ft (2,000 m)[1] and include the stratus (dense and grey). When stratus clouds contact the ground, they are called fog.



Clouds in Family C1 include:



Genus Stratocumulus (Sc)

Genus Stratus (St)

[edit] 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[2] (Cu)

Species Cumulus humilis (Cu hum)

Species Cumulus mediocris (Cu med)

Genus Nimbostratus (Ns)

[edit] Vertical clouds (Family D)



A typical anvil shaped Cumulonimbus incusThese clouds can have strong up-currents, rise far above their bases and form at many heights.



Clouds in Family D include:



Genus Cumulonimbus (associated with heavy precipitation and thunderstorms) (Cb)

Species Cumulonimbus calvus (Cb cal)

Species Cumulonimbus capillatus (Cb cap)

Variety Cumulonimbus incus (Cb inc)

Variety Cumulonimbus with mammatus (Cb mam) Official WMO term Cumulonimbus mamma.

Genus Cumulus (Cu)[3][4]

Species Cumulus congestus (TCu)

Pyrocumulus (No official abbreviation, not recognised by WMO as a distinct genus or species.)

[edit] 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.



[edit] 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.

[edit] 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.



[edit] 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.See also: Cloud cover and Cloud feedback

Understanding the role of clouds in regulating both weather and climate is at an early stage, and remains a critical unknown factor in predicting the extent of global warming.[citation needed]



[edit] Global brightening

New research indicates a global brightening trend.[5]



Global brightening is caused by decreased amounts of particulate matter in the atmosphere, leaving less surface area for condensation to occur. Less condensation in the atmosphere and more evaporation from increasing amounts of sunlight striking the surfaces of water causes more moisture to build in the air, creating fewer but more dense clouds.



[edit] Bacteria in clouds

Bacteria that live in clouds may have evolved the ability to promote rainstorms as a way to disperse themselves. These 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. They may rely on the rainfall to spread to new habitats, much as plants rely on windblown pollen grains, Christner said. [6]



[edit] Other planets

Main article: Extraterrestrial atmospheres

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.