Cirrus (cloud classification symbol:Ci) is agenus of highcloud made of icecrystals. Cirrus clouds typically appear delicate and wispy with white strands. In the Earth's atmosphere, cirrus are usually formed when warm, dry air rises, causingwater vapordeposition onto mineral dust and metallic particles at high altitudes. Globally, they form anywhere between 4,000 and 20,000 meters (13,000 and 66,000 feet) abovesea level, with the higher elevations usually in thetropics and the lower elevations in morepolar regions.
Cirrus clouds can form from the tops ofthunderstorms andtropical cyclones and sometimes predict the arrival ofrain or storms. Although they are a sign that rain and maybe storms are on the way, cirrus themselves drop no more thanfalling streaks of ice crystals. These crystals dissipate, melt, and evaporate as they fall through warmer and drier air and never reach the ground. The wordcirrus comes from theLatin prefixcirro-, meaning "tendril" or "curl".[1] Cirrus clouds warm the earth, potentially contributing toclimate change.[2] A warming earth will likely produce more cirrus clouds, potentially resulting in aself-reinforcing loop.[3]
Optical phenomena, such assun dogs andhalos, can be produced by light interacting with ice crystals in cirrus clouds. There are two other high-level cirrus-like clouds calledcirrostratus andcirrocumulus. Cirrostratus looks like a sheet of cloud, whereas cirrocumulus looks like a pattern of small cloud tufts. Unlike cirrus and cirrostratus, cirrocumulus clouds contain droplets ofsupercooled (belowfreezing point) water.
Cirrus are wispy clouds made of long strands of ice crystals that are described as feathery,[4] hair-like, or layered in appearance.[5] First defined scientifically byLuke Howard in an 1803 paper,[6] their name is derived from theLatin wordcirrus, meaning 'curl' or 'fringe'.[7] They aretransparent, meaning that the sun can be seen through them. Ice crystals in the clouds cause them to usually appear white, but the rising or setting sun can color them various shades of yellow or red.[5][8] Atdusk, they can appear gray.[8]
Intortus variety has an extremely contorted shape, withKelvin–Helmholtz waves being a form of cirrus intortus that has been twisted into loops by layers of wind blowing at different speeds, calledwind shear.[9]
Radiatus variety has large, radial bands of cirrus clouds that stretch across the sky.[9]
Vertebratus variety occurs when cirrus clouds are arranged side-by-side like ribs.[14]
Duplicatus variety occurs when cirrus clouds are arranged above one another in layers.[15]
Cirrus clouds often produce hair-like filaments calledfall streaks, made of heavier ice crystals that fall from the cloud. These are similar to thevirga produced in liquid–water clouds. The sizes and shapes of fall streaks are determined by the wind shear.[16]
Cirrus cloud cover variesdiurnally. During the day, cirrus cloud cover drops, and during the night, it increases.[17] Based onCALIPSO satellite data, cirrus covers an average of 31% to 32% of the Earth's surface.[18] Cirrus cloud cover varies wildly by location, with some parts of the tropics reaching up to 70% cirrus cloud cover. Polar regions, on the other hand, have significantly less cirrus cloud cover, with some areas having a yearly average of only around 10% coverage.[17] These percentages treat clear days and nights, as well as days and nights with other cloud types, as lack of cirrus cloud cover.[19]
Cirrus clouds are usually formed as warm, dry air rises,[5] causing water vapor to undergodeposition onto particles, including mostly mineral dust and metallic particles[20][21] at high altitudes. Particles gathered by research aircraft from cirrus clouds over several locations above North America and Central America included mineral dust (containing aluminum, potassium, calcium, iron, and silicon), metallic particles in elemental, sulfate and oxide forms (containing sodium, potassium, iron, nickel, copper, zinc, tin, silver, molybdenum and lead), possible biological particles (containing oxygen, carbon, nitrogen and phosphorus) and elemental carbon. The authors concluded that mineral dust contributed the largest number of ice nuclei to cirrus cloud formation.[20]
The average cirrus cloud altitude increases aslatitude decreases, but the altitude is always capped by thetropopause.[22] These conditions commonly occur at the leading edge of awarm front.[23] Becauseabsolute humidity is low at such high altitudes, this genus tends to be fairly transparent.[24] Cirrus clouds can also form insidefallstreak holes (also called "cavum").[25]
At latitudes of65° N orS, close topolar regions, cirrus clouds form, on average, only 7,000 m (23,000 ft) above sea level. In temperate regions, at roughly45° N orS, their average altitude increases to 9,500 m (31,200 ft) above sea level. Intropical regions, at roughly5° N orS, cirrus clouds form 13,500 m (44,300 ft) above sea level on average. Across the globe, cirrus clouds can form anywhere from 4,000 to 20,000 m (13,000 to 66,000 ft) above sea level.[22] Cirrus clouds form with a vast range of thicknesses. They can be as little as 100 m (330 ft) from top to bottom to as thick as 8,000 m (26,000 ft). Cirrus cloud thickness is usually somewhere between those two extremes, with an average thickness of 1,500 m (4,900 ft).[26]
Thejet stream, a high-level wind band, can stretch cirrus clouds long enough to cross continents.[27]Jet streaks, bands of faster-moving air in the jet stream, can create arcs of cirrus cloud hundreds of kilometers long.[28]
Cirrus cloud formation may be effected by organicaerosols (particles produced by plants) acting as additionalnucleation points for ice crystal formation.[29][30] However, research suggests that cirrus clouds more commonly form on mineral dust or metallic particles rather than on organic ones.[21]
A vast shield of cirrus clouds accompanying the west side ofHurricane Isabel
Sheets of cirrus clouds commonly fan out from theeye walls of tropical cyclones.[31] (The eye wall is the ring of storm clouds surrounding the eye of a tropical cyclone.[32]) Alarge shield of cirrus andcirrostratus typically accompanies the high altitudeoutflowing winds of tropical cyclones,[31] and these can make the underlyingbands of rain—and sometimes even the eye—difficult to detect in satellite photographs.[33]
Thunderstorms can form dense cirrus at their tops. As the cumulonimbus cloud in a thunderstorm grows vertically, the liquid water droplets freeze when the air temperature reaches thefreezing point.[34] Theanvil cloud takes its shape because thetemperature inversion at the tropopause prevents the warm, moist air forming the thunderstorm from rising any higher, thus creating the flat top.[35] In the tropics, these thunderstorms occasionally produce copious amounts of cirrus from their anvils.[36] High-altitude winds commonly push this dense mat out into an anvil shape that stretchesdownwind as much as several kilometers.[35]
Individual cirrus cloud formations can be the remnants of anvil clouds formed by thunderstorms. In the dissipating stage of a cumulonimbus cloud, when the normal column rising up to the anvil has evaporated or dissipated, the mat of cirrus in the anvil is all that is left.[37]
Contrails are anartificial type of cirrus cloud formed when water vapor from the exhaust of ajet engine condenses on particles, which come from either the surrounding air or the exhaust itself, and freezes, leaving behind a visible trail. The exhaust can trigger the formation of cirrus by providingice nuclei when there is an insufficient naturally occurring supply in the atmosphere.[38] One of theenvironmental impacts of aviation is that persistent contrails can form into large mats of cirrus,[39] and increased air traffic has been implicated as one possible cause of the increasing frequency and amount of cirrus in Earth's atmosphere.[39][40]
Random, isolated cirrus do not have any particular significance.[23] A large number of cirrus clouds can be a sign of an approachingfrontal system or upper air disturbance. The appearance of cirrus signals a change in weather—usually more stormy—in the near future.[41] If the cloud is acirrus castellanus, there might be instability at the high altitude level.[23] When the clouds deepen and spread, especially when they are of thecirrus radiatus variety orcirrus fibratus species, this usually indicates an approaching weather front. If it is a warm front, the cirrus clouds spread out into cirrostratus, which then thicken and lower intoaltocumulus andaltostratus. The next set of clouds are the rain-bearingnimbostratus clouds.[4][23][42] When cirrus clouds precede acold front,squall line ormulticellular thunderstorm, it is because they are blown off the anvil, and the next clouds to arrive are the cumulonimbus clouds.[42] Kelvin-Helmholtz waves indicate extreme wind shear at high levels.[23] When a jet streak creates a large arc of cirrus, weather conditions may be right for the development ofwinter storms.[28]
Within the tropics, 36 hours prior to the center passage of a tropical cyclone, a veil of white cirrus clouds approaches from the direction of the cyclone.[43] In the mid- to late-19th century, forecasters used these cirrus veils to predict the arrival of hurricanes. In the early 1870s the president of Belén College inHavana, FatherBenito Viñes, developed the first hurricane forecasting system; he mainly used the motion of these clouds in formulating his predictions.[44] He would observe the clouds hourly from 4:00 am to 10:00 pm. After accumulating enough information, Viñes began accurately predicting the paths of hurricanes; he summarized his observations in his bookApuntes Relativos a los Huracanes de las Antilles, published in English asPractical Hints in Regard to West Indian Hurricanes.[45]
Cirrus clouds cover up to 25% of the Earth (up to 70% in the tropics at night[46]) and have a net heating effect.[47] When they are thin and translucent, the clouds efficiently absorb outgoinginfrared radiation while only marginally reflecting the incoming sunlight.[48] When cirrus clouds are 100 m (330 ft) thick, they reflect only around 9% of the incoming sunlight, but they prevent almost 50% of the outgoing infrared radiation from escaping, thus raising the temperature of the atmosphere beneath the clouds by an average of 10 °C (18 °F)[49]—a process known as thegreenhouse effect.[50] Averaged worldwide, cloud formation results in a temperature loss of 5 °C (9 °F) at the earth's surface, mainly the result ofstratocumulus clouds.[51]
Cirrus clouds are likely becoming more common due toclimate change. As their greenhouse effect is stronger than their reflection of sunlight, this would act as aself-reinforcing feedback.[52]Metallic particles from human sources act as additional nucleation seeds, potentially increasing cirrus cloud cover and thus contributing further to climate change.[21] Aircraft in the upper troposphere can createcontrail cirrus clouds if local weather conditions are right. These contrails contribute to climate change.[53]
Cirrus cloud thinning has been proposed as a possiblegeoengineering approach to reduce climate damage due tocarbon dioxide. Cirrus cloud thinning would involve injecting particles into the upper troposphere to reduce the amount of cirrus clouds. The2021 IPCC Assessment Report expressed low confidence in the cooling effect of cirrus cloud thinning, due to limited understanding.[54]
Scientists have studied the properties of cirrus using several different methods.Lidar (laser-basedradar) gives highly accurate information on the cloud's altitude, length, and width. Balloon-carriedhygrometers[a] measure the humidity of the cirrus cloud but are not accurate enough to measure the depth of the cloud. Radar units give information on the altitudes and thicknesses of cirrus clouds.[55] Another data source is satellite measurements from theStratospheric Aerosol and Gas Experiment program. These satellites measure whereinfrared radiation is absorbed in the atmosphere, and if it is absorbed at cirrus altitudes, then it is assumed that there are cirrus clouds in that location.[56]NASA'sModerate-Resolution Imaging Spectroradiometer gives information on the cirrus cloud cover by measuring reflected infrared radiation of various specific frequencies during the day. During the night, it determines cirrus cover by detecting the Earth's infrared emissions. The cloud reflects this radiation back to the ground, thus enabling satellites to see the "shadow" it casts into space.[31] Visual observations from aircraft or the ground provide additional information about cirrus clouds.[56] Particle Analysis by LaserMass Spectrometry (PALMS)[b] is used to identify the type of nucleation seeds that spawned the ice crystals in a cirrus cloud.[21]
Cirrus clouds have an average ice crystal concentration of 300,000 ice crystals per 10cubic meters (270,000 ice crystals per 10cubic yards). The concentration ranges from as low as 1 ice crystal per 10 cubic meters to as high as 100 million ice crystals per 10 cubic meters (just under 1 ice crystal per 10 cubic yards to 77 million ice crystals per 10 cubic yards), a difference of eightorders of magnitude. The size of each ice crystal is typically 0.25 millimeters,[26] but they range from as short as 0.01 millimeters up to several millimeters.[59] The ice crystals in contrails can be much smaller than those in naturally occurring cirrus cloud, being around 0.001 millimeters to 0.1 millimeters in length.[38]
In addition to forming in different sizes, the ice crystals in cirrus clouds can crystallize in different shapes: solid columns, hollow columns, plates, rosettes, and conglomerations of the various other types. The shape of the ice crystals is determined by the air temperature,atmospheric pressure, and icesupersaturation (the amount by which therelative humidity exceeds 100%). Cirrus in temperate regions typically have the various ice crystal shapes separated by type. The columns and plates concentrate near the top of the cloud, whereas the rosettes and conglomerations concentrate near the base. In the northernArctic region, cirrus clouds tend to be composed of only the columns, plates, and conglomerations, and these crystals tend to be at least four times larger than the minimum size. InAntarctica, cirrus are usually composed of only columns which are much longer than normal.[59]
Cirrus clouds are usually colder than −20 °C (−4 °F).[59] At temperatures above −68 °C (−90 °F), most cirrus clouds have relative humidities of roughly 100% (that is they are saturated).[60] Cirrus can supersaturate, with relative humidities over ice that can exceed 200%.[61][60] Below −68 °C (−90 °F) there are more of both undersaturated and supersaturated cirrus clouds.[62] The more supersaturated clouds are probably young cirrus.[60]
Cirrus clouds can produce several optical effects likehalos around the Sun and Moon. Halos are caused by interaction of the light with hexagonal ice crystals present in the clouds which, depending on their shape and orientation, can result in a wide variety of white and colored rings, arcs and spots in the sky, includingsun dogs,[59] the46° halo,[63] the22° halo,[63] andcircumhorizontal arcs.[64][65] Circumhorizontal arcs are only visible when the Sun rises higher than 58° above the horizon, preventing observers at higher latitudes from ever being able to see them.[66]
More rarely, cirrus clouds are capable of producingglories, more commonly associated with liquid water-based clouds such asstratus. A glory is a set of concentric, faintly colored glowing rings that appear around the shadow of the observer, and are best observed from a high viewpoint or from a plane.[67] Cirrus clouds only form glories when the constituent ice crystals areaspherical; researchers suggest that the ice crystals must be between 0.009 millimeters and 0.015 millimeters in length for a glory to appear.[68]
Heights of various cloud genera including high-, mid-, and low-level clouds
Cirrus clouds are one of three different genera of high-level clouds, all of which are given the prefix "cirro-". The other two genera arecirrocumulus and cirrostratus. High-level clouds usually form above 6,100 m (20,000 ft).[4][69][70] Cirrocumulus and cirrostratus are sometimes informally referred to ascirriform clouds because of their frequent association with cirrus.[71]
In the intermediate range, from 2,000 to 6,100 m (6,500 to 20,000 ft),[4][69] are the mid-level clouds, which are given the prefix "alto-". They comprise two genera,altostratus andaltocumulus. These clouds are formed from ice crystals, supercooled water droplets, or liquid water droplets.[4]
Low-level clouds usually form below 2,000 m (6,500 ft) and do not have a prefix.[4][69] The two genera that are strictly low-level arestratus, andstratocumulus. These clouds are composed of water droplets, except during winter when they are formed ofsupercooled water droplets or ice crystals if the temperature at cloud level is below freezing. Three additional genera usually form in the low-altitude range, but may be based at higher levels under conditions of very low humidity. They are the generacumulus, andcumulonimbus, andnimbostratus. These are sometimes classified separately as clouds of vertical development, especially when their tops are high enough to be composed of supercooled water droplets or ice crystals.[72][4]
Cirrocumulus clouds form in sheets or patches[73] and do not cast shadows. They commonly appear in regular, rippling patterns[70] or in rows of clouds with clear areas between.[4] Cirrocumulus are, like other members of the cumuliform category, formed viaconvective processes.[74] Significant growth of these patches indicates high-altitude instability and can signal the approach of poorer weather.[75][76] The ice crystals in the bottoms of cirrocumulus clouds tend to be in the form of hexagonal cylinders. They are not solid, but instead tend to have stepped funnels coming in from the ends. Towards the top of the cloud, these crystals have a tendency to clump together.[77] These clouds do not last long, and they tend to change into cirrus because as the water vapor continues to deposit on the ice crystals, they eventually begin to fall, destroying the upward convection. The cloud then dissipates into cirrus.[78] Cirrocumulus clouds come in four species:stratiformis,lenticularis,castellanus, andfloccus.[75] They areiridescent when the constituent supercooled water droplets are all about the same size.[76]
Cirrostratus clouds can appear as a milky sheen in the sky[75] or as a striated sheet.[70] They are sometimes similar to altostratus and are distinguishable from the latter because the Sun or Moon is always clearly visible through transparent cirrostratus, in contrast to altostratus which tends to be opaque or translucent.[79] Cirrostratus come in two species,fibratus andnebulosus.[75] The ice crystals in these clouds vary depending upon the height in the cloud. Towards the bottom, at temperatures of around −35 to −45 °C (−31 to −49 °F), the crystals tend to be long, solid, hexagonal columns. Towards the top of the cloud, at temperatures of around −47 to −52 °C (−53 to −62 °F), the predominant crystal types are thick, hexagonal plates and short, solid, hexagonal columns.[78][80] These clouds commonly produce halos, and sometimes the halo is the only indication that such clouds are present.[81] They are formed by warm, moist air being lifted slowly to a very high altitude.[82] When a warm front approaches, cirrostratus clouds become thicker and descend forming altostratus clouds,[4] and rain usually begins 12 to 24 hours later.[81]
A composite black-and-white photograph showing cirrus clouds over the surface ofMarsCirrus clouds on Neptune, captured duringVoyager 2's flyby
Cirrus clouds have been observed on several other planets. In 2008, the Martian LanderPhoenix took atime-lapse photograph of a group of cirrus clouds moving across theMartian sky using lidar.[83] Near the end of its mission, the Phoenix Lander detected more thin clouds close to the north pole of Mars. Over the course of several days, they thickened, lowered, and eventually began snowing. The total precipitation was only a few thousandths of a millimeter. James Whiteway fromYork University concluded that "precipitation is a component of the [Martian]hydrologic cycle".[84] These clouds formed during the Martian night in two layers, one around 4,000 m (13,000 ft) above ground and the other at surface level. They lasted through early morning before being burned away by the Sun. The crystals in these clouds were formed at a temperature of −65 °C (−85 °F), and they were shaped roughly like ellipsoids 0.127 millimeters long and 0.042 millimeters wide.[85]
On Jupiter, cirrus clouds are composed ofammonia. When Jupiter'sSouth Equatorial Belt disappeared, one hypothesis put forward by Glenn Orten was that a large quantity of ammonia cirrus clouds had formed above it, hiding it from view.[86] NASA'sCassini probe detected these clouds on Saturn[87] and thin water-ice cirrus on Saturn's moonTitan.[88] Cirrus clouds composed ofmethane ice exist on Uranus.[89] On Neptune, thin wispy clouds which could possibly be cirrus have been detected over theGreat Dark Spot. As on Uranus, these are probably methane crystals.[90]
Interstellar cirrus clouds are composed of tiny dust grains smaller than amicrometer and are therefore not true cirrus clouds, which are composed of frozen crystals.[91] They range from a fewlight years to dozens of light years across. While they are not technically cirrus clouds, the dust clouds are referred to as "cirrus" because of their similarity to the clouds on Earth. They emit infrared radiation, similar to the way cirrus clouds on Earth reflect heat being radiated out into space.[92]
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