- Cirrus cloud
Cirrus clouds (cloud classification symbol: Ci) are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair. The strands of cloud sometimes appear in tufts of a distinctive form referred to by the common name of mares' tails.
Cirrus clouds generally appear white or light grey in color. They form when water vapor undergoes deposition at altitudes above 5,000 m (16,500 ft) in temperate regions and above 6,100 m (20,000 ft) in tropical regions. They also form from the outflow of tropical cyclones or the anvils of cumulonimbus clouds. Since these cirrus clouds arrive in advance of the frontal system or tropical cyclone, they indicate that the weather conditions may soon deteriorate. While they indicate the arrival of precipitation (rain), cirrus clouds themselves produce only fall streaks (falling ice crystals that evaporate before landing on the ground).
Jet stream-powered cirrus clouds can grow long enough to stretch across continents, but they remain only a few kilometers deep. When visible light interacts with the ice crystals in cirrus clouds, it produces optical phenomena such as sun dogs and haloes. Cirrus clouds are known to raise the temperature of the air beneath them by an average of 10 °C (18 °F). When they become so extensive that they are virtually indistinguishable from one another, they form a sheet of cirrus called cirrostratus. Convection at high altitudes can produce another form of cirrus called cirrocumulus, a pattern of small cloud tufts that contain droplets of supercooled water.
Cirrus clouds form on other planets, including Mars, Jupiter, Saturn, Uranus, and possibly Neptune. They have even been seen on Titan, one of Saturn's moons. Some of these extraterrestrial cirrus clouds are composed of ammonia or methane ice rather than water ice. The term cirrus is also used for certain interstellar clouds composed of sub-micrometer sized dust grains.
Cirrus clouds range in thickness from 100 m (330 ft) to 8,000 m (26,000 ft), with an average thickness of 1,500 m (4,900 ft). There are, on average, 30 ice crystals per liter (96 ice crystals per gallon), but this ranges from one ice crystal per 10,000 liters (3.7 ice crystals per 10,000 gallons) to 10,000 ice crystals per liter (37,000 ice crystals per gallon), a difference of eight orders of magnitude. The length of each of these ice crystals is usually 0.25 millimeters long, but they range from as short as 0.01 millimeters or as long as several millimeters. The ice crystals in contrails are much smaller than those in naturally-occurring cirrus clouds, as they are around 0.001 millimeters to 0.1 millimeters in length. Cirrus clouds can vary in temperature from −20 °C (−4 °F) to −30 °C (−22 °F).
The ice crystals in the cirrus clouds have different shapes in addition to different sizes. Some shapes include 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 ice supersaturation. Cirrus clouds in temperate regions typically have the shapes segregated by type: the columns and plates tend to be at the top of the cloud, whereas the rosettes and conglomerations tend to be near the base. In the northern Arctic 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. In Antarctica, cirrus clouds are usually composed of only the columns, and these columns are much longer than normal.
Scientists have studied the characteristics of cirrus clouds using several different methods. One, Light Detection and Ranging (LiDAR), gives highly accurate information on the cloud's altitude, length, and width. Balloon-carried hygrometers give information on the humidity of the cirrus clouds, but are not accurate enough to measure the depth of the cloud. Radar units give information on the altitudes and thicknesses of cirrus clouds. Another data source is satellite measurements from the Stratospheric Aerosol and Gas Experiment (SAGE) program. These satellites measure where infrared radiation is absorbed in the atmosphere, and if it is absorbed at cirrus altitudes, it is assumed that there are cirrus clouds in that location. The United States National Aeronautics and Space Administration's (NASA) MODerate resolution Imaging Spectroradiometer (MODIS) also 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 cloud cover by detecting the Earth's infrared emissions. Cirrus clouds reflect this radiation back to the ground, thus enabling satellites to see the "shadow" it casts into space. Visual observations from aircraft or the ground provide additional information about cirrus clouds.
Based upon data taken from the United States using these methods, cirrus cloud cover was found to vary diurnally and seasonally. The researchers found that in the summer, at noon, the cover is the lowest, with an average of 23% of the United States' land area covered by cirrus clouds. Around midnight, the cloud cover increases to around 28%. In winter, the cirrus cloud cover did not vary appreciably from day to night. These percentages include clear days and nights, as well as days and nights with other cloud types, as lack of cirrus cloud cover. When cirrus clouds are present, the typical cloud cover ranges from 30% to 50%. Based on satellite data, cirrus clouds cover an average of 20% to 25% of the Earth's surface. In the tropical regions, cirrus clouds cover around 70% of the region's surface area.
Many cirrus clouds produce hair-like filaments, similar to the virga produced in liquid–water clouds, called fall streaks, and they are made of heavier ice crystals that fall from the clouds. The sizes and shapes of fall streaks are determined by the wind shear.
Cirrus clouds come in six distinct subforms: Cirrus castellanus, cirrus fibratus, cirrus intortus, cirrus radiatus, cirrus spissatus, and cirrus uncinus. Cirrus castellanus is the form of a cirrus cloud that has a large shape rising up from the main cloud body. Cirrus fibratus looks striated and is the most common cirrus subform. Cirrus intortus is an extremely contorted shape, and cirrus radiatus has large, radial bands of cirrus clouds that stretch across the sky. Cirrus uncinus clouds are hooked and are the form that is usually called mare's tails. Kelvin-Helmholtz waves are a form of cirrus cloud that has been twisted by vertical wind shear.
Cirrus clouds are formed when water vapor undergoes deposition at high altitudes where the atmospheric pressure ranges from 600 mbar at 4,000 m (13,000 ft) above sea level to 200 mbar at 12,000 m (39,000 ft) above sea level. These conditions commonly occur at the leading edge of a warm front. Because humidity is low at such high altitudes, cirrus clouds tend to be very thin.
Cirrus clouds form from tropical cyclones, and they are commonly seen fanning out from the eyewalls of hurricanes. A large shield of cirrus and cirrostratus clouds typically accompanies the high altitude outflow of hurricanes or typhoons, and these can make the underlying rain bands—and sometimes even the eye—difficult to detect in satellite photographs.
Thunderstorms also form cirrus clouds. As the cumulonimbus cloud in a thunderstorm grows vertically, the liquid water droplets freeze when the air temperature reaches the freezing point. The anvil cloud takes its shape because the temperature inversion at the tropopause prevents the warm, moist air forming the thunderstorm from rising any higher, thus creating the flat top. In the tropics, these thunderstorms occasionally produce copious amounts of cirrus clouds from their anvils. High-altitude winds commonly push this dense mat out into an anvil shape that stretches downwind as much as several kilometers.
Cirrus clouds also are the remnant anvil clouds of 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.
Contrails are a manmade type of cirrus cloud formed when water vapor from the exhaust of a jet 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 also trigger the formation of cirrus clouds by providing ice nuclei when there is an insufficient naturally-occurring supply in the atmosphere. One of the environmental impacts of aviation is that persistent contrails can form into large mats of cirrus clouds, and increased air traffic has been implicated as one possible cause of the increasing number of cirrus clouds.
Use in forecasting
Random, isolated cirrus clouds do not have any particular significance. A large number of cirrus clouds can be a sign of an approaching frontal system or upper air disturbance. This signals a change in weather in the near future, which usually becomes stormier. If the cirrus cloud is a cirrus castellanus cloud, there might be instability at the cirrus level. When the cirrus clouds deepen and spread, especially when they are of the cirrus radiatus or cirrus fibratus forms, this usually indicates an approaching weather front. When the weather front is a warm front, the cirrus clouds spread out into cirrostratus clouds, which then thicken and lower into altocumulus and altostratus clouds. The next set of clouds are the rain-bearing nimbostratus clouds. When cirrus clouds precede a cold front, it is because they are blown off the anvil, and the next clouds to arrive are the cumulonimbus clouds. Kelvin-Helmholtz waves indicate extreme wind shear at high levels.
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. 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 Belen College (located in Havana, Cuba), Benito Vines, developed the first hurricane forecasting system, and he mainly used the motion of these clouds in formulating his predictions. He would observe the clouds hourly from 4:00 am to 10:00 pm. After accumulating enough information, Vines began accurately predicting the paths of hurricanes, and he eventually summarized his observations in his book, Apuntes Relativos a los Huracanes de las Antilles.
Effects on climate
Cirrus clouds cover up to 25% of the Earth and have a net heating effect. When they are thin and translucent, cirrus clouds efficiently absorb outgoing infrared radiation while only marginally reflecting the incoming sunlight. 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 cirrus clouds by an average of 10 °C (18 °F)—a process known as the greenhouse effect. Averaged worldwide, cloud formation results in a temperature loss of 5 °C (9 °F) at the earth's surface, mainly the result of cumulus clouds.
As a result of their warming effects when relatively thin, cirrus clouds have been implicated as a potential partial cause of global warming. Scientists have speculated that global warming could cause thin cirrus cloud cover to increase, thereby increasing temperatures and humidity. This, in turn, would increase the cirrus cloud cover, effectively creating a positive feedback circuit. A prediction of this hypothesis is that the cirrus clouds would move higher as the temperatures rose, increasing the volume of air underneath the cirrus clouds and the amount of infrared radiation reflected back down to earth. In addition, the hypothesis suggests that the increase in temperature would tend to increase the size of the ice crystals in the cirrus cloud, possibly causing the reflection of solar radiation and the reflection of the Earth's infrared radiation to balance out.
A similar hypothesis put forth by Richard Lindzen is the iris hypothesis in which an increase in tropical sea surface temperatures results in less cirrus clouds and thus more infrared radiation emitted to space.
Cirrus clouds produce several optical effects, including glories. A glory is a set of concentric, faintly-colored glowing rings that appear around the shadow of the observer. Cirrus clouds only form glories when the constituent ice crystals are aspherical, and researchers suggest that the ice crystals must be between 0.009 millimeters and 0.015 millimeters in length. Cirrus clouds also produce halos around the sun and sundogs, which are arcs of brightness near the sun.
Cirrus clouds can also produce colorful arcs such as the circumzenithal and circumhorizontal arcs. The top color of a circumhorizontal arc is red, followed by orange, then running through all the colors of the rainbow to violet on the bottom. The ice crystals required to produce one of these arcs must be shaped like plates, and the crystals must be oriented horizontally. Light comes from the sun and passes through a cirrus cloud. The sun must be either below 32° or more than 58° above the horizon to produce a circumzenithal or circumhorizontal arc, respectively. The sunlight enters one face of a crystal and refracts through it, exiting with its colors spread in a rainbow-like pattern.
Relation to other clouds
Cirrus clouds are one of three different types of high-étage (high-level) clouds. High-étage clouds are clouds that form at 5,000 m (16,500 ft) and above in temperate regions. The other two types, cirrocumulus and cirrostratus, are also cirriform clouds. Low-étage clouds, form at less than 2,000 m (6,500 ft). These clouds are composed of water droplets, except during winter when they are formed from ice crystals. The four types of low-étage clouds are cumulus clouds, stratus clouds, stratocumulus clouds—a hybrid of the two—and cumulonimbus clouds. In the intermediate range, from 2,000 m (6,500 ft) to 7,000 m (23,000 ft) in temperate regions, are the mid-étage clouds. They come in three varieties: altostratus clouds, altocumulus clouds, and nimbostratus clouds. These clouds are formed from ice crystals, supercooled water droplets, or liquid water droplets.
The altitudes of the high-étage clouds like cirrus clouds vary considerably with latitude. In the polar regions, they are at their lowest, with a minimum altitude of only 3,000 m (10,000 ft) to a maximum of 7,600 m (25,000 ft). In tropical regions, they are at their highest, ranging in altitude from about 6,100 m (20,000 ft) to around 18,000 m (60,000 ft). In temperate regions, they range in altitude from 5,000 m (16,500 ft) to 14,000 m (45,000 ft)—a variation in contrast to low-étage clouds, which do not appreciably change altitude with latitude.
There are other clouds which are subtypes of the cirrus cloud family. These clouds are referred to as "cirriform clouds", and they take the prefix "cirro-". There are three main cloud genera in the cirrus family: cirrus clouds, cirrocumulus clouds, and cirrostratus clouds. Cirriform clouds commonly produce halos because they are composed almost entirely of ice crystals.
Cirrocumulus clouds form in sheets or patches and do not cast shadows. They commonly appear in regular, rippling patterns or in rows of clouds with clear areas between. Cirrocumulus clouds are, like other members of the cumulus family, formed via convective processes. When these patches grow large, this indicates high-altitude instability and can signal the approach of poorer weather. The ice crystals in the bottoms of cirrocumulus clouds tend to be hexagonal cylinders. These cylinders 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. Cirrocumulus clouds do not last long, and they tend to convert to cirrus clouds because as the water vapor continues to deposit on the ice crystals, they eventually begin to fall, destroying the upward convection. The cirrocumulus cloud then dissipates into a cirrus cloud. Cirrocumulus clouds come in four species: stratiformis, lenticularis, castellanus, and floccus. Cirrocumulus clouds are iridescent when the constituent supercooled water droplets are all about the same size.
Cirrostratus clouds can appear as a milky sheen in the sky or as a striated sheet. They are sometimes similar to altostratus clouds; they are distinguishable because the sun or moon is always visible through a cirrostratus cloud, which is not the case with an altostratus cloud. Cirrostratus clouds come in two species, fibratus and nebulosus. The ice crystals in these clouds vary depending upon the height in the cloud. Towards the bottom, at temperatures of around −35 °C (−31 °F) to −45 °C (−49 °F), the crystals tend to be long, solid, hexagonal columns. Towards the top of the cloud, at temperatures of around −47 °C (−53 °F) to −52 °C (−62 °F), the predominant crystal types are thick, hexagonal plates and short, solid, hexagonal columns. Cirrostratus clouds produce halos very commonly, and sometimes the halo is the only thing that indicates the presence of the cloud. They are formed by warm, moist air being lifted slowly to a very high altitude. When a warm front approaches, cirrostratus clouds deepen and lower into altostratus, and rain usually begins 12 to 24 hours later.
Cirrus clouds have been observed on several other planets. On September 18, 2008, the Martian Lander Phoenix took a time-lapse photograph of a group of cirrus clouds moving across the Martian sky using LiDAR. Near the end of its mission, the Phoenix Lander detected more thick cirrus clouds close to the north pole of Mars. Over the course of several days, these clouds thickened, lowered, and eventually began snowing. The total precipitation was only a few thousandths of a millimeter. James Whiteway from York University concluded that "precipitation is a component of the [Martian] hydrologic cycle." 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. The clouds lasted through early morning before being burned away by the sun. The crystals in these cirrus 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.
On Jupiter, cirrus clouds are composed of ammonia. When Jupiter's South 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. NASA's Cassini probe detected cirrus clouds on Saturn and thin water ice cirrus clouds on Saturn's moon Titan. Cirrus clouds composed of methane ice exist on Uranus. On Neptune, thin wispy clouds which could possibly be cirrus clouds have been detected over the Great Dark Spot. As on Uranus, these are probably methane crystals.
Interstellar cirrus clouds are composed of tiny dust grains smaller than a micrometer. These clouds range from a few light 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 also emit infrared radiation, similar to the way cirrus clouds on Earth reflect heat being radiated out into space.
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- A cloud atlas with many photos and description of the different cloud genuses
- UIUC.edu's online guide to meteorology
- Artificial cirrus clouds
- International Cloud Atlas – Cirrus
Cloud genuses Extreme-level High-level Medium-level Low-level Vertical
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