Cirrus (cloud classification symbol: Ci) is a genus of high-altitude cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. In the Earth's atmosphere, cirrus are usually formed when warm, dry air rises, causing water vapor deposition onto mineral dust and metallic particles at high altitudes. Globally, they form anywhere between above sea level, with the higher elevations usually in the tropics and the lower elevations in more polar regions.
Cirrus clouds can form from the tops of thunderstorms and tropical cyclones and sometimes predict the arrival of rain or storms. Although they are a sign that rain and maybe storms are on the way, cirrus themselves drop no more than falling streaks of ice crystals. These crystals dissipate, melt, and evaporate as they fall through warmer and drier air and never reach the ground. The word cirrus comes from the Latin prefix cirro-, meaning "tendril" or "curl". Cirrus clouds warm the earth, potentially contributing to climate change.
Optical phenomena, such as sun dogs and halos, can be produced by light interacting with ice crystals in cirrus clouds. There are two other high-level cirrus-like clouds called cirrostratus and cirrocumulus. 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 of supercooled (below freezing point) water.
Cirrus clouds form in the atmosphere of the planets Mars, Jupiter, Saturn, Uranus, and Neptune, as well as of the satellite Titan, Saturn's largest moon. Some of these extraterrestrial cirrus clouds are made of ammonia or methane, much like water ice in cirrus on Earth. Some interstellar clouds, made of grains of dust smaller than a thousandth of a millimeter, are also called cirrus.
Description
Cirrus are wispy clouds made of long strands of water crystals that are described as feathery, hair-like, or layered in appearance. First defined scientifically by Luke Howard in an 1803 paper, their name is derived from the Latin word cirrus, meaning 'curl' or 'fringe'. They are transparent, meaning that the sun can be seen through them. Ice crystals in the clouds usually cause them to appear white, but the rising or setting sun can color them various shades of yellow or red. At dusk, they can appear gray.
Cirrus comes in five visually-distinct species: castellanus, fibratus, floccus, spissatus, and uncinus:
Cirrus castellanus has cumuliform tops caused by high-altitude convection rising up from the main cloud body. Cirrus fibratus looks striated and is the most common cirrus species. Cirrus floccus species looks like a series of tufts. Cirrus spissatus is a particularly dense form of cirrus that often forms from thunderstorms. Cirrus uncinus clouds are hooked and are the form that is usually called mare's tails.
Each species is divided into up to four varieties: intortus, vertebratus, radiatus, and duplicatus:
Intortus variety has an extremely contorted shape, with Kelvin–Helmholtz waves being a form of cirrus intortus that has been twisted into loops by layers of wind blowing at different speeds, called wind shear. Radiatus variety has large, radial bands of cirrus clouds that stretch across the sky. Vertebratus variety occurs when cirrus clouds are arranged side-by-side like ribs. Duplicatus variety occurs when cirrus clouds are arranged above one another in layers.
Cirrus clouds often produce hair-like filaments called fall streaks, made of heavier ice crystals that fall from the cloud. These are similar to the virga produced in liquid–water clouds. The sizes and shapes of fall streaks are determined by the wind shear.
Cirrus cloud cover varies diurnally. During the day, cirrus cloud cover drops, and during the night, it increases. Based on CALIPSO satellite data, cirrus covers an average of 31% to 32% of the Earth's surface. 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. These percentages treat clear days and nights, as well as days and nights with other cloud types, as lack of cirrus cloud cover.
Formation Cirrus clouds are usually formed as warm, dry air rises, causing water vapor to undergo deposition onto particles, including mostly mineral dust and metallic particles 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.
The average cirrus cloud altitude increases as latitude decreases, but the altitude is always capped by the tropopause. These conditions commonly occur at the leading edge of a warm front. Because absolute humidity is low at such high altitudes, this genus tends to be fairly transparent. Cirrus clouds can also form inside fallstreak holes (also called "cavum").
At latitudes of 65° N or S, close to polar regions, cirrus clouds form, on average, only above sea level. In temperate regions, at roughly 45° N or S, their average altitude increases to above sea level. In tropical regions, at roughly 5° N or S, cirrus clouds form above sea level on average. Across the globe, cirrus clouds can form anywhere from above sea level. Cirrus clouds form with a vast range of thicknesses. They can be as little as from top to bottom to as thick as , Cirrus cloud thickness is usually somewhere between those two extremes, with an average thickness of .
Convection can cause cirrus clouds to be thicker especially in the tropics where they can reach as thick as 8,000m (26,000ft)
The jet stream, a high-level wind band, can stretch cirrus clouds long enough to cross continents. Jet streaks, bands of faster-moving air in the jet stream, can create arcs of cirrus cloud hundreds of kilometers long.
Cirrus cloud formation may be effected by organic aerosols (particles produced by plants) acting as additional nucleation points for ice crystal formation. However, research suggests that cirrus clouds more commonly form on mineral dust or metallic particles rather than on organic ones.
Tropical cyclones Sheets of cirrus clouds commonly fan out from the eye walls of tropical cyclones. (The eye wall is the ring of storm clouds surrounding the eye of a tropical cyclone.) A large shield of cirrus and cirrostratus typically accompanies the high altitude outflowing winds of tropical cyclones, and these can make the underlying bands of rain—and sometimes even the eye—difficult to detect in satellite photographs.
Thunderstorms
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 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 from their anvils. High-altitude winds commonly push this dense mat out into an anvil shape that stretches downwind as much as several kilometers.
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.
Contrails Contrails are an artificial 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 trigger the formation of cirrus 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, and increased air traffic has been implicated as one possible cause of the increasing frequency and amount of cirrus in Earth's atmosphere.
Use in forecasting
Random, isolated cirrus 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. The appearance of cirrus signals a change in weather—usually more stormy—in the near future. If the cloud is a cirrus castellanus, there might be instability at the high altitude level. When the clouds deepen and spread, especially when they are of the cirrus radiatus variety or cirrus 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 into altocumulus and altostratus. The next set of clouds are the rain-bearing nimbostratus clouds. When cirrus clouds precede a cold front, squall line or multicellular thunderstorm, 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. When a jet streak creates a large arc of cirrus, weather conditions may be right for the development of winter storms.
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 Belén College in Havana, Father Benito Viñes, developed the first hurricane forecasting system; 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, Viñes began accurately predicting the paths of hurricanes; he summarized his observations in his book Apuntes Relativos a los Huracanes de las Antilles, published in English as Practical Hints in Regard to West Indian Hurricanes.
Effects on climate
Cirrus clouds cover up to 25% of the Earth (up to 70% in the tropics at night) and have a net heating effect. When they are thin and translucent, the clouds efficiently absorb outgoing infrared radiation while only marginally reflecting the incoming sunlight. When cirrus clouds are 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)—a process known as the greenhouse effect.
Cirrus clouds are likely becoming more common due to climate change. As their greenhouse effect is stronger than their reflection of sunlight, this would act as a self-reinforcing feedback. Metallic particles from human sources act as additional nucleation seeds, potentially increasing cirrus cloud cover and thus contributing further to climate change. Aircraft in the upper troposphere can create contrail cirrus clouds if local weather conditions are right. These contrails contribute to climate change.