Cirrus Clouds – Feathery Wisps That Can Foretell The Weather

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cirrus spissatus cloud
Cirrus spissatus cloud

What Are Cirrus Clouds?

Cirrus (cloud classification symbol: Ci) is a variety of atmospheric cloud frequently characterised by thin, wispy strands, giving the type its title from the Latin term cirrus, meaning a twist or curling lock of hair.

Such a cloud can appear at an altitude within 5,000 and 13,700 m (16,500 and 45,000 ft) over sea level. The strands of cloud sometimes develop in tufts of a characteristic form referred to by the popular name of “mares’ tails”.

Mares tails clouds cirrus
Mares tails

From Earth’s surface, cirrus clouds typically look white, or a light grey, in colour; they appear when water vapour experiences deposition (a transition from water vapour directly to ice crystals without the normal intermediate stage of being a liquid) at elevations above 5,500 m (18,000 ft), in temperate regions and higher than 6,400 m (21,000 ft) in tropical ones.

They also develop from the outflow of tropical cyclones and the anvils of cumulonimbus clouds and thunderstorms. They also appear in advance of storms associated with frontal systems, likely foretelling a deterioration in weather conditions.

Though showing the arrival of precipitation, these clouds create, at most fall streaks, whose ice crystals evaporate in more heated and drier air (also known as virga), without touching ground level.

Jet stream-powered cirrus can develop long enough to reach across continents while remaining only several kilometres deep. Interaction of light that is visible with the ice crystals creates optical phenomena such as sun dogs and halos below.

Jet stream cirrus clouds
Jet stream cirrus clouds as seen from outer space

Cirrus is understood to raise the temperature (due to the heat discharged as water vapour freezes) of the air underneath the main cloud layer, by an average of 10 °C (18 °F). When the cloud grows so large and becomes indistinguishable from other cirrus clouds, they create a sheet of high cloud named cirrostratus.

Convection at high altitudes can create another high-based genus of cloud, cirrocumulus, with a design of small cloud tufts comprising droplets of supercooled water. Some polar stratospheric clouds can mirror Cirrus. Noctilucent clouds typically grow structured in ways similar to those of Cirrus.

Cirrus clouds can also form in the atmospheres of other planets, including Mars, Jupiter, Saturn, Neptune and Uranus, and have also been seen on Titan, one of Saturn’s more substantial moons.

Some of these extraterrestrial cirrus clouds are formed of ammonia or ices of methane, much like terrestrial water ice. (The title cirrus also applies to certain interstellar clouds, formed of sub-micrometre-sized grains of dust.

What Do Cirrus Clouds Look Like?

Cirrus clouds range in depth from 100 m (330 ft) to 8,000 m (26,000 ft), with a common thickness of 1,500 m (4,900 ft). The distance of each ice crystal is normally 0.25 millimetres long. Still, they range from as little as 0.01 millimetres or as long as several millimetres.

The ice crystals in contrails are much tinier than those in typically occurring cirrus cloud. They are about 0.001 millimetres to 0.1 millimetres in length. Cirrus can vary in temperature of −20 °C (−4 °F) to −30 °C (−22 °F).

The ice crystals in cirrus clouds have various shapes in addition to different sizes. Some forms comprise solid columns, hollow columns, plates, rosettes, and conglomerations of the numerous other types. The ice crystals’ shape is defined by the air temperature, atmospheric pressure, and ice supersaturation.

Cirrus clouds weather

Cirrus in temperate zones typically have the forms segregated by type: the columns and plates tend to be at the cloud’s tip, whereas the rosettes and conglomerations tend to be near the bottom.

In the north Arctic region, cirrus clouds tend to be formed up of only the columns, plates, and conglomerations. These crystals tend to be at above four times greater than the smallest size. In Antarctica, Cirrus is usually formed of only the columns, and these columns are significantly longer than usual.

Scientists have examined the features of Cirrus using many different applications. Light Detection and Ranging (LiDAR), gives highly detailed information on the cloud’s altitude, length, and width.

Balloon-carried hygrometers provide data on the humidity of the cirrus cloud. Still, they are not precise enough to measure the depth of the cloud. Radar units give data on the altitudes and thicknesses of cirrus clouds.

Another data source is satellite measures from the Stratospheric Aerosol and Gas Experiment (SAGE) program. These satellites measure wherever infrared radiation is absorbed in the atmosphere. If it is absorbed in cirrus altitudes, then it is believed that there are cirrus clouds in that position.

The United States National Aeronautics and Space Administration’s (NASA) MODerate Resolution Imaging Spectroradiometer (MODIS) gives data on the cirrus clouds by measuring reflected infrared radiation of different specific frequencies during the day. During the night, it defines cirrus cover by detecting the Earth’s infrared emanations.

The cloud reflects this particular radiation back to the surface, allowing satellites to see the “shadow” it projects into space. Visual observations from aircraft or the ground give further information about cirrus clouds.

Based on data obtained from the United States practising these methods, cirrus cloud cover was observed to vary daily and seasonally. The researchers discovered that the cover is the lowest at noon in the summer, with a proportion of 23% of the United States’ land area covered by Cirrus. About midnight, the cloud cover extends to around 28 per cent.

Cirrus floccus cirrus uncinus x

In the wintertime, the cirrus cloud cover did not vary appreciably from day to night. These percentages incorporate clear days and nights and days and nights with other cloud types, as lack of cirrus cloud cover.

When these clouds are present, the average coverage ranges from 30% to 50%. Based on satellite data, Cirrus comprises an average of 20% to 25% of the Earth’s surface. In the tropical areas, this cloud covers around 70% of the region’s exterior area.

Virga – Falling Ice Crystals From Cirrus Clouds

Virga cirrus clouds weather
Virga – cirrus clouds where the ice crystals become too heavy and fall and evaporate before they hit the ground

Cirrus clouds often create hair-like filaments—similar to the virga produced in liquid–water clouds—termed fall streaks. They are formed of heavier ice crystals that descend from the cloud. The dimensions and shapes of fall streaks are defined by the wind shear.

Cirrus occurs in four distinct species; Cirrus castellanus, fibratus, spissatus, and uncinus, divided into four types: intortus, vertebratus, radiatus, and duplicatus. Cirrus castellanus is a variety that has cumuliform tops produced by high-altitude convection building up from the main cloud body.

Cirrus fibratus seems striated and is the most frequent cirrus species. Cirrus uncinus clouds are curved and are the form that is regularly called mare’s tails. Of the variations, Cirrus intortus has a notably contorted shape, and cirrus radiatus has large, radial zones of cirrus clouds that spread across the sky.

Kelvin–Helmholtz waves are a type of cirrus intortus that has been distorted into loops by vertical wind shear.

Cirrus castellanus clouds
Cirrus-castellanus clouds
Cirrus fibratus clouds
Cirrus fibratus clouds
Cirrus spissatus cloud
Cirrus spissatus cloud
Cirrus uncinus clouds
Cirrus uncinus clouds

How Do Cirrus Clouds Form?

Cirrus clouds are produced when water vapour experiences deposition at high altitudes. The atmospheric pressure varies from 600 mbar at 4,000 m (13,000 ft) to 200 mbar at 12,000 m (39,000 ft) beyond sea level. These conditions regularly happen at the leading edge of a warm front.

Because humidity is low at such high elevations, this genus-type tends to be very thin. Cirrus clouds are formed of ice crystals that begin from the freezing of supercooled water droplets in zones where the air temperature is less than -20 °C or -30 °C. Cirrus typically develop in fair weather.

They are formed when it is elevated enough to be cold and freeze the water drops into ice. They sometimes may be produced by turbulence and wind shear, or by upper-tropospheric convection. Sometimes they are like inflated balls flattening from the top of a dying cumulonimbus. These types of clouds are called mammatus.

Mammatus clouds
Mammatus clouds over nebraska after a tornado. Photographer by jorn c. Olsen

Cirrus Clouds In Cyclones, Tropical Storms and Hurricanes

Cirrus grows from tropical cyclones and is regularly seen fanning out from the eyewalls of hurricanes and cyclones. A large shield of Cirrus and cirrostratus normally follows the high altitude outflow of hurricanes or typhoons. These can make the rainbands underneath—and sometimes also the eye—challenging to detect in satellite images.

Cirrus clouds and cyclones hurricanes weather
Satellite image of cirrus clouds emanating out of a tropical disturbance

Thunderstorms and Cirrus Clouds

Thunderstorms can form thick Cirrus at their tops. As the cumulonimbus cloud in a thunderstorm arises vertically, the liquid water droplets freeze while the air temperature approaches the freezing point.

The anvil cloud takes its appearance because the temperature inversion at the tropopause stops the warm, moist air creating the thunderstorm from rising any higher, thus producing the flat top.

In the tropics, these thunderstorms infrequently produce copious quantities of Cirrus from their anvils. High-altitude winds regularly push this dense mat out into an anvil frame that stretches downwind as much as several kilometres.

Individual cirrus cloud formations can be the remains of anvil clouds produced by thunderstorms. In the dying or dissipating stage of a cumulonimbus cloud, while the regular column rising up to the anvil has evaporated or disappeared, the mat of Cirrus in the anvil is all that is left.

Cumulo nimbus with anvil cirrus
Cumulonimbus (thunderstorm) with anvil cirrus

Contrails

Contrails are an artificial type of cirrus cloud formed while water vapour from the exhaust of a jet engine condenses on particles of dust and other airborne material, which come from either the encompassing air or the exhaust itself, and freezes, leaving behind a visible path.


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The exhaust can also trigger cirrus formation by providing ice nuclei when there is an inadequate naturally-occurring accumulation in the atmosphere.

One of the environmental impacts of aviation is that large masses contrails can develop into huge mats of Cirrus. Increased air traffic has been linked as one possible cause of the growing frequency and amount of Cirrus in Earth’s atmosphere.

Can Cirrus Clouds Mean Bad Weather?

Irregular, isolated Cirrus does not have any particular importance. Many cirrus clouds can be an indication of an impending frontal system or upper air disturbance. This indicates a change in weather in the near future, which ordinarily becomes more stormy. If the cloud is a cirrus castellanus, there may be instability at a high altitude level.

Cirrus castellanus cloud
Cirrus castellanus cloud

When the clouds expand and spread, particularly when they are of the cirrus radiatus variety or cirrus fibratus species, this usually symbolises an incomming weather front. If it is a warm front, the cirrus clouds spread out into cirrostratus, which also thickens and lowers into altocumulus and altostratus.

The following set of clouds are the rain-bearing nimbostratus clouds. When cirrus clouds lead a cold front, squall line or multicellular thunderstorm, they are swept off the anvil. The next to arrive is the cumulonimbus clouds. Kelvin-Helmholtz waves mean extreme wind shear at high levels.

In the tropics, 36 hours before the centre passage of a tropical cyclone, a veil of white cirrus clouds advances from the cyclone’s direction. In the mid to late 19th century, forecasters and meteorologists used these cirrus veils to foretell a cyclones’ arrival.

In the early 1870s, Belén College’s leader in Havana, Cuba, Father Benito Viñes, formed the first hurricane forecasting system. He mainly used the movement of these clouds in forming his predictions.

He would watch the clouds hourly from 4:00 am to 10:00 pm. After accumulating sufficient information, Viñes began correctly predicting hurricanes’ paths. He eventually summarised his investigations in his book, Apuntes Relativos a los Huracanes de las Antilles.

Effects on climate

Cirrus clouds cover from 0% to 25% of the Earth (up to 70% in the tropics) and have a net warming effect. When they are thin and translucent, the clouds absorb outgoing infrared radiation while just marginally reflecting the incoming sunlight. When cirrus clouds are 100 m (330 ft) deep, they reflect only about 9% of the incoming sunlight.

Still, they block almost 50% of the outgoing infrared radiation from leaving, thus raising the temperature of the atmosphere below the clouds by an average of 10 °C (18 °F), a process identified as the greenhouse effect. Averaged globally, cloud formation occurs in a temperature loss of 5 °C (9 °F) at the Earth’s surface, mainly due to stratocumulus clouds.

As a consequence of their warming effects when comparatively thin, cirrus clouds have been linked as a potential partial basis of global warming. Scientists have theorised that global warming could produce high thin cloud cover to increase, raising temperatures and humidity.

This, in turn, would enhance the cirrus cloud cover, effectively causing a ripple effect or creating a more extensive impact from a smaller effect. A prediction of this theory is that the Cirrus would climb higher as the temperatures rose, raising the air volume beneath the clouds and the amount of infrared radiation bounced back down to Earth.

The hypothesis also implies that the increase in temperature would increase the ice crystals’ dimensions in the cirrus cloud, perhaps causing the reflection of solar radiation and the reflection of the Earth’s infrared radiation to balance out.

A comparable hypothesis put forth by Richard Lindzen is the iris hypothesis in which an expansion in tropical sea surface temperatures ends in fewer cirrus clouds and thus more infrared radiation transmitted to space.

Weather and Optical Phenomena – Halos

Sun halo cirrus clouds weather
Sun halo caused by cirrus clouds

Cirrus clouds, cirrus clouds can give several optical effects, such as halos around the sun and moon. Halos are produced by the light’s interaction with hexagonal ice crystals already in the clouds, which, depending on their shape and orientation, can occur in a wide variety of white and coloured rings, arcs, and spots sky. Well-known halo varieties are the 22° halo, the circumzenithal arc, sun dogs, and the circumhorizontal arc (also recognised as fire rainbows).

Fire rainbows
Fire rainbows – weather phenomena

Halos generated by cirrus clouds tend to be more noticeable and colourful than those produced by cirrostratus.

More infrequently, cirrus clouds can produce glories, more generally associated with liquid water-based clouds such as stratus. A glory is a set of concentric, faintly-coloured gleaming rings that develop around the observer’s shadow and are best seen from a high viewpoint or plane.

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Cirrus clouds only form glories when the forming ice crystals are aspherical. Researchers suggest that the ice crystals need to be between 0.009 millimetres and 0.015 millimetres in length.

Relation to Other Clouds

Cirrus clouds are one of three distinct varieties of high-étage (high-level) clouds. High-étage clouds appear at 5,000 m (16,500 ft) and higher in temperate regions. The other two varieties, cirrocumulus and cirrostratus, are also high clouds.

In the middle range, from 2,000 to 7,000 m (6,500 to 23,000 ft) in temperate areas, are the mid-étage clouds. They include two or three varieties depending on the height classification system being applied: altostratus, altocumulus, and, according to WMO classification, nimbostratus. These clouds are created from ice crystals, supercooled water droplets, or liquid water droplets.

Low-étage clouds appear at less than 2,000 m (6,500 ft). The two genera that are low-étage are stratus and stratocumulus. These clouds are formed of water droplets, except throughout winter when formed of supercooled water droplets or ice crystals if the temperature at the cloud level is below freezing.

Two different genera usually start in the low altitude range. Still, they may be based at higher levels under circumstances of very low humidity. They include the classes cumulus, and cumulonimbus, which along with nimbostratus, are often categorised separately as clouds of vertical growth, mainly when their tops are high sufficient to be composed of supercooled water droplets or ice crystals.

The heights of high-étage clouds like Cirrus vary considerably with latitude. They are at their lowest in the polar regions, with a minimum altitude of 3,000 m (10,000 ft) to a maximum of 7,600 m (25,000 ft). They are at their highest in tropical regions, ranging in altitude from approximately 6,100 m (20,000 ft) to about 18,000 m (60,000 ft).

In temperate areas, they range in height from 5,000 m (16,500 ft) to 14,000 m (45,000 ft)—a change in contrast to low-étage clouds, which do not change altitude with latitude.
Summary of high cloud varieties

There are three principal classes in the family of high clouds: Cirrus, cirrocumulus, and cirrostratus. Cirrostratus clouds generally produce halos because they are formed almost entirely of ice crystals. Cirrocumulus and cirrostratus are sometimes informally attributed to as “cirriform clouds” because of their common connection with Cirrus.

They are given the prefix “cirro-“, but this applies more to their altitude range than their physical structure. Cirrocumulus in its proper form is really a high cumuliform genus, and cirrostratus is stratiform, like altostratus and deeper based sheet clouds.

Cirrocumulus

Cirrocumulus
Cirrocumulus

Cirrocumulus clouds appear in sheets or patches and do not cast shadows. They usually appear in uniform, rippling patterns or in rows of clouds with clear areas within. Cirrocumulus are, like other branches of the cumuliform class, formed via convective methods.

Significant growth of these patches means high-altitude instability and can indicate the approach of unsettled weather. The ice crystals in the bottoms of cirrocumulus clouds mostly tend to be in the order of hexagonal cylinders. They are not solid, but rather, tend to have stepped funnels developing in from the ends.

Towards the top of the clouds, these crystals have an inclination to clump together. These clouds do not last long. They tend to transform into Cirrus because as the water vapour continues to accumulate on the ice crystals, they ultimately begin to fall, ending the upward convection. The cloud then evaporates into Cirrus.

Cirrocumulus clouds come in four varieties: stratiformis, lenticularis, floccus and castellanus. They are iridescent when the integral supercooled water droplets are all about the same size.

Cirrostratus

Cirrostratus fibratus undulatus halo x
Cirrostratus clouds

Cirrostratus clouds can occur as a milky sheen in the sky or as a striated layer. They are sometimes comparable to altostratus and are distinguishable from the latter because the sun or moon is clearly visible through transparent cirrostratus, in contrast to altostratus, which tends to be opaque or semitransparent.

Cirrostratus occur in two species, fibratus and nebulosus. The ice crystals in these clouds can change depending upon the height in the cloud. Towards the base, at temperatures of around −35 to −45 °C (−31 to −49 °F), the crystals tend to be long, dense, hexagonal columns.

Towards the top of the cloud, at temperatures of about −47 to −52 °C (−53 to −62 °F), the dominant crystal types are deep, hexagonal plates and short, solid, hexagonal columns. These clouds usually produce halos, and sometimes the corona is the only evidence that such clouds are near.

They are created by warm, moist air being lifted gradually to a very high altitude. When a warm front nears, cirrostratus clouds become thicker and descend, creating altostratus clouds. Rain usually commences 12 to 24 hours later.

Cirrus Clouds On Other Planets

Cirrus clouds have been seen on numerous other planets. On the 18th September 2008, the Martian Lander Phoenix caught a time-lapse photograph of a collection of cirrus clouds drifting across the Martian sky using LiDAR.

Near to the end of its mission, the Phoenix Lander discovered more thin clouds near the north pole of Mars. Over several days, they thickened, lowered, and finally began snowing. The total water content or precipitation was only a few thousandths of a millimetre. James Whiteway from York University reasoned that “precipitation is a component of the [Martian] hydrologic cycle.”

During the Martian night in two layers, these clouds formed one around 4,000 m (13,000 ft) above ground and the other at ground level. They lasted through the early morning before being cooked away by the sun.

Sol clouds color
Curiosity rover captures cirrus type clouds on the planet mars

The crystals in these clouds were produced at a temperature of −65 °C (−85 °F). They were shaped irregularly like ellipsoids 0.127 millimetres long and 0.042 millimetres wide.

On Jupiter, cirrus clouds are formed of ammonia. When Jupiter’s South Equatorial Belt vanished, one theory put forward by Glenn Orten was that a large amount of ammonia cirrus clouds had appeared above it, hiding it from view.

NASA’s Cassini probe discovered these clouds on Saturn and thin water-ice Cirrus on Saturn’s moon Titan. Cirrus clouds made of methane ice exist on Uranus. On Neptune, thin wispy clouds that could be Cirrus have been discovered over the Great Dark Spot. As on Uranus, these are presumably methane crystals.

Interstellar cirrus clouds are made of tiny dust grains tinier than a micrometre. Therefore, they are not real clouds of this variety, composed of ice crystals or other frozen liquids. They vary 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 they resembled Earth’s clouds. They also release infrared radiation, similar to how cirrus clouds on Earth reflect heat being radiated out toward space.

People Also Ask

What Are Cirrus Clouds a Sign Of?

Cirrus clouds can mean different weather patterns depending on the type of clouds that are present. Some kinds of these clouds can mean fine and mild weather while other types of this cloud can herald unsettled or stormy weather or even cyclones and hurricanes.

Do Cirrus Clouds Bring Rain?

Cirrus appear very high in the atmosphere. But if you see that Cirrus starts to cover more of the sky, and gets deeper and thicker, this is a good sign that a warm front is approaching or unsettled weather is approaching.

Why Are Cirrus Clouds So Thin?

As humidity is low at such high elevations, this cloud tends to be very thin. Cirrus clouds form ice crystals that start from the freezing of supercooled rain droplets in areas where the air temperature is lower than -20 °C or -30 °C. These clouds can appear thin to someone on the ground but are actually several kilometres high and long.

What Causes Cirrus Clouds To Form?

Cirrus clouds form from the ascent of dry air, causing the small quantity of water vapour in the air to undergo displacement into ice (to change from a gas straight into a solid). Cirrus is created solely of ice crystals, which gives their white colour and form in a wide variety of forms and sizes.

Further Reading

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