Cirrus clouds are very thin, wispy clouds which form in the upper levels of the troposphere. They are composed primarily of ice crystals, reflecting the extreme cold at this height, and they can take a variety of forms and shapes. As a general rule, cirrus clouds are thin enough to be transparent or very close to it, and they form in fair weather, although an especially heavy layer of cirrus can indicate an incoming storm system.
These clouds generally form above 23,000 feet (6,000 meters), and they are often accompanied with streaking tails of ice crystals which enhance the wispy, ethereal appearance of cirrus clouds. They may appear as isolated clouds, or in a large mass, depending on weather conditions and ambient moisture, and they can also appear in association with other types of clouds.
One famous form of cirrus cloud is cirrus fibratus, the classic “mare's tails” of wispy, trailing cloud. When cirrus clouds appear in a very puffy form, they are known as cirrocumulus clouds, while transparent sheets of cirrus which stretch across large chunks of the sky are known as cirrostratus. Cirrus duplicatus forms stacked layers of cloud which may be linked by strands which pass between the layers, and cirrus Kelvin-Helmholtz forms distinctive spiral patterns in the sky.
Cirrus clouds can also be seeded by passing aircraft, as they expel moisture and other materials from their engines. These clouds are known as “cirrus aviaticus” or “contrails,” and they are familiar to many people who live or work near major airports, where contrails may criss-cross the sky in certain weather conditions. Depending on conditions, contrails may linger, or slowly melt away from the sky; contrails can be used to estimate the direction a plane is headed to or away from, as observers on the ground can see which direction the cloud runs.
Among the various types of clouds which people can identify, cirrus clouds appear at the highest point in the troposphere, the section of the Earth's atmosphere which comes into contact with the ground. The height of cirrus clouds can vary, depending on the region of the Earth they form over, as the troposphere is thinner in some places than in others. By looking at the direction of the tails associated with a patch of cirrus clouds, people can determine which way the wind is going at the level of the troposphere inhabited by the clouds.
What do Cirrus Clouds Look Like?
The word "cirrus" is derived from a Latin word meaning "curl" or "lock of hair," since cirrus clouds often resemble a ringlet or wisp of curly hair. Generally, cirrus clouds look like wispy streaks of white or grey painted across the sky. This appearance can be misleading, though. While they may look thin and flat, cirrus clouds are about 1.5 kilometers or close to one mile thick.
The characteristic feathery look of cirrus clouds comes from two main factors. As cirrus clouds form in the high levels of the troposphere, the relative moisture level is low. This means that fewer ice crystals can form. In addition, winds at this high altitude are strong, and their speeds spread out the ice crystals present, thus forming the delicate, frothy wisps recognized as cirrus clouds.
These clouds, due to their relative thinness, appear transparent, and they do not have internal shadows that can lead to dramatic effects sometimes seen in larger, more dense clouds. However, they can throw shadows onto external objects.
Cirrus clouds are fascinating because they are not limited to Earth's atmosphere. They have been observed on other planets, including Jupiter, Saturn, Mars, Neptune, and even the distant Uranus, cold in the outer reaches of the solar system.
Types of Cirrus Clouds
The characteristic wispiness of cirrus clouds can be seen throughout the year. Cirrus is a genus of cloud. Some aspects of appearance are fairly standard, but there are multiple types of cirrus clouds within that genus. The different types of cirrus are termed as species, but within the species are multiple varieties.
Cirrus Cloud Species
- Cirrus fibratus - This is a common phenomenon in cirrus cloud formation. The ice crystals in these cirrus clouds create a halo effect. Furthermore, the filaments in these clouds are relatively straight. They can signal fair weather or the approach of a warm front. The name is derived from a Latin word meaning "fibrous." These clouds may also be called Cirrus filosus.
- Cirrus spissatus - This is a thick type of cloud, and is the highest type of the major cloud genera, sometimes forming in the lower stratosphere. These clouds can veil the sun or even cover it. Other names for this type include Cirrus densus and Cirrus northus. They frequently form from an upper area on Cumulonimbus clouds.
- Cirrus castellanus - This generally uncommon species features turret-like formations rising from a general base. They sometimes give the appearance of castle battlements, hence the Latin name meaning "of a fort or castle." These cirrus generally form in lines and are tall rather than wide.
Cirrus Cloud Varieties
- Cirrus vertebratus - This type of cloud, exclusive to the Cirrus genus, resembles ribs, spinal column vertebra, or the skeleton of a fish. These dramatic clouds are a relatively unusual type of cirrus, and they are linked to the location of the jet stream.
- Cirrus radiatus - These clouds are formed in neat, parallel rows. These cirrus clouds can cover the entire sky and may appear to meet at a single point.
- Cirrus intortus - The name for this type of cirrus is derived from Latin terms meaning "wound" or "twisted." These clouds look like cirrus jumbled in a random pattern. Sometimes they may resemble spider webs.
- Cirrus uncinus - These cirrus clouds get their name from a Latin term meaning "curly hooks." They look like mares' tails with ends that flip up.
- Cirrus duplicatus - These cirrus clouds appear to be positioned in overlaid layers. Often, Cirrus fibratus and Cirrus uncinus combine to make this type of cloud formation.
Despite varied species and varieties, Cirrus clouds have commonalities. They are generally attractive, and they all form high in the atmosphere.
How Are Cirrus Clouds Formed?
Water levels in the atmosphere decrease as altitude increases. There is very little water vapor present at the levels where cirrus clouds form. Temperatures at such high levels are cold, and the water droplets present freeze.
Cirrus clouds are unique. Other clouds form by condensing onto organic matter in the air. Cirrus clouds, however, are composed entirely of ice crystals, and large masses and groupings of ice crystals blown by high upper winds create the distinctive appearance of cirrus clouds.
Because cirrus clouds form so high in the atmosphere, even though they appear around the planet, there has been a mystery about why they form. New research, however, has discovered more about these mysterious clouds. Metal, chemical, and mineral particles at high altitudes contribute to the formation of cirrus clouds, but their makeup is solely of ice particles.