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.