Cirrus Cloud - Description

Description

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 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 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 are usually composed of only the columns, and these columns are much longer than normal.

Scientists have studied the characteristics of cirrus 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. The 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 these clouds are present, the typical coverage ranges from 30% to 50%. Based on satellite data, cirrus covers an average of 20% to 25% of the Earth's surface. In the tropical regions, these 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 five distinct species; Cirrus castellanus, cirrus fibratus, cirrus radiatus, cirrus spissatus, and cirrus uncinus; which are each divided into four varieties: Cirrus intortus, cirrus vertebratus, cirrus radiatus, and cirrus duplicatus. Cirrus castellanus is a species that 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 uncinus clouds are hooked and are the form that is usually called mare's tails. Of the varieties, Cirrus intortus has an extremely contorted shape, and cirrus radiatus has large, radial bands of cirrus clouds that stretch across the sky. Kelvin-Helmholtz waves are a form of cirrus intortus that has been twisted into loops by vertical wind shear.