Cirrus clouds (abbreviated “Ci”) are thin, wispy clouds that usually form in the upper stratosphere, at a height of 18,000 feet (6000 meters) above sea level and at temperatures in the range of -20 to -30 degree centigrade (-4 to -22 Fahrenheit). They occur at all latitudes and accounts for almost a quarter of the total cloud cover of the world. They are most dense near the tropics, where such clouds constitute almost three-fourth of the total cloud cover. The strong westerly winds blow these clouds from east to west into streamers.
APPEARANCE
Cirrus means ‘curl’ in Latin, and as the name indicates they are curly clouds. However they take many shape and size, and are classified into many species and varieties depending on the size, shape, and form of the elements.
Uncinus means “curly hooks” in Latin, and Ci Unicius type of clouds have thin, wisp-like strands, often accompanied by tufts, giving it the appearance and an unofficial name of “Mare’s tails”. They are generally sparse in the sky, and very thin.
Ci Spisstaus, previously known as densus or nothus, and also called thunderstorm cirrus and false cirrus is a cloud species of with a very high optical thickness. These clouds are gray in color and conceal or even hide the sun.
Ci Floccus is a cloud species where each cloud element is a small tuft with a cumuli form or rounded appearance, with the lower part more or less ragged and often accompanied by virga.
Ci Castellanus is a cloud species that has a crenellated or turreted appearance because of some of the upper parts of the cloud having vertically developed cumuliform protuberances. They have a common base and seem arranged in lines.
Ci Radiatus is a cloud variety where the elements arrange in straight parallel bands. These bands however seem to converge toward a point on the horizon or toward two opposite points owing to the effect perspective.
Ci Duplicatus is a cloud variety composed of superimposed layers, sheets, or patches. The stratified parts at slightly different levels partly merge at times.
Ci Intortus are the variety of cirrus where the filaments are very irregularly curved, or more or less zigzag, and often apparently entangled.
Ci Vertebratus is a cloud variety where the elements arrange in a manner suggestive of vertebrae, ribs, or a fish skeleton.
Many cirrus clouds produce hair like filaments made of the heavier ice crystals that precipitate from them. These “fall streaks”, a form of virga, often indicate the difference in the motion of air between the upper part of the cirrus cloud and the air below it. Sometimes the top of the cirrus cloud moves rapidly above a slower layer of air, or the streak falls into a faster moving lower layer. The directions of these winds can also vary.
Cirrus clouds undergo continuous changes in area coverage, thickness, texture, and position. Sometimes these clouds are so extensive that they are virtually indistinguishable from one another, when shortly thereafter most of the clouds would have dissipated.
HOW THEY FORM
As the sun heats the earth, bubbles of hot air called thermals rise upward from the warm surface. These thermals of air parcels undergo convention. In other words, they continue to rise as long as the air within the parcel is warmer than the surrounding air. When cool air dominates the air parcel, moisture condense and form clouds. Cirrus clouds form when such a process takes place above an altitude of 18000 feet. Because of the extremely low temperatures, the water vapor condense into ice crystals directly. Due to the sparse moisture at such high altitudes, they tend to be very thin.
When the hot exhaust of aircrafts leaves trails of water that freeze at high altitudes, they too form cirrus clouds. If wind is absent when this process takes place, a streak of cirrus uncinus clouds form.
COMPOSITION
Unlike clouds in the lower and middle altitudes, which are made largely of water droplets, cirrus clouds, being high up in the atmosphere contain only ice crystals. These ice crystals vary substantially in shape and size.
HOW THEY AFFECT THE ATMOSPHERE
A high number of cirrus clouds may be a sign of an approaching frontal system or upper air disturbance. This usually signals the weather becoming increasingly stormy in the near future.
Cirrus clouds can also be the remnants of a thunderstorm. A large shield of cirrus and cirrostratus typically accompany the high altitude outflow of hurricanes or typhoons.
However, the biggest impact of the cirrus clouds on the atmosphere is in the arena of global warming and climate change. When the sun’s energy, after touching the earth reflects back to outer space, the ice crystals in the cirrus clouds capture some of it. This contributes to greenhouse effect, and makes the earth warm and habitable. Conversely, in a phenomenon called albedo, the cirrus cloud also reflects some of the sunlight back to space without allowing it to touch the earth’s surface. For this reason, it has not yet been determined whether the net effect of cirrus clouds is to warm or cool the earth. Much of the difficulty lies in modeling the albedo effect of clouds composed of various size and shape crystals. The relative significance of the solar-albedo effect that cools the earth versus infrared greenhouse effects that warms the earth is clearly dependent on the ice crystal size and the amount of ice in the cloud.
Scientists postulate that cirrus cloud seeding can help manipulate global warming, but the research is not yet conclusive.
HOW THEY DISSIPATE
Cirrus clouds at times form a sheet and transform to cirrostratus clouds. Sometimes convection at high altitudes produces another cloud form called cirrocumulus, a pattern of small cloud tufts, which include droplets of super cooled water.
Cirrus clouds do not produce any rain. However, on days with thunderstorm activity, dissipating thunderstorms continue to produce precipitation even with thick cirrus clouds. Precipitation here usually consists of medium to large drops and gradually decreases. The rain area may occur from the rear of the cloud and hence rainbows are common with a lowering of the sun towards the horizon.
REFERENCES
http://www.newton.dep.anl.gov
http://www.eo.ucar.edu
http://www.sciencedaily.com
http://www.env.leeds.ac.uk
http://www.geo.arc.nasa.gov
http://ww2010.atmos.uiuc.edu
http://www.amsglossary.allenpress.com
http://www.australiasevereweather.com
