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Cirrus
Cirrus floccus and Cirrus spissatus at sunrise.jpg
Abbreviation	Ci
Altitude	above 6000 m (above 20,000 ft)
Precipitation	no

Introduction

The most common form of high-level clouds are thin and often wispy cirrus clouds.Cirrus (Ci) - the name is derived from the Latin cirrus = curl of hair, tuft or wisp.Cirrus cloud is a member of the ten fundamental cloud types (or cloud genera).In fact, they are the highest of the main cloud genera, popularly known as 'mares' tails they may even form in the upper troposphere.

Occurrence

Mostly you see cirrus clouds very high up in the sky.Cirrus clouds are composed of ice crystals that originate from the freezing of super cooled water droplets and are found at heights greater than 20,000 feet (6,000 meters).Cirrus usually occur in fair weather.They are formed when it is high enough to be cold and freeze the water drops into ice.Cirrus are composed of minute ice crystals, in regions where air temperature is lower than -20°C or -30°C. They may be caused by turbulence and wind shear, or by upper-tropospheric convection. Sometimes they are just blown out ice-crystals spreading from the top of a dying cumulonimbus or dissolving altocumulus.

Sometimes people think they see cirrus clouds when it's really only the trail of an airplane, half blown away and spreading across the sky. But in any way, if the sky is blue with only these thin cirrus clouds high up in it, you can predict it's going to be a nice day.If they appear irregular and patchy, slowly shifting from easterly directions, often dissolving, they are indicating increasing high pressure and dry, sunny and quiet weather.

Apperarance

They look like someone pulled a bigger cloud apart into small bits of cloud.Cirrus (SIR-russ) clouds look thin because they are made of ice crystals but not water drops.They point in the direction of air movement at their elevation. Cirrus can form from almost any cloud that has undergone glaciation and can be seen in a variety of shapes and sizes. The possibilities range from the "finger-like" appearance of cirrus fall streaks to the uniform texture of more extensive cirrus clouds associated with an approaching warm front.

cirrus uncinus

Fall streaks

Fall streaks form when snowflakes and ice crystals fall from cirrus clouds. The shapes and sizes the fall streaks attain are determined by the change in wind with height and how quickly these ice crystals fall .Fall streaks tend to be stretched out horizontally as well as vertically since ice crystals fall much more slowly than raindrops, . Cirrus streaks may be straight, shaped like a comma, or seemingly all tangled together.

Virga
Virga is similar to fall streaks which appears as streamers suspended in air beneath the base of precipitating clouds. Virga is developed when precipitation falls through a layer of dry air and evaporates before reaching the ground. Cirrus cloud frequently exhibit some halo phenomena, particularly mock suns and parts of haloes, shimmering in rainbow colors.The only cloud type that develops from cirrus is cirrostratus. Cirrus may be confused with cirrostratus, but actual cirrus always occurs in relatively small patches.

Cirrus uncinus duplicatus vertebratus

Common types and varieties of cirrus

The common types of cirrus clouds are as follows

• Ci fibratus: characterized by long fine striations;
  
• Ci uncinus :look like a hook or comma;
  
• Ci spissatus: as dense cirrus patches;
  
• Ci vertebratus:look like ribs or fish bone and
  
• Ci radiatus: parallel bands apparently radiating from one point of the sky.
  

citations

http://www.weatheronline.co.uk/reports/wxfacts/Cirrus.htm

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/cldtyp/hgh/crs.rxml

http://quatr.us/physics/weather/cirrus.htm