Cirrus cloud
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Cirrus (Lat. 'wisp of hair') is a type of cloud.
Cirrus clouds are composed of ice crystals and shaped like hairlike filaments. They are formed at an altitudes above 16,500 feet. If there are many cirrus clouds in the sky it may be a sign that a frontal system is approaching. Cirrus clouds can also be the remnants from a thunderstorm.
Cirrus clouds form on other planets, including Mars, Jupiter, Saturn, Uranus, and possibly Neptune. They have even been seen on Titan, one of Saturn's moons. Some of these extraterrestrial cirrus clouds are composed of ammonia or methane ice rather than water ice. The term cirrus is also used for certain interstellar clouds composed of sub-micrometer-sized dust grains.
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Formation
Cirrus clouds are formed when water vapor undergoes deposition at high altitudes where the atmospheric pressure ranges from 600 mbar at 4,000 m (13,000 ft) above sea level to 200 mbar at 12,000 m (39,000 ft) above sea level.[13] These conditions commonly occur at the leading edge of a warm front.[14] Because humidity is low at such high altitudes, this genus-type tends to be very thin.[2] Cirrus clouds are composed of ice crystals that originate from the freezing of super cooled water droplets in regions where air temperature is lower than -20 °C or -30 °C.Cirrus usually occur in fair weather. They are formed when it is high enough to be cold and freeze the water drops into ice. They sometimes may be caused by turbulence and wind shear, or by upper-tropospheric convection. Sometimes they are like blown out ice-crystals spreading from the top of a dying cumulonimbus.
Thunderstorms
Thunderstorms can form dense cirrus at their tops. As the cumulonimbus cloud in a thunderstorm grows vertically, the liquid water droplets freeze when the air temperature reaches the freezing point.[16] The anvil cloud takes its shape because the temperature inversion at the tropopause prevents the warm, moist air forming the thunderstorm from rising any higher, thus creating the flat top.[17] In the tropics, these thunderstorms occasionally produce copious amounts of cirrus from their anvils.[18] High-altitude winds commonly push this dense mat out into an anvil shape that stretches downwind as much as several kilometers.
Forcasting
A large number of cirrus clouds can be a sign of an approaching frontal system or upper air disturbance. This signals a change in weather in the near future, which usually becomes stormier.[22] If the cloud is a cirrus castellanus, there might be instability at the high altitude level.[14] When the clouds deepen and spread, especially when they are of the cirrus radiatus variety or cirrus fibratus species, this usually indicates an approaching weather front. If it is a warm front, the cirrus clouds spread out into cirrostratus, which then thicken and lower into altocumulus and altostratus. The next set of clouds are the rain-bearing nimbostratus clouds When cirrus clouds precede a cold front, squall line or multicellular thunderstorm, it is because they are blown off the anvil, and the next to arrive are the cumulonimbus clouds