<P> Cirrus clouds often 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 cloud . The sizes and shapes of fall streaks are determined by the wind shear . </P> <P> Cirrus comes in four distinct species; Cirrus castellanus, fibratus, spissatus, and uncinus; which are each divided into four varieties: intortus, vertebratus, radiatus, and 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 . </P> <P> 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 . These conditions commonly occur at the leading edge of a warm front . Because humidity is low at such high altitudes, this genus - type tends to be very thin . 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 . </P> <P> Cirrus forms from tropical cyclones, and is commonly seen fanning out from the eyewalls of hurricanes . A large shield of cirrus and cirrostratus typically accompanies the high altitude outflow of hurricanes or typhoons, and these can make the underlying rain bands--and sometimes even the eye--difficult to detect in satellite photographs . </P>

High level clouds that are mainly composed of ice crystals