<P> Cirrus (cloud classification symbol: Ci) is a genus of atmospheric cloud generally characterized by thin, wispy strands, giving the type its name from the Latin word cirrus, meaning a ringlet or curling lock of hair . This cloud can form at any altitude between 16,500 ft (5.0 km) and 45,000 ft (14 km) above sea level . The strands of cloud sometimes appear in tufts of a distinctive form referred to by the common name of "mares' tails". </P> <P> On planet Earth, cirrus generally appears white or light grey in color . It forms when water vapor undergoes deposition at altitudes above 5,500 m (18,000 ft) in temperate regions and above 6,400 m (21,000 ft) in tropical regions . It also forms from the outflow of tropical cyclones or the anvils of cumulonimbus clouds . Since cirrus clouds arrive in advance of the frontal system or tropical cyclone, it indicates that weather conditions may soon deteriorate . While it indicates the arrival of precipitation (rain), cirrus clouds only produce fall streaks (falling ice crystals that evaporate before landing on the ground). </P> <P> Jet stream - powered cirrus can grow long enough to stretch across continents while remaining only a few kilometers deep . When visible light interacts with the ice crystals in cirrus cloud, it produces optical phenomena such as sun dogs and halos . Cirrus is known to raise the temperature of the air beneath the main cloud layer by an average of 10 ° C (18 ° F). When the individual filaments become so extensive that they are virtually indistinguishable from one another, they form a sheet of high cloud called cirrostratus . Convection at high altitudes can produce another high - based genus called cirrocumulus, a pattern of small cloud tufts that contain droplets of supercooled water . </P>

When do cirrus clouds usually appear in the sky
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