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.The strands of cloud sometimes appear in tufts of a distinctive form referred to by the common name of "mares' tails". Cirrus generally appears white or light gray in color. It forms when water vapor undergoes deposition at altitudes above 5,000 m (16,500 ft) in temperate regions and above 6,100 m (20,000 ft) in tropical regions. It also forms from the outflow of tropical cyclones or the anvils of cumulonimbus cloud. Since cirrus clouds arrive in advance of the frontal system or tropical cyclone, it indicates that weather conditions may soon deteriorate. 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 haloes. Cirrus is known to raise the temperature of the air beneath the main cloud layer by an average of 10 °C (18 °F). 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 thin, wispy clouds blown by high winds into long streamers. They are considered "high clouds" forming above 6000 m (20,000 ft). Cirrus clouds usually move across the sky from west to east. They generally mean fair to pleasant weather. Cirrostratus clouds are thin, sheetlike high clouds that often cover the entire sky. They are so thin that the sun and moon can be seen through them.