Cirrus clouds (cloud classification symbol: Ci) are a genus of atmospheric clouds generally characterized by thin, wispy strands, giving them their 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 clouds generally appear white or light grey in color. They form 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. They also form from the outflow of tropical cyclones or the anvils of cumulonimbus clouds. Since these cirrus clouds arrive in advance of the frontal system or tropical cyclone, they indicate that the weather conditions may soon deteriorate. While they indicate the arrival of precipitation (rain), cirrus clouds themselves produce only fall streaks (falling ice crystals that evaporate before landing on the ground).

Jet stream-powered cirrus clouds can grow long enough to stretch across continents, but they remain only a few kilometers deep. When visible light interacts with the ice crystals in cirrus clouds, it produces optical phenomena such as sun dogs and haloes. Cirrus clouds are known to raise the temperature of the air beneath them by an average of 10 °C (18 °F). When they 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.