Cirrus cloud

Cirrus (cloud classification symbol: Ci) is a genus of high-altitude cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. In the Earth's atmosphere, cirrus are usually formed when warm, dry air rises, causing water vapor deposition onto mineral dust and metallic particles at high altitudes. Globally, they form anywhere between 6,000 and 20,000 meters (20,000 and 66,000 feet) above sea level, with the higher elevations usually in the tropics and the lower elevations in more polar regions. Cirrus clouds can form from the tops of thunderstorms and tropical cyclones and sometimes predict the arrival of rain or storms. Although they are a sign that rain and maybe storms are on the way, cirrus themselves drop no more than falling streaks of ice crystals. These crystals dissipate, melt, and evaporate as they fall through warmer and drier air and never reach the ground. The word cirrus comes from the Latin prefix cirro-, meaning "tendril" or "curl". Cirrus clouds warm the earth, potentially contributing to climate change. Optical phenomena, such as sun dogs and halos, can be produced by light interacting with ice crystals in cirrus clouds. There are two other high-level cirrus-like clouds called cirrostratus and cirrocumulus. Cirrostratus looks like a sheet of cloud, whereas cirrocumulus looks like a pattern of small cloud tufts. Unlike cirrus and cirrostratus, cirrocumulus clouds contain droplets of supercooled (below freezing point) water. Cirrus clouds form in the atmosphere of the planets Mars, Jupiter, Saturn, Uranus, and Neptune, as well as of the satellite Titan, Saturn's largest moon. Some of these extraterrestrial cirrus clouds are made of ammonia or methane, much like water ice in cirrus on Earth. Some interstellar clouds, made of grains of dust smaller than a thousandth of a millimeter, are also called cirrus.