Cirrus (cloud classification symbol: Ci) is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition onto rocky or metallic dust particles at high altitudes. Globally, they form anywhere between 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 ground. Cirrus clouds warm the earth, potentially contributing to climate change. A warming earth will likely produce more cirrus clouds, potentially resulting in a self-reinforcing loop. 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 atmospheres of Mars, Jupiter, Saturn, Uranus, and Neptune; and on Titan, one of Saturn's larger moons. Some of these extraterrestrial cirrus clouds are made of ammonia or methane ice, 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. Description Cirrus are wispy clouds made of long strands of ice crystals that are described as feathery, hair-like, or layered in appearance. First defined scientifically by Luke Howard in an 1803 paper, their name is derived from the Latin word cirrus, meaning 'curl' or 'fringe'. They are transparent, meaning that the Sun can be seen through them. Ice crystals in the clouds cause them to usually appear white, but the rising or setting Sun can color them various shades of yellow or red. At dusk, they can appear gray. Cirrus comes in five visually-distinct species: castellanus, fibratus, floccus, spissatus, and uncinus: Cirrus castellanus has cumuliform tops caused by high-altitude convection rising up from the main cloud body;