Cirrus 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 above sea level, with the higher elevations usually in the tropics and the lower elevations in more polar regions.