Cirrus clouds are high-altitude clouds that form above 20,000 feet (6,000 meters). They are composed of ice crystals and appear as thin, wispy, feathery clouds. This distinguishes them from low clouds (stratus, cumulus under 6,500 feet), medium clouds (altostratus, altocumulus 6,500-20,000 feet), and very low clouds (fog, mist).