Clouds are good indicators of the moisture content and atmospheric stability. Cumulus are puffy clouds of vertical development formed by rising air currents in unstable air. Stratus are layered clouds formed when a layer of moist air is cooled below its saturation point and remains stable. Cirrus are high wispy clouds. The Nimbus prefix indicates that the clouds produce precipitation. The Fracto prefix means that the clouds are fragmented. The Alto prefix indicates that the clouds occupy middle layers of the troposphere. The high clouds are found between 16,500-45,000 ft and are generally made up of ice crystals. The common ones are Cirrus(Ci), Cirrocumulus(Cc), and Cirrostratus(Cs). The middle clouds are seen from 6,500-20,000 ft and are generally formed from water droplets and ice crystals. Typical middle clouds are the Altocumulus(Ac), Altostratus(As), and Nimbostratus(Ns). Low clouds form from the surface up to 6,500 ft. Common low clouds are the Stratus(St), Stratocumulus(Sc), and Cumulus(Cu) types.