Cumulus clouds are one of the most recognizable cloud types, known for their fluffy, puffy appearance and flat bases. They form at low altitudes, typically between 1,000 and 6,500 feet (300 to 2,000 meters), and are made up of water droplets. Cumulus clouds have a classic cotton-ball shape and are usually white, although they can appear gray when they grow larger or contain more moisture. These clouds form when warm, moist air rises due to convection, cooling and condensing to create visible cloud masses. While small cumulus clouds usually indicate fair weather, larger cumulus clouds can develop into more significant storm clouds if the atmosphere becomes unstable.