When a cumulus cloud develops vertically, it may grow into a cumulonimbus-a thunderstorm cloud-like the one in this slide. The dark cloud base is usually composed of water droplets, while both water droplets and ice crystals are found higher up. Toward the cold top of the cloud are ice crystals, often blown by winds into an anvil shape. The pouchlike formations protruding from the underside of the storm top are mammatus clouds. They are formed by downdrafts in which the air is cooler than the surrounding air. The dynamics, microphysics, and societal impacts of thunderstorms are key research areas for scientists at the National Center for Atmospheric Research.