High clouds, called cirrus, cirrostratus, and cirrocumulus, are formed entirely of ice crystals. Other clouds are mainly water droplets. A cirrostratus cloud is a thin sheet of cloud. It often causes a halo to appear around the sun or moon. This halo is a good way to recognize a cirrostratus cloud.
Cirrocumulus clouds look like many small tufts of cotton hanging high in the sky.
These clouds are rare. Clouds that grow vertically. Cumulus and cumulonimbus clouds may grow to great heights while their bases are near the ground. Cumulus clouds are heaped-up piles of clouds. They can look like small cotton balls, or they can grow into the most spectacular of all clouds, the cumulonimbus.