Photograph of cumulus clouds in the mountains of Colorado. Click on image for full size version (107K GIF) Courtesy of Lisa Gardiner

Clouds can come in all sizes and shapes, and can form near the ground or high in the atmosphere. Clouds are groups of tiny water droplets or ice crystals in the sky and are formed by different processes. They can make different kinds of precipitation depending on the atmosphere's temperature.

Cloud types are classified by height and appearance. The shape depends on the way the air moves around the cloud. If air moves horizontally, clouds form spread-out layers. Clouds grow upward if air is moving vertically near the cloud.

At any given time, clouds cover about 50% of the Earth. We would not have rain, thunderstorms, rainbows or snow without clouds. The atmosphere would be boring if the sky was always clear!