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 produce different kinds of precipitation dependent 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 in spread-out layers. However, clouds grow upward if air movement is vertical. Clouds reach higher heights near the equator and develop at lower heights near the poles.

At any given time, clouds cover about 50% of the Earth. We would not have rain, thunderstorms, rainbows, or snow without clouds. Clouds make up some of the atmospheric optics we can see in the sky. The atmosphere would be quite boring if the sky was always clear!