Clouds are the visible collections of water droplets or ice particles in the atmosphere. Meteorologists classify cloud types according to shape and altitude.
Fog is a ground-level cloud. Stratus clouds, which are flat or layered, are much longer and wider than they are tall. Altostratus is a stratus cloud about 2 miles above the Earth. When these clouds rain or snow, they are called nimbostratus.
Cirrostratus clouds lie at an altitude ofabout miles. The cumulus type is puffy. True cumulus clouds form about a mile above the surface of the Earth; they often develop as sunlight heats the ground and the ground, in turn, heats the air. This cloud type literally bubbles upward. If it becomes very tall, it can form a cumulonimbus or thunderstorm cloud, with its top reaching an altitude of7 miles or more.
Cumulus clouds can also develop in layers. Stratocumulus is a layered cumulus cloud about a mile above the ground. Altocumulus is a similar cloud at an altitude of 2 miles. Its greater distance from the ground makes the cumulus puffs appear smaller than those of stratocumulus clouds. The cirrocumulus type (with smaller puffs still) is found about 4 to 5 miles up.
Cirrus clouds occur at an altitude of 4 miles or more, where the temperature is always below freezing; hence, these clouds are always filled with ice crystals.