The differences among cloud types depend on their altitude and their appearance. There are a few basic types of clouds. They are cirrus, stratus, cumulus and nimbus.
Cirrus clouds are found higher up in the atmosphere than other clouds. They are made of ice and tend to be white, curly or wispy.
Stratus clouds are found only a few hundred feet above the Earth. They are thin clouds that are most often seen in the early morning or the late evening, when the air is calm.
Cumulus clouds are fluffy clouds that are seen floating in the sky on bright days. They increase in size and number during the middle of the day, when the air warms. A large number of cumulus clouds often presage rain. In the evening, when the air cools, cumulus clouds tend to give way to stratus clouds. Nimbus clouds are dark, shapeless rain clouds.
Clouds can also combine to create altocumulus clouds, which are small, white or grayish clouds found high in the sky and packed closely together. Altostratus clouds form gray sheets, while cirrostratus clouds form thin, white sheets made of crystals of ice. Cumulonimbus clouds are enormous, cauliflower-like clouds that are tall enough to rise through all the cloud layers.