The water that they bring as rain or snow is necessary to all forms of life. Clouds can also bring destruction or even death, in the form of hail or tornadoes. Some clouds are great fleecy masses, and others look like giant feathers. Still others are gray or black sheets that darken the sky. Most clouds change shape continuously, as new droplets form in some parts of the cloud and droplets evaporate in others.
Clouds also change because of the action of winds and other moving air.
Kinds of clouds Scientists give names to clouds that describe their appearance. For example, the prefix strato- means layerlike or sheetlike. Clouds that appear as layers or sheets are called stratus clouds. The prefix cirro- means curl, and cirrus clouds are curly white clouds. These terms and a few others are used to form the names of the most common clouds.