Classifying Clouds
Clouds seem to come in an endless variety of forms, constantly shifting shapes as they move through the sky. Yet, different clouds have some similarities. Scientists classify clouds on the basis of form and height.
The first cloud classification system was developed in 1803 by the British naturalist Luke Howard. His classification system and the terms he coined for different types of clouds are the basis for what scientists use today. 
There are three basic cloud forms: stratus, cumulus, and cirrus. Scientists use these three forms to name and describe a variety of common cloud types. Notice that clouds are generally grouped into families by the altitude at which they occur. Photos of common types of clouds are shown on this and the following page.
Fog is a cloud that is near or touching the ground. Fogs often form when warm, moist air passes over land. Other fogs form when the ground cools at night, cooling the humid air immediately above the ground to the dew point.
Stratus Clouds
Flat layers of clouds that cover much or all of the sky are known as stratus clouds (STRAT US). The word stratus comes from the Latin word meaning “to spread out.” Nimbostratus clouds are low stratus clouds that produce steady and widespread rain or snow. When nimbo- or -nimbus is added to a cloud's name, it means that the cloud produces precipitation.
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