They are classified by altitude and the patterns they form.
Explanation:
Clouds are classified by their altitude in the troposphere. Some clouds are found beyond the troposphere. The classification as surface level, low, middle, high and multi-level. The multi-level are massive storms clouds, known as cumulonimbus that rise from the low altitudes to the top of the troposphere. A surface cloud can be classified as fog or mist. The low level can have stratus clouds. The middle level can contain nimbostratus and the high level can contain cirrus clouds. The names are given based on their altitude but the way in which their form.
