Clouds are classified by their general appearance and level. Cloud names are Cirrus, Stratus and Cumulus. To indicate the level of a cloud, a prefix is given to the cloud name, like Cirro and Alto. Cirro indicates high clouds with bases above 20.000 feet. Alto indicates mid-level clouds between 7.000 and 20.000 feet. There is no prefix for low clouds. Low clouds are found from near the ground to about 7.000 feet. Often you can find the word “Nimbo” added to the beginning or “Nimbus” added to the end of a cloud name. This means that the cloud is producing precipitation.