Clouds are found only in the troposphere. Those that form in blanketlike layers are stratus clouds. Cirrus clouds form at very high altitudes out of ice crystals. They have a featherlike shape. Puffy clouds that seem to rise up from a flat bottom are called cumulus clouds. When snow or rain falls from a cloud, the term nimbo is added to the cloud’s name. Clouds are further grouped by form and height. For example, cumulonimbus clouds develop upward. Clouds called fog can form at ground level.