Cloud the bottom of the hierarchy. Clouds in the troposphere assume five physical forms based on structure and process of formation. These forms are commonly used for the purpose of satellite analysis. They are given below in approximate ascending order of instability or convective activity. "Non-convective" stratiform clouds appear in "stable" airmass conditions and, in general, have flat sheet-like structures that can form at any altitude in the troposphere. The stratiform group is divided by altitude range into the genera cirrostratus (high-level), altostratus (mid-level), stratus (low-level), and nimbostratus (multi-level). Fog is commonly considered a surface-based cloud layer. The fog may form
Sheet erosion some limited unevenness in the sheet flow arising from clods of earth, rock fragments, or vegetation. Sheet erosion occurs in two steps; first rainsplash dislodges small particles of the substrate and then the particles are carried away, usually short distances, by a thin and uniform layer of water known as sheetflow. Transport by the sheetflow is usually small distances meaning sheet erosion is a low magnitude process. However the frequency in time with which the occur may be high, compensating for the small change observed in each individual episode of sheet erosion. A sheetflood can be distinguished from ordinary an