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
Scud (cloud) tornado, landspout, or waterspout. The difference is determinable by observing the presence or absence of rotation (not just movement) of the scud clouds. If rotation is present, then a tornado, landspout, or waterspout is possible, and the more intense the rotation, the more likely. Viewing scud clouds means that there is a cumulonimbus parent cloud. Rain, hail, strong wind, and lightning can be expected from the parent cloud. Scud (cloud) Pannus, or scud clouds, is a type of fractus cloud at low height above ground, detached, and of irregular form found beneath nimbostratus or cumulonimbus clouds. These clouds are often