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
Altostratus cloud nimbostratus. Altostratus most often takes the form of a featureless sheet of cloud but can be wavy (undulatus) as a result of wind shear through the cloud. It can also be fragmented (fibratus) with clear sky visible, which often the approach of a weakened or upper level warm front. Altostratus can be composed of ice crystals. In some ice crystal altostratus, very thin, rapidly disappearing horizontal sheets of water droplets appear at random. The sizes of the ice crystals in the cloud tended to increase as altitude decreased. However, close to the bottom of the cloud, the particles decreased in