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 (sailing) flat, thin surface, such as a sheet of paper or a bed sheet. In nautical usage the term "sheet" is applied to a rope or chain attached to the lower corners of a sail for the purpose of extension or change of direction. The connexion in derivation with "shoot" is more clearly seen in "sheet-anchor", one that is kept in reserve, to be "shot" in case of emergency. Fore-and-aft rigs comprise the vast majority of sailing vessels in use today, including effectively all dinghies and yachts. The sheet on a fore-and-aft sail controls the angle of the sail to the