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 vinyl flooring has environmental concerns in manufacturing, use, and disposal. The three basic categories of vinyl sheet flooring are homogeneous, inlaid, and layered composite. Vinyl flooring differs in manufacturing process and polyvinyl chloride content. Composite manufacturing begins with a fibrous backing sheet. This was often felt or paper (before 1980, often with asbestos); more modernly, it is generally fiberglass. This sheet is coated in vinyl and plasticizer. The resulting sheet is printed and possibly embossed (the print layer). Then the sheet is coated again with one or more protective wear layers, the topmost of which may be polyurethane, to avoid waxing. Cushioned