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 pan steel core (in this process, the aluminum surface is typically 90% aluminum and 10% silicon, which is not quite the same as pure aluminum). Common features that may be found in sheet pans include: one or more flat edges to assist food removal, one or more raised edges (lips) to retain food, a contiguous rim to retain either food or shallow liquid, handles to assist in moving the pan into and out of the oven, a layer of insulation (typically air) designed to protect delicate food from burning (air bake pan), or perforations to aid in speeding cooking (pizza tray).