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
Cloud condensation nuclei in the cold bottom layers while most of the light needed for photosynthesis in the warm top layer. Under this scenario, deprived of nutrients, marine phytoplankton would decline, as would sulfate cloud condensation nuclei, and the high albedo associated with low clouds. This is known as the CLAW hypothesis (named after the authors' initials of a 1987 "Nature" paper) but no conclusive evidence to support this has yet been reported. Cloud condensation nuclei Cloud condensation nuclei or CCNs (also known as cloud seeds) are small particles typically 0.2 µm, or 1/100th the size of a cloud droplet on which water