Station model
Above or below the circle for manned stations (automatic stations do not report cloud types) that indicates sky cover can lie one or more symbols indicating cloud types in any of the low, middle, and high-étages for tropospheric clouds. One predominant cloud type may be depicted for each of three étages, if known. The middle and high-étage types are depicted above the sky cover circle of the station model, while the main low-étage cloud type is indicated below the circle. Since the station model has limited room, it makes no special provision for vertical or multi-level clouds that can occupy more than one étage at a particular time. Consequently, cloud genera with significant vertical development may be coded and plotted as low or middle depending on the altitude at which they normally form. Cumulus and cumulonimbus usually form in the low étage of the troposphere and achieve vertical extent by growing upward into the middle or high étage. Conversely, nimbostratus usually forms in the middle étage of the troposphere and becomes vertically developed by growing downward into the low étage. Although the SYNOP code has no separate formal group classification for vertical or multi-level clouds, the observer procedure for selecting numerical codes is designed to give high reporting priority to those genera or species that show significant vertical development.
Cloud physics
Clouds in the troposphere, the atmospheric layer closest to Earth, are classified according to the height at which they are found, and their shape or appearance. There are five "forms" based on physical structure and process of formation. "Cirriform" clouds are high, thin and wispy, and are seen most extensively along the leading edges of organized weather disturbances. "Stratiform" clouds are non-convective and appear as extensive sheet-like layers, ranging from thin to very thick with considerable vertical development. They are mostly the product of large-scale lifting of stable air. Unstable free-convective "cumuliform" clouds are formed mostly into localized heaps. "Stratocumuliform" clouds of limited convection show a mix of cumuliform and stratiform characteristics which appear in the form of rolls or ripples. Highly convective "cumulonimbiform" clouds have complex structures often including cirriform tops and stratocumuliform accessory clouds.