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.
Overcast
Sometimes clouds can be different colors such as black or white, but overcast usually refers to darker skies. In some cases, it can be impossible to see distinct borders of clouds or the sky may be covered by a single type of cloud, such as stratus and the whole sky will be a dull white.