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.
Castellanus
Castellanus that are unfavourable to soaring can be easily identified and are nicknamed ""rocket clouds"."
The visual difference between a cumulus and a castellanus is that the former has a flat base while the latter generally has no clearly defined base. However, a visual observation of the cloud may not be sufficient, since in some cases the hybrid castellanus (pseudo cumulus) may have a flat base. The only sure way to distinguish between these two types of clouds is through an atmospheric sounding preceding the flight. If the base of these false cumulus is higher than the convective condensation level, the pilot is probably close to unwanted cumulonimbus. Moreover, when a cumulus breaks down, there may still exist an updraft column under the cloud base that did not originate from the ground. In particular, at the end of the day, the airmass can be stable up to 2000 feet and unstable above this stable layer. The cumulus turn into stratocumulus or altocumulus depending on their height. Before the 1956 version of the International Cloud Atlas, these clouds were called stratocumulus or altocumulus "vesperalis". Directly under these clouds, updrafts are still present. The meteorologist Corfidi explains that these seemingly innocuous clouds that break down rapidly can be a harbinger of nocturnal thunderstorms.