1941. Cloud cover Along with wind direction, cloud cover is one of the oldest atmospheric conditions to be coded on a station model. The circle in the middle of the station model represents cloud cover. In the United Kingdom, when the observation is taken from an automated weather observation site, the shape is a triangle. If the shape is completely filled in, it is overcast. If conditions are completely clear, the circle or triangle is empty. If conditions are partly cloudy, the circle or triangle is partially filled in. The cloud cover shape has different looks depending upon how many oktas (eighths of the sky) are covered by cloud. A sky half full of clouds would have a circle that was half white and half black. Below the shape indicating sky cover, the station model can indicate the coverage of low clouds, in oktas, and the ceiling height in hundreds of feet. The ceiling height is the height at which more than half the sky is covered by clouds. For pilots, knowledge of the sky cover helps determine if visual flight rules (VFR) are being met. Knowing the degree of cloud cover can help determine whether or not various weather fronts, such as cold fronts or warm fronts, have passed by a location. A nephanalysis, contouring areas that are cloudy with scalloped lines, can be performed to indicate a system's cloud and precipitation pattern. This technique is rarely performed nowadays, due to the prevalence of satellite imagery worldwide. Cloud types 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