The visible channel at the same nominal date-time is scanned separately within 30 minutes. It is designated in the product fields by the value -1. The directional radiance that would be emitted from a cloud free area. It is assumed to be adequately calculable using radiative transfer theory from an atmospheric profile expressing temperature, humidity, and ozone for a cell as functions of pressure, with a black underlying surface. The clear sky radiances in all channels are estimated for each cell as the observed average from those pixels within the square which are deemed to be clear, or by interpolation from surrounding squares.
Clear sky is determined by the cloud mask.
The effect of clouds on the directional radiance in a given channel. Operationally, it is the difference between the observed radiance in a cell and the estimated clear sky radiance. It corresponds roughly to the temperature of the air at a level within the cloud above which the emission optical depth is unity. This temperature is assumed to be the same as that of the surrounding clear sky elsewhere in the cell at the same pressure. The pressure in a cell profile which corresponds to the cloud top temperature.