High opaque clouds depict in dark red colours in the Night Microphysics RGB images.
The red colour stems from the brightness temperature difference (BTD) of MSG channels IR12.0 and IR10.8. The used colour scale reaches from -4 (black) to +2 degrees (intensive red). As both channels show approximately the same temperature (i.e. BTD ~ 0), the resulting red colour is of medium intensity.
The contribution of the green colour is small as the BTD between the window channel IR10.8 and the absorption channel IR3.9 is small. As most of the measured radiance comes from high levels, the CO2 absorption at 3.9 micrometer is small.
The blue contribution from channel IR10.8 is small due to the very cold cloud tops.
Explanation of the dark red colour of high opaque clouds in the Night Microphysics RGB (see also the recipe on the left side):
• In case of high opaque clouds, only the BTD (IR12.0 - IR10.8) contributes with the red colour beam.