Thick ice clouds appear brown in the Ash RGB images.
The image below shows frontal cloudiness and deep convective clouds.
Explanation of the colour of thick ice clouds (see also the recipe on the left side):
• For thick ice cloud the brightness temperature values are close to each other in all three channels the Ash RGB is composed from. Both the
(IR12.0 - IR10.8) and the (IR10.8 - 8.7) brightness temperature differences are around zero. However, the red signal will be higher than
the green signal due to the different ranges of the linear stretching (see the recipe on the left).
• The blue signal is low, most often zero, as the IR10.8 brightness temperature of a thick ice cloud is usually less than the lower limit of the appropriate range (-30 Celsius degree).
As a consequence the thick ice cloud appears brown in the Ash RGB images.