Clouds which prediminantly comprise ice crystals appear cyan in the Natural Colour RGB images.
High reaching, cold clouds like frontal or Cumulonimbus clouds but also thin Cirrus clouds appear cyan in the Natural Colour RGB. Ice crystalls strongly absorb solar radiation at 1.6 micrometer, while it is reflected for a large extent at 0.6 and 0.8 micrometer. With changing sun zenith angles and uneven cloud heights, the illuminated parts of the clouds may cause an inhomogenous cyan colour pattern. Parts of the cloud which reflect more solar radiation might appear brighter than shadowed regions.
The Natural Colour RGB is nevertheless not able to distinguinsh ice from water clouds under all situations. Warm water clouds with large drop sizes also appear cyan in the Natural Colour RGB as can be seen in Himawari-8 images (see image below, "mixed?" area).
If Himawari channel 2.3 micrometer is used instead of VIS0.8, some parts of the cyan colour in the Natural Colour RGB reveal as being water clouds.
A difficulty may arrise from the fact that snow and ice on the ground show similar cyan colour scales than ice clouds (see green and red arrow in the left hand image). In practise, ice clouds are easy to distinguish from the first by their more intense colour and often clumpy appearance.
Explanation of the colours of ice clouds in the Natural Colour RGB (see also the recipe on the left side):
• Radiation mesured by the SEVIRI instrument of MSG in channel 1 (0.6 micron) and channel 2 (0.8 micron) contributes much more to the Natural Colour RGB than radiation measured in channel 3 (1.6 micron). Solar radiation is strongly absorbed by snow and ice crystals at 1.6 micrometer while it is strongly reflected in both visible channels at 0.6 and 0.8 micrometer. Cyan is resulting from a mixture of blue and green.