Thick ice clouds
Thick ice clouds usually appear orange in the Snow RGB images. Thick ice clouds with small ice crystals on the top appear light orange, or light yellow.
The left image and the next two images show frontal cloudiness. Snow is seen in the Alps, Carpathian Mountains, Russia, Scandinavia, Iceland and Dinaric Alps.
Meteosat, SEVIRI Snow RGB, 15 March 2014 09:25 UTC
Meteosat, SEVIRI Snow RGB, 07 March 2014 09:25 UTC
Thick ice clouds usually have large ice crystals on their top appearing orange in the Snow RGB. In case the ice crystals on the cloud top are small the thick ice cloud appears light orange (closer to grey). In case the ice crystals are very small then the cloud is depicted in light yellow occasionally with some greenish tones, see the thicker part of the high level lee clouds south and southwest of the Alps in the image below. High-level lee clouds typically consist of very small ice crystals. (For comparison the Day Microphysics RGB is visualised.)
High-level lee clouds with small ice particles in the Snow RGB (up) and Day Microphysics RGB (below) south and southwest of the Alps for 16 March 2014, 09:40 UTC
Explanation of the colours for thick ice clouds (see the recipe):
• The thick ice clouds usually have large ice crystals on their top. The reflectivity of a thick ice cloud is high in the 0.8 micrometer channel, and much lower in the 1.6 and in 3.9 micrometer channels, because ice absorbs the radiation at these wavelengths. The absorption is much stronger at 3.9 than at 1.6 micrometer. Even after the linear stretching and gamma correction the signal is still strongest in the red and weakest in the blue colour beam. As a result the thick ice cloud appears orange.
• If the cloud top ice crystals are small then the green and blue component will be closer to the red component, but still lower. Such a thick ice cloud will appear light orange (closer to grey). In case the crystals are very small than the green component may became equal or even slightly higher than the red component causing light yellow colour occasionally with some green shades.