Thin semitransparent ice clouds
Thin cirrus clouds over sea are pale blue in the Snow RGB images.
Meteosat, SEVIRI Snow RGB image for 07 March 2014 09:25 UTC (left) and 11 March 2015 09:10 UTC (right)
Note that thicker semitransparent ice clouds are orange even over sea.
Explanation of the colours of the thin semitransparent clouds over sea (see the recipe):
In case of semitransparent clouds the satellite measures mixed radiation consisting of the radiation emitted or reflected by the semitransparent cloud itself and the radiation originated from below and transmitted through it. That is why the colour of a semitransparent cloud depends not only on the characteristics of the cloud itself, but also on the transparency and the colour of the underlying surface.
• The colour of the almost thick semitransparent cloud approximates the colour of the thick ice clouds (orange, characterised by relatively low blue component).
• The colour of the very thin cirrus clouds approximates the colour of the underlying clouds or surface (dark blue in case of sea).
• The colour of the thin cirrus is between the colour of the thick ice cloud and that of the underlying cloud or surface. Over the bluish sea, the colour of the very thin cirrus ‘loses’ its orange characteristic.