Warm water clouds

Warm water clouds appear white in the Natural Colour RGB images.

Low and optically thick water clouds appear whithish in the Natural Colour RGB because the mesured reflected short wave radiation at 0.6, 0.8 and 1.6 micrometer are nearly equal. The image below shows fog patches over the black sea.

Natural Colour RGB for 30 April 2012 06:00 UTC

Very low and thin water clouds tend to have a more redish tone because the green and the blue colour beam from the visible channels at 0.6 and 0.8 micrometer decrease in intensity (see image below, Mediterranean Sea around Sicily). The reflection of short wave solar radiation in both visible channels correlates with the optical thickness of the cloud.

Natural Colour RGB for 29 January 2016 12:00 UTC

When ice crystals form at the top of the water clouds solar radiation at 1.6 micrometer is absorbed by ice and the intensity of the red colour beam decreases. Ice cystals do not affect the amount of refelcted solar radiation at 0.6 and 0.8 micrometer, water clouds mixed with ice crystalls are therefore overlayed by a cyan veil (upper left part of the above image).

Explanation of the colours of warm water clouds in the Natural Colour RGB (see the recipe):

• For optically thick warm water clouds, reflected solar radiation measured in all 3 channels is about the same. The resulting colour is white, sometimes grey depending on the solar angle.
• For optically thin warm water clouds, reflected solar radiation in channel 3 (1.6 micrometer) outbalances the measured radiance in the visible channels (0.6 and 0.8 micrometer). The resulting colour is reddish.
• Mixed ice-water clouds appear with a cyan veil above the white cloud feature. Similar to pure ice clouds, the solar radiation at 1.6 micrometer is absorbed by ice crystals (only slightly because of the fewer ice crystals), resulting in a reduction of the red colour beam.