Thin fog or low clouds over land appear usually pinkish grey at mid-latitudes in the VIIRS Night Microphysics RGB images.
Thin fog/low clouds over land may appear in many colour shades:• The colour shade depends on the transparency of the thin fog/low cloud, and on its microphysical properties.
The images below show examples of thin fog/low clouds over the north-eastern part of Hungary and neighbouring counties.
The central region of the image below shows thin fog over the south-eastern part of Poland.
The image below shows thin fog/low clouds over the Po Valley. Some parts of the fog is evidently transparent. The thin fog here has more blue shades as it is relatively warm (~10°C). Note that in the lower part of the image thick clouds are also present.
The image below shows thin and thick fog/low cloud at the coast of the Baltic countries, both over sea and land, see the encircled areas. The cloud top temperature was relatively warm (~10°C). The thin fog has slightly different colour shade over sea and land as the background has different colour.
Note that if thin fog/low cloud are much colder than the warm land below then the thin clouds appear in a different shade (weaker red component). However, this happens rarely over land. This effect is discussed in the example of ‘thin fog/low cloud over sea’.
Note that thick fog or low water clouds consisting of large droplets usually appear also pinkish grey.
Explanation of the colour of thin fog/low water clouds over land at mid-latitudes in the Night Microphysics RGB (see also the recipe on the left hand side):
In case of a thin cloud the measured radiance is a mixture of the radiance coming from below it and the radiance emitted by the cloud. Note that the IR channel differences depend on the difference between the surface and on the cloud top temperatures as well. However, in case of fog/low clouds over land this difference is usually low. So, the colour of the thin cloud usually will be a ‘simple weighed mixture’ of the colour of the underlaying surface and the colour of the (thick) cloud. (The weights depend on the transparency of the cloud)
• In case the land is pinkish (typical colour of cool land) thin fog/low cloud over land will be pinkish grey.
• In case bluish land surface, the thin fog/low cloud may appear bluish grey.