Cloud free vegetated land depicts in blue in the Day Microphysics RGB images.
The red colour beam (VIS0.8) is smaller compared to the blue colour contribution. The reflectivity in the VIS0.8 channel depends mainly on the sun angle.
The reflection of short wave solar radiation measured by channel IR3.9 is higher for bare soils (sandy desert) than for vegetated surfaces. Vegetated surfaces have a small albedo at 3.9 micrometer.
The intensity of the blue colour beam increases with temperature. Vegetated surfaces do not reach such high temperatures as sandy deserts do, therefore, the blue colour contribution is around average.
With all 3 colour beams not contributing very much over vegetated soils, the combination of them results in a darker blue, sometimes aiming into violet when the red beam contribution of channel VIS0.8 is of the same amount then IR10.8. When vegetation cover become scarse, the combined colour turns into a lighter blue (see Spain in the image below).
HRV Cloud RGB from 7 April 2015, 12:00 UTC
Explanation of the blue colour of vegetated soils in the Day Microphysics RGB (see the recipe):
• With all 3 colour beams contribution with an average value, the red and the blue colour slightly dominate. This results in a darker blue to violet colour for vegetated soils.