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I work with Sentinel-2 images (Level-1C products (top-of-atmosphere reflectances)). I would like to know why some of the images do have this blue(?) undertone/color cast. I think those images are from the winter months.
Is there another explanation than different light conditions?
I would like to know the possible reasons for this effect.
Image as expected:
Color cast:
Blue color cast is caused by atmospheric scattering (= light reflecting off particles in the atmosphere is added to the light reflected from the ground), and it is always present in top-of-atmosphere images.
Its strength varies with atmospheric conditions (e.g. more aerosols or water vapour scatter more light, hence the blue cast will be stronger) and sun angle (lower sun angle will cause more light to be deflected in the direction of the sensor).
The reason the additional light is blue is related to the reason the sky is blue - light with shorter wavelengths is scattered more.
There are several potential reasons for observing differences in the images.
First, most likely you are observing differences in the level of atmospheric aerosols which causes scattering. To rectify this, you need to use the Sentinel-2 Sen2Cor processor to convert top-of-atmosphere (TOA) reflectance (Level 1C) to bottom-of-atmosphere (BOA) reflectance (Level 2A).
Second, if the scene is from a different time period, there may be differences in the amount of green vegetation or snow--thus producing visual differences.
Third, you may need to adjust the histogram so that the image renders properly. you can do this directly in the Sentinel-2 SNAP viewer. Adjust the slider so that the majority of your image pixel values are encompassed in the interval for all the bands. In this case, you would want to move the upper slider to the left to "clip" the histogram.
