The Question of GreenLooking at a collection of different images, we can readily see that sunsets can have almost any color found in the color wheel - blue, hues of violet and pink, red, orange, yellow or white - with one rather notable exception. Green.
So - are there green sunsets? Internet searches for green sunsets produce a fair share of images, but many of them appear to be simply post-processed by a green filter for artistic reasons. Why is green difficult to generate at all? Perhaps you might think of a watercolor kit where one can mix yellow and blue to get green - but that is subtractive color mixing, it only works for pigments that absorb a certain wavelength. For low sun colors, we deal with additive color mixing, i.e. with colored light, and here green is one of the pure colors, it can't be generated by mixing any of the colors we have (see Appendix A for an explanation). Thus, to get green light, one would have to remove both the long wavelength (red) and the short wavelength (blue) part of the original sunlight spectrum such that only the green spectral range (roughly in the center of the visible wavelength range) remains. With Rayleigh scattering, there is a process which readily removes the blue part. But we have no corresponding scattering process to remove the red part. Thus, there are really only two possibilities how green can arise in the sky: First, a different physical process which is not related to scattering removes the red part of the light, and this light propagates through the atmosphere which then removes the blue part via Rayleigh scattering. This can indeed happen and leads to the Green Flash phenomenon. Second, conceivably the blue skylight can serve as the sole illumination of a distant white cloud or haze - thus the cloud would appear blue. If the view ray to that object is shadowed by a cloud layer such that there is no in-scattering, Rayleigh out-scattering will remove the red part and make the object appear greenish. This would generate a true green sunset.
The Green FlashThe speed of light is different in vacuum and a medium, and that difference is wavelength dependent. As a result, light passing through a medium is refracted into a rainbow of different colors - the most striking illustration is an optical prism. In principle, the same thing happens when light passes through the atmosphere - the uppermost part of the low sun disc is blue, the lowermost part is red. Usually we can't see that because the still white central part of the sun disc is much brighter and makes it tricky to observe colors.
However, when the sun is actually below the horizon, the refracted uppermost part of the disc may still appear above the horizon for a few moments (usually no longer than seconds). And that part of the light does not contain the red wavelengths, because for them the refraction is less prominent and so the red light is already below the horizon. In very clear air the result is a Blue Flash, but more usual is that Rayleigh scattering on the view ray changes the appearance to green by removing the short wavelength light, and a Green Flash appears.
As in the example, the Green Flash can be made even more prominent by Mirage-reflections which double or even triple the green sun disc sliver. The Green Flash is an extremely transient phenomenon, and it affects only the last visible sliver of the sun disc itself, it does not illuminate any clouds in green for instance (because for higher clouds, the sun would not be below the horizon when it is for the observer on the ground). Moreover, it requires an unobstructed view to the horizon.
Green SunsetsThe second possibility to create green is the combination of blue skylight with out-scattering on the view ray. That requires a day with strong Rayleigh scattering, i.e. lots of dry haze and a generally 'dirty' sky appearance.
Generally the effect isn't very prominent (and the actual color is more brown than green), but it can be visually enhanced by the contrast with red clouds. The scene here shows originally blue sky seen through relatively strong Rayleigh cattering with an obscuring cloud layer preventing in-scattering of blue light. Can it be as prominent as this?
I don't really have a good explanation for this sky, it doesn't look obviously post-processed and conceivably the faint clouds could be illuminated by the blue sky above and then shifted to green by out-scattering, but the amount of dry haze would have to be exceptionally strong for this explanation to work. Since this is obviously not my picture, I hesitate to make any strong conclusions - but if such a sky really exists, it must be exceptionally rare.
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