SpssWhat causes this?
When light from the Sun reaches earth, it's wavelengths are preferentially scattered by atoms and particles and molecules in the atmosphere. But, due to the different energy levels within the visible light spectrum and the geometry of the particles the wavelengths are eventually colliding with (in the atmosphere), the light is not scattered equally. The gradient of the light dispersing in the sky, or at a certain point, and thus reaching our eyes is an important factor we must consider too.
So with these components we can explain the green flash, which usually occurs at the suns very last visible position at sunset, and only for a moment or two. The atmosphere at which the suns' light travels through, from it's extreme angle, towards our eyes preferentially scatters particles from the green wavelengths and it's closest relatives in terms of energy levels and physical shape.