The Green Flash

The reason for green flash optical phenomena lies in refraction of light (as in a prism) in the atmosphere: light moves more slowly in the lower, denser air than in the thinner air above, so sunlight rays follow paths that curve slightly, in the same direction as the curvature of the Earth. Higher frequency light (green/blue) curves more than lower frequency light (red/orange), so green/blue rays from the upper rim of the setting sun remain visible after the red rays are obstructed by the curvature of the earth.
Green flashes are enhanced by mirage, which increase the density gradient in the atmosphere and therefore, increase refraction. A green flash is more likely to be seen in clear air, when more of the light from the setting sun reaches the observer without being scattered. One might expect to see a blue flash, but the blue is preferentially scattered out of the line of sight, and remaining light ends up looking green.
With slight magnification a green rim on the top of the solar disk may be seen on most clear-day sunsets, although the flash or ray effects require a stronger layering of the atmosphere and a mirage, which serves to magnify the green for a fraction of a second to a couple of seconds.


Bruce and I are at Venice Beach California. We are taking a cruise through the Panama Canal that leaves south of here. Tonight we watched the sun set at the beach on the Pacific Ocean and saw the green flash. We've seen the flash several times when we lived aboard Arcadia. When a sailor sees the green flash it means safe travels for him. It is amazing and exciting to see this phenomena. We should have safe travels on our cruise.