A Green Flash is an atmospheric refraction phenomenon which shows up as a short green f]are at the upper rim of the sun. It is rarely observed with the naked eye, mainly because it requires special conditions, but also because many people do not know when and where to look for it.
There are three different kinds of the Green Flash:
The basic reason for the Greer Flash to appear is the refraction of the sunlight. As refraction is more prominent near the horizon, the last segment of light from the setting sun is there split up into its spectral colours. There is a red, a green ands a blue rim of the sun, This splitting increases towards the horizon, but even there it is only a few arcseconds small (one arcsecond is 1/3600 degree). As the red rim sinks first below the horizon, there are only the green and the blue rim above the horizon for a very short time. The blue light gets dimmed a lot by the pollution of the atmosphere, and for a few seconds the green rim really becomes visible. Under extraordinary good conditions also the blue light can be seen in very few cases.
But what conditions are necessary to see a green flash? A green flash can be observed well when there is a free sight to the horizon and the air is very clear and clean. This is also the reason why a green flash is often observed on the endless horizon over seas and oceans where there is an almost parallel geometrical sight to the horizon. On a mountain top there are also good conditions for observing a green flash as observers there often are above the inversion layer.
To observe the green flash well it is important to take scrupulous precautions in order to avoid severe eye damage. Although the sun is near the horizon and its brightness is dimmed by the atmosphere, the sun is still very bright and longer observation with the naked eye or through optical devices will in any case damage your eyes.
A green flash can occur not only at the sun, but, of course, also at the moon. It has even been observed at Venus, Jupiter and Saturn.
There is not only a green flash and a blue flash, but - just like it can be concluded - also a red one. This occurs when the lower rim of the sun or the moon is just rising above the horizon or when the sun has sunk behind a sharply confined cloud bank near the horizon and its lower part is just coming out be1ow the cloud bank. This phenomenon, however, is even more transient than the green flash. Furthermore, it is very seldom that a real flash appears. So it is very difficult to observe the red flash.
The appearing of the green segment requires a calm atmosphere with several layers of air masses of different density. When the sun is just a few degrees above the horizon, it appears distorted and vertically flattened. The nearer it gets to the horizon the more flattened it appears. Soon the rim of the sun seems to be notched at both sides. These notches caused by refraction and reflection seem to “ride“ on the upper or lower rim of the sun and then suddenly turn green. The intensity of the colour depends on several factors. When the sun is too bright the colours fade. But when it is too faint, colours are also hardly visible. Thus, unmistakable photographs of the green flash are very difficult to obtain.
Atmospheric refraction effects also cause the “green rim“ or the so-called Nowaya Zernlya Effect which has been named after an archipel which extends from the Ural mountains in Russia into the Arctic Sea. A narrow green line appears at the upper rim of the sun and sometimes is still visible when the sun itself has already sat. This narrow green line of light moves along the horizon like the sun does below it. This effect mainly appears at higher latitudes where the angle between the sun‘s path in the sky and the horizon sometimes can be very small. But even if this effect can be observed at medium latitudes, it is visible for up to a few minutes after sunset at very few occasions.
Have you ever seen the sun set behind the horizon? - Yes, of course! - Did you follow him with your eyes until the upper rim of his disk just touched the horizon wanting to dive behind it? - Yes, very probably. - But did you take notice of the phenomenon that occurs at the very last ray of sunlight, when the sky is without any mist and completely clear? - Maybe not. - So, the next time there is an occasion for this observation (it is very seldom) make yourself sure that it is no red ray that you are going to see, but a green ray, of a wonderful green, a green that no painter would ever get on his palette, a green that nature did not yield anywhere else, not among the variety of colours of the plants nor in the colour of the clearest oceans! If there is a green in paradise, then it cannot be another green than this one, the real green of hope.
[Jules Verne: Le Rayon vert]
Terrace upon a dune, looking far over the North Sea. Deep blue waters,
cloudless skies, no cloudiness, no haze, horizon tense and sharp. White veils
show up in a light blue, all shadows are conspicuous in a special blue. Sun,
big clear disk of copper, gleaming in bright golden air, sinking into the sea
like a ball of fire, a fisherman passes, doesn´t go up in flames. The sun keeps
his colour of copper, now he has almost gone, a spot, clear, blue green flame
like a jewel surrounded by gold on the dark blue sea, the Green Ray is over.
[J.P.F. van der Mieden van Opmeer: Hemel en Dampkring 30, 234f, 1932]