A green flash belongs to the group of photometeors. It is a predominantly green coloration of short duration, often in the form of a flash, seen at the extreme upper edge of a luminary (sun, moon, or sometimes even a planet) when disappearing below or appearing above the horizon. Although the color of the phenomenon is predominantly green, blue and violet may also be visible, particularly when the air is very transparent. It is more frequently seen over sea than over land. A satisfactory explanation for the "green flash" has so far not been given but it is most probable that the different refrangibility of light of different colors plays an important role in its formation.  

The picture was taken on a sunset at sea. On the upper part the green flash did appear on several short occasions. On the under part another photometeor can be seen, a so-called inferior mirror effect on which the sun seems to run into sea. This effect is due to different density of the atmosphere layers: in this case a colder atmosphere layer is on top of a warmer surface layer of the sea.