A rare optical effect in which the tip of the Sun's
disk suddenly and briefly changes to a vivid green, either as the last remnant
of the setting Sun vanishes or just at the moment of sunrise. Caused by
the preferential refraction of light, it requires a distant, sharply-defined
low horizon (preferably the sea) and special atmospheric conditions, including
cool weather. The period of visibility tends to increase in summer months
with increasing latitude, i.e. as the angle of descent of the Sun decreases.
In the Antarctic, it has been observed for as long as 30 minutes. Occasionally
it takes the form of a white flash followed by a deep blue one. A similar
phenomenon, the red flash, is sometimes seen as the lower edge of
the Sun emerges from a dark cloud near the horizon.
| Green flash observed in Finland.
Image credit: Pekka Parvianinen
PHENOMENA AND STRUCTURES