The Green Flash
Green flashes are a rare optical atmospheric phenomena that occur immediately before suns sets completely or immediately after sun rise begins. This small green spot is visible just above the upper rim of the disk of the sun and only when the conditions are right. The green appearance usually lasts only for a second or two and is very hard to photograph.
Green flashes occur because the atmosphere can cause the light from the sun to separate out into different colors. Green flashes are a group of phenomena which stem from slightly different causes, and therefore some types of green flashes are more common than others
Even rarer, the green flash can resemble a green ray shooting up from the suntouch point.
The Green Flash,
photo credit: Brocken Inaglory
How to see a Green Flash
Green flashes may be observed from any altitude but are usually seen at an unobstructed horizon, such as over the ocean. Some have bee seen over cloud tops and mountain tops.
A Green Flash Explained
Green flashes are enhanced by mirage, which increase refraction. A green flash is more likely to be seen in stable, 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, since blue light is refracted most of all, and the blue component of the sun′s light is therefore the very last to disappear below the horizon, but the blue is preferentially scattered out of the line of sight with the remaining light appearing green.