Green flash

Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible, usually for no more than a second or two, above the sun, or it may resemble a green ray shooting up from the sunset point. Green flashes are a group of phenomena stemming from different causes, and some are more common than others.^[1] Green flashes may be observed from any altitude (even from an aircraft). They usually are seen at an unobstructed horizon, such as over the ocean, but are possible over cloud tops and mountain tops as well.
A green flash also may be observed in association with the Moon and bright planets at the horizon, including Venus and Jupiter.^[2][3]

A Green flash(National Geographic Society)

Explanation

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.

[edit] Types

The "green flash" description relates to a group of optical phenomena, some of which are listed below:^[1]

Type	Characteristics	Conditions	Best seen from...
Inferior-mirage flash	Joule's "last glimpse"; oval, flattened below; lasts 1 or 2 seconds	Surface warmer than the overlying air	Close to sea level
Mock-mirage flash	Indentations seem to "pinch off" a thin, pointy strip from the upper rim of the Sun; lasts 1 or 2 seconds	Atmospheric inversion layer below eye level; surface colder than air	The higher the eye, the more likely; flash is most obvious when the eye is just above the inversion.
Sub-duct flash	Large upper part of an hourglass-shaped Sun turns green for up to 15 seconds;	Observer below a strong atmospheric inversion	In a narrow height interval just below a duct (can occur at any height)
Green ray	Green beam of light either shooting up or seen immediately after sundown; usually few degrees long, lasting several seconds	Hazy air and a bright green flash acting as a light source	Sea level

The majority of flashes observed are inferior-mirage or mock-mirage effects, with the others constituting only 1% of reports. Some types not listed in the table above, such as the cloud-top flash (seen as the sun sinks into a coastal fog, or at distant cumulus clouds), are not understood.^[1]

[edit] Blue flashes

Very occasionally, the amount of blue light is sufficient to be visible as a "blue flash".^[4] The term should not be confused with the similar usage of blue flash referring to the blue light seen in nuclear criticality accidents.

[edit] Green rim

As an astronomical object sets or rises in relation to the horizon, the light it emits travels through Earth's atmosphere, which works as a prism separating the light into different colors. The color of the upper rim of an astronomical object could go from green to blue to violet depending on the decrease in concentration of pollutants, as they spread throughout an increasing volume of atmosphere.^[5] The lower rim of an astronomical object is always red.
A green rim is very thin, and is difficult or impossible to see with the naked eye. In usual conditions, a green rim of an astronomical object gets fainter, when an astronomical object is very low above the horizon because of atmospheric reddening,^[6] but sometimes the conditions are right to see a green rim just above the horizon. The following quote describes what was probably the longest observation of a green rim, which at times could have been a green flash. It was seen on and off for 35 minutes by members of the Richard Evelyn Byrd party from the Little America exploration base in 1934:

There was a rush for the surface and as eyes turned southward, they saw a tiny but brilliant green spot where the last ray of the upper rim of the sun hung on the skyline. It lasted an appreciable length of time, several seconds at least, and no sooner disappeared than it flashed forth again. Altogether it remained on the horizon with short interruptions for thirty-five minutes.
When it disappeared momentarily it seemed to have been shut off by a tiny spurt, an inequality in the skyline caused by the barrier surface.
Even by moving the head up a few inches it would disappear and reappear again and after it had finally disappeared from view it could be recaptured by climbing up the first few steps of the antanea [sic] post.^[7]

For the explorers to have seen a green rim on and off for 35 minutes, there must have been some mirage effect present.
A green rim is present at every sunset, but it is too thin to be seen with the naked eye. Often a green rim changes to a green flash and back again during the same sunset. The best time to observe a green rim is about 10 minutes before sunset.^[6] That is too early to use any magnification like binoculars or a telescope to look directly at the Sun without potential harm to the eyes. (Of course, a magnified image might be projected onto a sheet of paper for safe viewing.) When the sun gets closer to the horizon, the green rim is becoming fainter because of atmospheric reddening.^[6] According to the above, it is probably correct to conclude that although a green rim is present during every sunset, a green flash is rarer because of the required mirage.
Green rim flashes