Green flashes can be observed from any altitude (even from an aircraft). They are usually seen at an unobstructed horizon, such as over the ocean, but are possible over cloud-tops and mountain-tops as well.
Green flashes are enhanced by atmospheric inversions, 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. We might expect to see a blue flash, but the blue is preferentially scattered out of our line of sight and remaining light ends up looking green.
With slight magnification a green rim on the top limb of the solar disk can be seen on most clear-day sunsets. However 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.
|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 limb 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||Unknown|
The majority of flashes observed are inferior-mirage or mock-mirage ones, 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.