blue flash

Green flash

Green flashes and green rays are rare optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible for a short period of time above the sun, or a green ray shoots up from the sunset point.

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.

Explanation

The reason for a green flash 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 limb of the setting sun remain visible after the red rays are obstructed by the curvature of the earth.

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.

Types of green flashes

The green flash is actually a group of phenomena, some of which are listed below:

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.

Blue flashes

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

Green flashes in fiction

This phenomenon features as a major plot device in the Walt Disney Pictures film Pirates Of The Caribbean: At World's End. In the movie's back story, the green flash is fictionalised, becoming an occurance of legendary quality that is rumoured to signify a soul returning from the dead. Later in the film, this concept is further expanded, and the flash is revealed to signal when a trapped spirit escapes from its imprisonment in Davy Jones' Locker (see Geography of Pirates of the Caribbean), a feat achieved by the film's main characters. However, the size and scale of the flash depicted in the film is greatly exaggerated.

There is also reference to mysterious green light in F. Scott Fitzgerald's The Great Gatsby.

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