Green flash

From Free net encyclopedia

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. It is usually observed from a low altitude where there is an unobstructed view of the horizon, such as on the ocean.

Image:Rayon vert observatoire de La Silla.jpg

1 Explanation
2 Types of green flashes
- 2.1 Blue Flashes
3 In the media
4 External links

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Explanation

Its explanation lies in refraction of light (as in a prism) in the atmosphere and is enhanced by atmospheric layering. Whilst we would expect to see a blue light, the blue is dispersed (this is why the sky is blue) and only the green light remains visible.

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.

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Types of green flashes

The green flash is actually a group of phenomena, some of which are listed below ([1]):

Phenomenon	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; duration 1 or 2 sec.	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 sec.	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 that seems to shoot up from a green flash, or is seen immediately after sundown. Usually only a few degrees long; lasts no longer than a couple of seconds	hazy air and a bright green flash of one of the kinds named above that acts as a light source	unknown

Most observations involve inferior-mirage flashes, while the rest are mostly mock-mirage flashes. The two other kinds are rare, constituting only about 1% of all reported flashes. The green flash phenomena that are not listed here - such as the cloud-top flash, which is usually seen as the Sun sinks into the coastal fog and sometimes also at distant cumulus clouds — are omitted because they are not fully understood. Moreover, Alistair Fraser's flashes, which are seen in hilly country, are a variant of the mock-mirage flash in places where inversions are pushed up over hills and are subsequently not listed in the chart.

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Blue Flashes

Very occasionally, the amount of blue light is sufficient to be visible as a "blue flash". [2] (This is not to be confused with the blue flash reported to occur during nuclear criticality accidents.)

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In the media

Jules Verne, the early French science-fiction writer, wrote a book named Le rayon vert (The green ray) whose hero is chasing this elusive phenomenon.

Éric Rohmer, a French film director, made a movie named Le rayon vert where one gets to see a green ray in the last scene.

Carl Hiassen, an American author, wrote a novel called Flush, in which the characters see a green flash at the end of the book.

The green flash features a couple of times in the works of Arthur C. Clarke, UK science-fiction writer.

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