Green flash
From Wikipedia, the free 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.
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[edit] Explanation
Its explanation lies in refraction of light (as in a prism) in the atmosphere and is enhanced by atmospheric inversions. 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 (caution: never point binoculars or a telescope at the sun), 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.
[edit] 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.
[edit] 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.)
[edit] 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 The Green Ray (film). The green ray is visible 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.
Raymond Feist, a fantasy writer, mentions it once to demonstrate one of the rare treasures of a life of traveling.
Sonic Youth's song "Jams Run Free" describes the occurrence in its chorus.
The green flash is mentioned in Middlesex by Jeffrey Eugenides
There is a beautiful fragment (in Spanish) by Julio Cortazar, on the 'Green Ray', see www.cortazartextual.com.ar/fragmentos/rayo.html
Roger Penrose, in his book "The Road to Reality" Epilogue includes the 'Green Flash'
[edit] External links
- A Green Flash Page, Andrew T. Young's page with comprehensive explanations and simulations.
- Green Flash - Atmospheric Optics, Explanations and image gallery, Les Cowley's Atmospheric Optics site.
- Straight Dope Article on the Green Flash from Cecil Adams
- Green flash photos
- A Green Flash Gallery
- A Green Flash from Astronomy Picture of the Day, NASA
- Hyperphysics
- Fact Monster
