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.
Green flashes can be observed from any altitude (even from an airplane). They are usually seen at an unobstructed horizon, such as over the ocean, but are possible over cloud-tops and mountain-tops as well.
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[edit] 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.
[edit] Types of green flashes
The green flash is actually a group of 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 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.[1]
[edit] Blue flashes
Very occasionally, the amount of blue light is sufficient to be visible as a "blue flash".[2] The term should not be confused with the similar usage of blue flash referring to the blue light seen in nuclear criticality accidents.
[edit] References
- ^ a b Young, A. "Green flashes at a glance" (2006), San Diego State University page. Retrieved on 09/05/07.
- ^ "The Green Flash, BBC Weather online. Retrieved on 09/05/07.
[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.
- A Green Flash from Astronomy Picture of the Day, NASA.
- Green flash picture released by the European Southern Observatory (ESO) May 2, 2008. Conditions for seeing green flashes are nearly ideal at the ESO in Chile, and they are seen regularly.
[edit] Gallery
 Mock mirage sunset and green flash |
 Some green flashes observed in San Francisco in 2007 |
 Some green flashes observed in San Francisco in 2007 |
 Sunset sequence with two green flashes in the second and fourth frames |
 Sunset sequence with green flashes |
 Inferior mirage green flash |
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 Mock mirage green flashes in the same sunset |
