Development of Green Flash

Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible, usually for no more than a second or two, above the sun, or a green ray shoots up from the sunset point. Green flashes are actually a group of phenomena stemming from different causes, and some are more common than others.^[1] 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.

A larger than usual green flash in Santa Cruz

[edit] Explanation

Stages of a green flash

Mock Mirage green flash observed in San Francisco

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 mirage, 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]

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] Green rim

Upper rim is green and lower rim is red while the sun is setting behind the Golden Gate Bridge

As an astronomical object sets or rises, the light it emits travels through the atmosphere, which works as a prism separating the light into different colors. The color of the upper limb of an astronomical object could go from blue to green to violet depending on the decrease in concentration of pollutants as they spread throughout an increasing volume of atmosphere.^[3] The lower limb of an astronomical object is always red.
A green rim is very thin, and is difficult or impossible to see with the naked eye. In usual conditions a green rim of an astronomical object gets fainter, when an astronomical object is very low above the horizon because of atmospheric reddening,^[4] but sometimes the conditions are right to see a green rim just above the horizon. The following quote describes probably the longest green rim, which at times could have been a green flash, observation. It was seen on and off for 35 long minutes by members of the Richard Evelyn Byrd party from the Little America exploration base in 1934. Often a green rim changes to a green flash and back again during the same sunset. The image below is an illustration of what members of the Richard Evelyn Byrd party from the Little America exploration base may have seen.^[5][6]

Green rim and green flashes of the setting sun

SEEN FOR HALF HOUR
"There was a rush for the surface and as eyes turned southward, they saw a tiny but brilliant green spot where the last ray of the upper limb of the sun hung on the skyline. It lasted an appreciable length of time, several seconds at least, and no sooner disappeared than it flashed forth again. Altogether it remained on the horizon with short interruptions for thirty-five minutes.
When it disappeared momentarily it seemed to have been shut off by a tiny spurt, an inequality in the skyline caused by the barrier surface.
"Even by moving the head up a few inches it would disappear and reappear again and after it had finally disappeared from view it could be recaptured by climbing up the first few steps of the antanea [sic] post."^{Citation needed for quotation.}

To see green rim on and off for 35 minutes there had to be some mirage present.

There's an interesting situation with green rim observing. Green rim is present at every sunset, but it is too thin to be seen with the naked eye. The best time to observe a green rim is about 10 minutes before sunset.^[4] It is too early to use any magnification like binoculars or a telescope to look right at the Sun. (Of course, a magnified image might be projected onto a sheet of paper for safe viewing.) When the sun gets closer to the horizon, the green rim is becoming fainter because of atmospheric reddening.^[4] According to the above it is probably correct to conclude that although a green rim is present during every sunset, a green flash is more rare because it requires a mirage to be present.

[edit] References

[edit] See also

• Mirage of astronomical objects

[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.

[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
	Mock mirage green flashes in the same sunset	Mock mirage green flashes in the same sunset	Mock Mirage green flash observed in San Francisco
Mock Mirage green flash observed in San Francisco	Mock Mirage green flash observed in San Francisco	Mock Mirage green flash observed in San Francisco	Sub-duct green flash followed by small mock mirage green flash