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I lived a long time in the tropics and often heard of, but never saw, the "green flash" that is said to occur just at sundown. Does it actually exist, and if so, what causes it?

Jerry Nelson of Lick Observatory and the University of California at Santa Cruz passed along the following information about the mysterious "green flash":

"The green flash does indeed exist. It occurs when the sun is rising or setting. It is a consequence of the dispersion of light by air. Light from the sun is bent (refracted) by the Earth's atmosphere; the shorter the wavelength, the greater the amount of bending. Hence, blue light is bent more than green, which is bent more than yellow, which is bent more than red. Further, the amount of bending increases as the sun approaches the horizon.

"By the time the apparent sun (that is, the sun as we see it in the sky) just starts to set, the actual sun (as we would see it if there were no atmospheric refraction) has completely set. The bending due to refraction at sunset is just about 0.5 degree--roughly the same as the angular diameter of the disk of the sun. One can imagine the image of the sun in the various colors of the spectrum, each deflected a slightly different amount by refraction. At the horizon, the blue apparent sun is highest and the red apparent sun is the lowest. This color shift is rather small, even at the horizon, but it is key to understanding the green flash.

"Note also that the atmosphere scatters blue light much more effectively than red light. (That is why the sky appears blue.) When the sun is setting, its light passes through a lot of atmosphere. As a result, the light of the blue sun is strongly scattered and attenuated. The green sun, however, is less diminished by scattering.

"In the evening, the red sun sets first, so there is a period when the red sun has set but the green sun--or at least a tiny sliver of it--is still above the horizon. Thus, at the last moments of a sunset we can get a green image of the upper edge of the sun: the green flash. If it were not for the scattering of the atmosphere, we could get blue flashes. At sites that have very clear air, one sometimes can see turquoise flashes.

"All of this happens much more clearly when the sunset is truly at the horizon, so sunsets over the ocean in a cloudless sky offer the best chance of catching the green flash. Sunsets occur more quickly near the equator than at higher latitudes, so the green flash is a shorter phenomenon in the tropics. It is sometimes easier to see the green flash through binoculars, but DO NOT STARE AT THE SETTING SUN. Wait until just a few seconds before the end of the sunset to look with binoculars.

"The green flash is gorgeous, and well worth the patience it takes to find an appropriately clear day during sunset (or sunrise)."

Jay M. Pasachoff, Field Memorial Professor of Astronomy at Williams College in Williamstown, Mass., adds some details to the above explanation:

"The green flash is visible at sunrise and sunset when you have a perfect view of the horizon; it is usually best seen when you are looking over the ocean. Atmospheric refraction spreads out the sun's image, with red at the bottom and blue at the top. Think of an overlapping set of disks in the colors ROY G BIV (red orange yellow green blue indigo violet).

"The blue-indigo-violet colors do not come through the atmosphere because of intense Rayleigh scattering when the sun is low above the horizon. Further, when the sun is low, the orange and yellow are absorbed by water vapor. So we get a green image above the red image, with the two largely overlapping. When the red image entirely sets, the top of the green image is visible, but only briefly--for a second or so. The brevity of the green flash is one of the reasons it is so rarely seen.

"A photograph of the green flash appears in the book 'A Field Guide to the Stars and Planets' by Jay M. Pasachoff and Donald H. Menzel (Peterson Field Guide Series, Houghton Mifflin, 2nd ed., 1992) on page 54, with an explanation of the phenomenon on page 422.

(We also call interested readers' attention to "The Green Flash," by D. J. K. O'Connell, in Scientific American, January 1960, pp. 112-122.)

Andrew T. Young of San Diego State University adds:

"I'd like to correct a minor slip in Jay Pasachoff's comment about green flashes. The orange light in sunsets isn't really much absorbed by water vapor; ozone is the main absorber in that part of the spectrum.

"It might also be worth pointing out why a sea horizon favors the visibility of green flashes. Most green flashes--certainly, all that are visible to the unaided eye--are associated with mirages, or mirage-like phenomena. By an old theorem of atmospheric optics, inverted images can only occur in the sky below the astronomical horizon. So to see a green flash, you need an apparent horizon that is as far below eye level as possible; the sea serves nicely.

"I have some nice green-flash images on my Sunsets site; there are others on the Mount Wilson Web site."

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