Our Window on Nature

. . . exploring the world around us

A Flash of Green

Filed under: Sky — Lowell Christie -- July 15, 2016 @ 11:12 am

Green_flashJules Verne, among others, wrote about the “green flash,” but the first time we read about it, we concluded that it seemed about as likely as traveling 20,000 leagues under the sea. It was called “an intense spark of colored light visible at sunrise or sunset” – sure, with Nessie rising to greet it, no doubt. However, after considerable research we decided that if the green flash does not exist, it is a hoax that has survived for a couple thousand years.

Assuming that the right atmospheric conditions are present, along with a view of a distant horizon, and the sun sinking toward its nighttime rest, scientists explain the phenomenon as follows. Consider a prism and the way it splits light into its component colors. The green color comes from a leftover ray of sunlight still hanging in the sky after the red, orange, and yellow have dropped below the horizon. The blue and violet rays are scattered invisibly in the atmosphere, leaving a clear emerald green just at the edge of the sun – the same green rim we’ve seen many times when ignoring parental admonitions to “Stop looking at the sun. It will ruin your eyes! ”

On rare occasions, those green rays do more than line the rim of the sun – they actually light up a small portion of the sky. In extreme cases, such as the singular sunset experienced by the Byrd Antarctic Expedition in 1929, this green ray can hover at the horizon for a half-hour or more. The reason those lucky explorers were able to view it for such a lengthy period is that on the first sunrise after the long polar night, the sun never rises fully above the horizon. A few green seconds is all the rest of us can hope to experience – or, maybe a few blue seconds. This even rarer event results when the atmosphere is both abnormally refractive (able to bend light rays) and abnormally clear. This time the flash can be blue instead.

One of the best places to view the green (or blue) flash is on the coastline, where one can most easily find the requirements of a distant horizon and a clear sky. People living or traveling along the west coast of Florida, and especially in Key West, claim the most frequent sightings, although numerous sightings have been reported along the Pacific coast also. Because most people view the flash when these coastal conditions are present, it was first assumed to be caused by the sun shining through a bulge of (green) ocean water at the curve of the horizon. Now we know this is not so.

Another early theory explained that the green flash resulted from retinal fatigue – i.e., our eyes become overwhelmed by looking at the red colors of sunset, so that upon the disappearance of the red light, we see its complement, green. That would be an acceptable explanation if the flash occurred only at sunset, but the theory falls apart when you consider that at sunrise the flash appears just as the sun arrives.
One might also assume that the green flash occurs only along the coastline; at least that’s where we looked for it over the years. Yet, the only time we’ve experienced the emerald flash was in the desert, hundreds of miles from open water. This ought not to surprise us when we consider that the earliest writers to comment upon the phenomenon were the ancient Egyptians. Their sun rose and set behind desert hills, not coastal waters. Our desert peaks were in Arizona.

We were driving west from Tucson as the sun slipped toward the horizon. It was during the summer monsoons, a time of extravagant sunsets and grandiose lightning shows, but this was quite different. Lowell kept his mind on the traffic. As usual, Kaye’s eyes wandered across the scene, and they fixed on a peak just as the sun edged below it. For no more than a couple of seconds, a brilliant burst of emerald green set the crest into sharp contrast with the pinks and browns of its surroundings. And, alas, it was gone so fast that Lowell missed it entirely. After all the days we had stood at the edge of the continent at sunset, waiting for, hoping for, a green flash that never appeared, it was waiting for us in the desert.


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