Green-flash Fallacies and Misconceptions

Introduction

An extraordinary amount of nonsense has been written about green flashes, and an astonishing amount of it has been promulgated and repeated by people who should have known better. Here are some of the commoner falsehoods, errors, and misconceptions, debunked:

The “old Scottish legend”

No story is more widespread, nor more false, than the “ancient legend” Jules Verne introduces in his 1882 novel Le Rayon-Vert, according to which, one who has seen the Green Ray is incapable of being “deceived in matters of sentiment,” so that “he who has been fortunate enough once to behold it is enabled to see closely into his own heart and to read the thoughts of others.”

Well, bunk! This is the stuff of 19th-Century romantic French novels, not Scottish folklore. Anyone who spends an afternoon in the folklore section of a good library will find not only that no such legend exists; but that the Scots, along with the rest of the Celtic people, regard green as a color associated with evil spirits, death, and misfortune. (They aren't known as the “dour Scots” for nothing.)

You'd think that, coming as it does in the paragraph after a passage of purple prose such as only the French can produce, and which is incongruously attributed to an actual English newspaper, the Morning Post, the “legend” might have seemed equally implausible. Yet the story is so appealing — surely, a testimony to Verne's skill as a novelist — that people want to believe it. And, in some perverse way, the fact that the seemingly incredible “Green Ray” really exists, after all, somehow seems to stay most readers' sense of disbelief; so they swallow it all, including the “legend.”

The after-image fiasco

The notion that green flashes at sunset are after-images, or due to contrast effects in the eye, seems plausible to many people who are unaware of green flashes at sunrise, which in fact were observed from the earliest times. Today, of course, it takes only a color photograph to show that real green flashes exist.

Nevertheless, there is a physiological component to sunset flashes (but not to sunrise ones); and this may help explain the persistence of this faulty explanation. The effect is primarily bleaching of the red-sensitive retinal pigment, though there is also some neural adaptation. Looking at the orange-red low Sun for a few seconds is enough to make most flashes appear green at the stage where they photograph as yellow on color film. That's why it's hard to get good color photographs: most people trip the shutter too soon at sunset.

I have published a paper on the subject; but perhaps the report of a simple experiment may suffice to convince doubters that bleaching is a significant effect.

Thus, the appearance of sunset green flashes is usually a mixture of physics and physiology; but sunrise flashes are almost pure physics. And, even when physiology plays an important part, the flash is NOT an after-image.

The Sun shining through the waves?

Another popular incorrect explanation attributes the flash to the Sun shining through the waves on the ocean. Apart from the fact that this is contrary to the laws of optics — light entering a wave is bent downward, into the water, and cannot escape again (and if this isn't obvious to you, see my more detailed explanation) — observations of flashes over land horizons show this is not a correct explanation. (Perhaps casual observers are misled by the Sun-glints on the waves; but these are light reflected from the surface, not light transmitted through the water.)

Furthermore, the mock-mirage flashes appear well above the horizon, when a considerable part of the Sun is still up (here's an example if you need a reminder); clearly, this green light has nothing to do with the waves. Indeed, close examination shows that even the inferior-mirage flashes are slightly above the apparent horizon, and thus cannot be due to light shining through sea-water. You can see this both in a photograph and in a simulation of such a flash.

Supposed restriction in latitude

Some people have heard that green flashes are only visible in some remote part of the world. “You can only see them in Hawaii/the tropics/the polar regions/over the ocean/anywhere but here,” is the typical impression they have. A similar misconception is that flashes are only seen at sunset, or only at sunrise.

No. Flashes are seen almost everywhere, both morning and evening. That said, there's no denying that they're more commonly seen over a water horizon (though large lakes work as well as the ocean) than over land, because the large heat capacity of water almost guarantees considerable thermal contrast with the overlying air, and so favors some kind of mirage.

Because the inferior-mirage flash requires water warmer than the air, these are seen more commonly at low latitudes, and in fall and winter. The mock-mirage flashes, and those associated with ducts and other thermal-inversion features, on the other hand, are more common at high latitudes, where the surface is usually colder than the air (especially when ice-covered); late spring and early summer are best for these. Nevertheless, the ranges overlap; I have seen both kinds here in San Diego, and many of each kind at the “wrong” time of the year. See my advice page for recommendations on where and when to look.

(The notion that flashes are restricted to low latitudes is refuted by spectacular observations made in the polar regions; see the next item.)

Sunglasses: an urban myth

I recently heard that some tourists are being told you can't see a green flash if you're wearing Polaroid sunglasses. Sunglasses in general decrease the brightness of the retinal image, so they might diminsh the shift in perceived color due to retinal bleaching, and make some flashes appear yellow rather than green. But the effect is probably not large.

The Polaroid angle is a strange one. The setting Sun is essentially unpolarized, and so is the nearby sky. Polaroids might darken the sea surface slightly; but this would, if anything, help improve the contrast between small flashes and their surroundings.

I'm not in the habit of wearing sunglasses (they get in your way if you're looking through a camera viewfinder), but my best guess on all this is that sunglasses, whether polarized or not, make little difference.

The Byrd Expedition canard

One often reads (or, these days, hears) that Admiral Byrd saw a green flash lasting 35 minutes. This story has a factual basis; but the story, as usually told, isn't a fact.

Byrd's own very brief description of this episode says merely that the polar sunset was “prolonged” by the Sun

. . . appearing and reappearing from behind the barrier surface.

Hence we had a series of sunsets. The “green sun,” as our men called it, lasted 35 minutes.

Even from this brief account it is obvious that the green was not seen continuously for 35 minutes, but only off and on. A fuller account was published by the expedition's meteorologist, W. C. Haines, who explained that

. . . the sun was skirting the southern horizon, its disk disappearing at intervals only to reappear again a few moments later. . . . The irregularities in the snow surface permitted the upper limb of the sun to appear in one or more starlike points of light from adjacent notches. These points or flares of light would sometimes have a greenish color on their appearance or disappearance. The length of time during which the green flare was visible varied from a fraction of a second to several seconds.... When the sun sank too low to be seen from the ground, it was still visible from elevated points such as the anemometer post or radio towers. The above effect was seen at intervals during a period lasting over half an hour.

Conditions were more favorable for its occurrence when first observed. Later the green appeared for shorter and less frequent intervals, and the orange and red flares increased in frequency.

This is a far cry from

Richard Byrd, on one of his expeditions to the south pole, saw the green flash for an amazing 35 minutes, as the rising sun slid across the horizon at the end of the long, dark winter.

as broadcast by the Mount Washington Observatory's radio program, “The Weather Notebook” on Sept. 29, 1999. (Notice that they also say it was a sunrise, when the actual observation was at sunset. This is cut and pasted from the transcript they posted on their Web page at http://www.mountwashington.org/notebook/ , so don't think I am mis-quoting them.)

I complained to them about this the day before, when they asked to use my GF photograph, and was told

Unfortunately, we are not able to correct the show at this point in time. We have considered changing the text for airing on the website tomorrow, but that would conflict with the pre-recorded audio.

Now you see why scientists don't trust journalists. I'm not saying these media people have no regard for the truth; but if it walks like a duck, and quacks like a duck ....

The path-length confusion

Many people think the flash has something to do with a long air path. One often sees remarks to the effect that it's caused by the large amount of air you have to look through at the horizon.

It's hard to see where this idea came from. Maybe these people are thinking of the removal of the orange light (which they often mistakenly attribute to water-vapor alone — actually, it's mostly removed by ozone, which is in the upper stratosphere).

The long air path certainly explains why flashes are more often green than blue or violet: a combination of Rayleigh (i.e., molecular) and aerosol scattering selectively weakens the shortest wavelengths, so that green is usually the last color seen at sunset, and the first to appear at sunrise. Unfortunately, in polluted urban air, the long air path often removes the green as well, so that the flash is an unspectacular yellow.

Indeed, the refraction at the horizon varies considerably from day to day; but on the days when the refraction is largest, so is the atmospheric reddening, so the possibility of seeing a green flash is reduced. (These are the “Type B” sunsets of Fisher; they usually end in a red line, not a green flash).

Actually, in the inferior-mirage flash, the path length through which the flash is seen is somewhat less than the path at the horizon in the Standard Atmosphere. What should be emphasized here is the peculiar refraction, rather than the mere length of the air path. Green flashes are by-products of mirages; so it is the mirage, rather than the path length, that should receive attention.

The textbook fallacy

Though the connection with mirages was apparent to several careful observers, it never got into the textbooks. They uniformly give the impression that “normal” refraction (i.e., what you get from the Standard Atmosphere) alone will produce a flash. The idea was just that the normal green rim would be cut off by a sharp edge, such as a distant horizon; the remaining “green segment” would be the flash itself. This picture was reinforced by Mulder's name “green segment” for one of his three main classes of flashes. This is still the explanation you find on most Web pages, too.

But this notion was soundly refuted by Gerhard Dietze, whose theoretical treatment of this textbook model in 1955 showed that it produces a green flash ten times smaller than the naked eye can see. Unfortunately, Dietze published in an East German meteorological journal, so his work was overlooked by O'Connell (an astronomer) and indeed by everyone else for some 40 years.

You might think that the arguments of experienced observers like Evershed and Fisher might have had some effect. Even Mulder, who had seen only a couple of flashes, but at least read many of the reports published up to his day, was aware that the standard explanation was not adequate to explain the phenomena. However, physicists are so fond of simple explanations that they brushed aside the evidence that failed to fit their favorite green-flash model.

To see just how inadequate the textbook model is, take a look at my simulations made for the Standard Atmosphere.

The “upper atmosphere” error

A mistake that runs through the literature of refraction and green flashes is the notion that the upper atmosphere plays an important part in phenomena observed near the horizon. It doesn't. In fact, the closer you get to the horizon, the more important the lower atmosphere becomes. In the interesting zone of sky between the astronomical horizon and the apparent horizon where astronomical mirages (and most green flashes) appear, it's the air below eye level that's solely responsible for these phenomena.

Unfortunately, O'Connell pushed the idea that the upper atmosphere was important, both in his book and in his articles in Scientific American (1960) and Endeavour (1961). While his error was caused mostly by a misunderstanding of basic optics, he may have been influenced by Newcomb, whose similar error was due to a mathematical misunderstanding of refraction theory, as I have shown in print. (The mathematical demonstration requires calculus; but even without it, I hope the figure here will indicate the relative importance of the upper and lower regions.)

Diffraction

Occasionally, I see a Web page or a newsgroup posting in which someone tries to explain green flashes with “diffraction.” I suppose these people had a brief exposure to a physics book once upon a time, and acquired the word, but not an understanding of it.

Diffraction is the spreading out of light after passing through a small aperture, such as a pinhole or a narrow slit, or around a narrow obstruction, such as a wire or a twig. It's a consequence of the wave nature of light.

Needless to say, there are no pinholes or narrow slits involved in the production of green flashes. Maybe the people who conjure up this word are really thinking of refraction, not diffraction. Or maybe they remember there's something called a “diffraction grating” that produces spectra, and suppose that might be involved in the separation of the spectral colors in sunset flashes.

It's the regular, periodic spacing of the diffraction grating's slits or grooves that makes it separate spectral colors cleanly, not diffraction per se. Wave optics has nothing to do with green flashes. If someone tries to invoke diffraction in connection with our flashes, you can be sure they're confused.

If you're confused, maybe a look at my optics pages will help clear things up.


© 1999 – 2006, 2012 Andrew T. Young


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