Explanations of Green Flashes

Warning!

Before reading the explanations below, or in the recommended reading list, you should be aware that green flashes are more complex than the usual simplistic explanations admit. That means that many common observations are not correctly explained — a circumstance that helps explain why the topic remains controversial.

You might like to take a look at my own explanation page, which is supported by some simulations of the “textbook” model and the common inferior-mirage and mock-mirage flashes, before reading the standard oversimplified explanations. And have a look at the list of common fallacies and misconceptions so you'll know what not to believe.

A major problem with the GF literature is that theoreticians who have never seen a green flash have been allowed to spout off in print on the subject, drawing conclusions that are not supported by observational evidence. Plainly put, these people literally don't know what they are talking about. Many otherwise reputable scientists have been guilty of this transgression, including the younger Lord Rayleigh.

A second problem is that there are several distinctly different forms of flash that have been mingled together; Mulder's three classes do not encompass all the types. Thus, the classical explanation partly explains the flash associated with the inferior mirage, but does nothing to explain that associated with the the mock mirage, which is at least as commonly photographed. (A sidebar in my Zenit article tabulates the commonest types.)

A third problem is that the GF literature is huge (over a thousand publications). Yet many people, who are unaware of the extent and complexity of the situation, get it into their heads that they can make a useful comment on it after a single glance at either a flash or the literature. As Havinga wrote in 1934:

All the writers have heard of the “green flash” now and then, don't know what and how much has already been written about this subject, have seen the phenomenon a single time and suppose that they have made an important discovery worth reporting.

A fourth problem is that green flashes are highly interdisciplinary phenomena, so no one comes to them knowing all the necessary background. In particular, sunset flashes involve some rather peculiar visual-adaptation problems. These frighten some physicists, who really want to believe that everything is “out there” and nothing is “in here”. Nevertheless, there is perfectly solid evidence that green flashes viewed at sunset involve visual physiology very deeply. I've written a paper on this topic that appeared in the December, 2000 issue of the Journal of the Optical Society of America. While GFs are not afterimages, they can evoke unusual responses from the visual system. The altered state of color perception in visual observations of green flashes at sunset is partly responsible for the difficulty of photographing them: film does not adapt, but the eye does. Unfortunately, this aspect of the problem has been ignored or misinterpreted by nearly everyone who has worked on it.

A fifth problem is that half-baked explanations and errors have been passed on uncritically from one writer to another, without any of them taking the trouble to check the original literature to verify facts. Sloppy scholarship has plagued the subject, partly because nobody has taken it seriously enough. In fact, it often turns out that newer publications are not as well done as older papers; there has been a steady decline in the quality of work on green flashes since the 1930s, and particularly since WW II.

Part of the problem is that Americans no longer learn to read foreign languages; but some of the most important GF literature is in German, Dutch, and French (with a little Italian as well). If you're too busy to learn to read these languages, fine; just don't suppose you can write an informed paper on the green flash.

The result is that a considerable fraction of what is in print (not to mention Web pages!) is baloney. So get out a large block of salt, not just a grain, before you take in the following explanations. Most of them are partly true....

Explanations

http://sciastro.astronomy.net/sci.astro.3.FAQ

Part of the sci.astro FAQ. Frequently posted to sci.astro, sci.answers, and news.answers. The GF part is down near the end of this 52kb file.

"The Green Flash"

The Physics Gearbag in its new location. Pretty much the standard textbook explanation (i.e., the right general idea, but oversimplified). I see the reference to Sir George Back's account has been corrected — thanks!

'Ask Jack' FAQ

There are a number of interesting phenomena explained here in addition to green flashes.

The Green flash

Mike Dworetsky's account follows the standard textbook story, and includes his personal observations of some green flashes.

Explanations in French:

Good explanations by Eric Frappa, with fine photographs taken at Pic du Midi.

Thierry Lombry's GF pages are very nicely done.

http://www.meteo.org/phenomen/ray-vert.htm

Le rayon vert

Explanation in German:

Der Grüne Strahl

These folks have a fine set of pages illustrating many other phenomena of atmospheric optics — well worth a look, even if you don't read German.

Explanations in Italian:

Paolo Candy's solar-eclipse green flash

Paolo Candy has combined a beautiful inferior-mirage flash here with a solar eclipse, photographed from Madagascar. There's also a good description of what's going on (though I wish he'd drop the part about Verne's “legend”).

Another Italian green-flash page

Explanation in Russian:

Зеленый луч

Yurii Pidopryhora has put together a Russian-language page on green flashes, mirages, and related refraction phenomena (linking to my pages here, among others — including the Russian version of Pekka Parviainen's mirage pages).


For better accounts, with more details, see the reading list. You might also have a look at some simulations I have prepared for the commoner types of flash.


There are still many loose ends to the Green Flash business. I have a list of unsolved problems you might be able to help with. Take a look and see if you know something I don't.

© 1999 – 2002, 2005, 2006, 2008, 2012, 2016 Andrew T. Young


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