Green Rays

A different form of green flash

Very rarely, a type of green-flash display is seen that really does light up the atmosphere, and deserve the name of “flash.” These are the displays of the type Mulder called “the green ray.”

Mulder himself became interested in green flashes by seeing a “green ray” display, which he describes in his book:

I had myself the privilege, in July of 1907, of seeing a “green ray” at sunset at Ilfracombe, on the coast of the Bristol Channel … .

The sea was quite smooth. not a cloud was visible on the horizon which, for the observation of the green ray, seems to be essential. The edge of the sun was sharply defined till the last moment, without greenish colouring of the last segment, as appears on other occasions. I followed with great interest from a slight elevation, with the naked eye, the setting of the last segment, because I hoped, under such favourable circumstances, to see the green ray, of which I had no proper idea. And indeed, at the same moment, at which the last tip of the sun disappeared below the horizon, or immediately afterwards, there suddenly shot, straight upwards, a clear narrow green ray, which vanished as suddenly as it appeared. I am not able to give an exact description of its form, because it lasted only a moment, but I estimated the length of the ray to be about 1/10 or 1/8 of the diameter of the sun, an estimate of which I cannot guarantee the accuracy. Concerning the duration I can only say, that the phenomenon lasted but a fraction of a second, about as long as the visibility of a lightning flash. It almost made the impression, that the ray was ejected from the disappeared sun, but it also resembled somewhat a rocket or the light from a lighthouse below the horizon, throwing a ray of light upwards . …

Sometimes the “ray” really looks like a searchlight beam; several observers have compared these to the beam from a lighthouse below the horizon. Some displays are very vivid, lighting up a large part of the sky; others appear to shoot out along the horizon around the sunset point. Others are just a small, brief glow of green above the sunset. All usually appear (at sunset) just as the last bit of the Sun is disappearing (often as a “green flash” of the ordinary kind), or immediately afterward.

As most observers have seen only a single flash of any kind, it is often difficult to tell, from an isolated account, whether a green ray or an ordinary green flash was seen. However, there are a few cases in which observers who are familiar with the common “green dot” form of flash have also seen a green ray. In these cases, the distinction between the two classes of flash is usually emphasized.

Other published accounts

The earliest such account I have found in print is in a letter published under the pseudonym of “Another Engineer Officer” in T.P.'s Weekly in 1904. This experienced observer relates:

In the course of sixteen years sailing the seven seas I have very frequently seen the green flash and also the green ray. Fine weather and an absolutely clear horizon are, or course, essential, but this form of refracted light requires other atmospheric conditions, which have not yet been scientifically explained, and the flash may be often looked for in vain when the conditions seem to be most favourable. The flash appears like a magnificent emerald the moment the sun sinks — sometimes vanishes instantly and sometimes fades in three or four seconds. Occasionally, in the latter case, a luminous green ray springs from the flash and shoots vertically upwards to a height of about five of the sun's apparent diameters.

There are only a handful of such reports in the literature, though accounts of well over a thousand green flashes have been published. However, a drawing of such a flash made by D. P. Lagaaij was published by P. Feenstra Kuiper in his thesis. This drawing was later reproduced (in color) by Rudolf Meyer, and (in black and white) in various editions of Minnaert's well-known book.

Another good report was made by the geologist B. G. Escher [brother of the better-known artist, M. C. Escher]. Like the “Engineer Officer” quoted above, he draws a sharp distinction between the two classes of green phenomena:

In the first place, a sharp separation must be made between the phenomena that are seen on the Sun itself, while it still (or already) stands above the horizon, and the phenomena that appear immediately after the Sun has set or just before it has risen.

[The latter are the green-ray phenomena of interest here.] Escher goes on to describe a display he himself has seen, and which

. . . I have named a green column, which I have observed on 6 July 1929 at sunrise in the Gulf of Suez. It was easy to predict where the Sun would rise on the pink illuminated parts of the mountains on the Sinai peninsula. Just above the profile line, where the Sun rose later on, appeared a bright light-yellow rim and simultaneously above that a light-blue segment. This segment rose and changed to green below. Suddenly the Sun itself appeared, absolutely not red or orange, as it was with the green flash at sunset, but intensely shining and white, surrounded by a light yellow shine. At the same time the mountain silhouette became light colored. The light between the mountain horizon and the sea was obviously laden with dust particles, which is hardly surprising in the Sinai desert. I estimate the duration of this long-lasting, broad and high green flash at 2 seconds. That abnormal refraction and dispersion occur here is certain; they were apparently caused by mineral particles in the air.

The diameter of the green column was estimated to be about half that of the Sun, while the height of the green column with blue cap was estimated by me at 2/3 of the diameter of the Sun."

We can discount Escher's remark about the mineral particles having anything to do with the refraction and dispersion of the air. However, it's obvious that the dust played a part in making the sunrise green beam visible in the air through which it passed. The fact that this was a sunrise observation shows that retinal bleaching is not necessary to make these rays appear green; indeed, it's remarkable that he even saw blue in it.

Green rays are undoubtedly one of the rarest green-flash displays. The Dutch sea captain E. E. Havinga, who must have seen more than a thousand flashes himself, reported in 1934 that he had never seen one of the green-ray type:

I made my first observation on 25 July 1902, the last on 20 Oct. 1905. This series, 303 observations, formed the basis of the dissertation of Dr. P. Feenstra Kuiper for the doctoral degree in physical and natural science. From that time I traveled another sixteen years, only interrupted by an interval of one year … . How many observations have I made in all these years? I do not know, as I kept no notes … . A “green ray” such as was observed by D. P. Lagaay on 22 July 1911, and afterward reproduced in the thesis of Dr. F. K., I have never seen, a proof that it occurs extremely seldom; one sees such a thing by chance.

A recent observation

Recently, I received a fresh account of a green-ray observation from an experienced local observer here in San Diego, Barney McComas. He says:

I have lived within an hour of the Pacific, in Southern California, all my life, and have always been a huge fan of watching the sunset whenever possible, but it was only about five years ago that I first learned of the green flash. Still, until sunset on Saturday, April 27, 2002, I thought I'd seen the flash several dozen times. Frequently, on days when the air was clear, and there were no clouds on the horizon, I would watch as the last bit of the rim of the setting sun dipped behind ocean and turn a lighter yellow, then whitish, then go green, just before disappearing.

When I first started seeing this, I thought, "Flash? What flash? I see 'green', but no 'flash'." Upon reading some of the material on the websites to which you contributed, I have the impression that it is really that "blip" of green light seemingly suspended above the setting sun that is actually the flash. Still, not quite the event one expects from a word like "flash" – maybe "dot", or "blip", but not "flash".

However, last Saturday, sailing back north to San Diego from a day sail to Los Coronados Islands, in Mexico, six out of seven of us (the seventh was seasick, and wasn't watching the sunset) saw what could truly be described as a "Green Flash"! In all the pictures I've ever seen – from those on websites, to the ones hung on the walls of a local restaurant named after the phenomenon – I've never seen anything like what we saw. Six pairs of eyes intently watched for the green rim I'd described to everyone (some of whom had seen it before several times), and were pleased not to have been disappointed. Then, at the moment the sun disappeared, there was an actual flash of green above the horizon! It was about one half the width of the sun at the horizon, and maybe half that in height. It lasted only half a second, but left all of us self-proclaimed green flash veterans with our jaws on the deck, realizing we'd never before really seen a green flash.

This modern green-ray observation shows that the green-ray phenomenon is still occasionally seen, in spite of urban air pollution. As these reports are quite rare, I asked for a little more information, and was told that the sky was clear, except for a few clouds on the horizon (but not near the Sun); the waves were moderate (3 to 6 foot swells), and the wind was estimated as 12 or 13 knots. (Probably this brisk breeze caused enough spray to provide the necessary scattering.)

Another modern observation is reported on the Web by Matt Marinkovich, who says he has seen several of these green (or blue) ray displays, including one at sunrise. (It sounds very much like Escher's sunrise “green column”.)

As such observations really are extremely rare, it helps if observers who see such things report them, or at least write down all the details, as soon as possible after the observation. In addition to the circumstances of wind, weather, and height of the eye, it's useful to estimate the duration of the flash, its angular extent, and (if possible) the relative brightness of the flash and the surrounding sky.

Possible explanation

As these “green ray” displays seem to come in a variety of forms, it's not clear that a one-size-fits-all explanation is justified. However, my general impression is that what is seen here is the surface-layer haze (background aerosol, and probably spray from whitecaps over the ocean) illuminated by a bright green flash. As the Sun sets, the layer of green light from the flash rises through the atmosphere above the observer's head. Because coarse aerosols are strongly forward-scattering, the part of the layer between the eye and the horizon that is illuminated by green light is visible against the background sky.

This idea would require just the right amount of aerosol: enough to be visible, but not enough to cause so much extinction that the green flash is dimmed much by it. Only rarely would one expect to have nearly the optimal amount and kind of aerosol present, which might account for the rareness of these displays.

As for the “green beam” sometimes seen to shoot up from the sunset, I suggest that the modulation of the flash by the waves on the sea, and/or by turbulence in the surface layer, which often produces a “beaded” appearance of the ordinary inferior-mirage flash, might account for the striated or ray-like appearance. The apparent radiation of the rays away from the Sun's position would then be the perspective effect familiar in displays of crepuscular rays. Thus, one might call these displays a special kind of crepuscular ray, illuminated by an unusually bright green flash.

If you see one of these rare displays, I'd certainly like to hear from you about it!

Copyright © 2002 – 2006, 2010, 2013, 2014 Andrew T. Young

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