Green Flash: CR4 Challenge (03/11/08)

I don't know the answer for certain (my estimate follows) - but I think this explanation must be incomplete. This is partly on the basis that it would be a long-lasting phenomenon if it were due purely to the increased curvature of green light through normal atmospheres, and partly that the increased atmospheric refraction of shorter-wavelength light is universal - if correct we should see this regularly.

I expect the reason to be that stratification of the atmosphere** causes the green components of the refracted light to be focussed to a specific location, thus temporarily offsetting losses due to higher short-wavelength scattering. Such an effect is inevitably highly localised, as far more red light can penetrate the scattering atmosphere than green (see almost any article on Rayleigh scatter); extreme localisation means that the effect must normally be short-lived, as either the rotation of the Earth or atmospheric changes will move the viewing location relatively rapidly (or even cancel the effect altogether).
N.B. Scattering losses in the blue are higher still, and refraction not that much greater, so I doubt we could ever see a blue flash.

**I think a temperature inversion would required for a "good" green flash - but a minor one could possibly be created by a sharp edge between warm air below colder air.

Fyz

I see that it is mostly seen of over the ocean and on the top of clouds, do you in your opinion think that water plays some part in it? eg: rainbow effect?

Regards JD.

I fear I wrote about the wrong effect - what is known as a "green flash" - which is an effect on the image of the sun; the first sentence in the question specifically asks about a green sky - so I'll address that separately.

Returning to your question: in neither case to I believe that there is any direct optical contribution from the water. But there will be good reasons that it is seen in these environments.

I can immediately think of three:
i) Low dust content in the atmosphere: dust further increases the scattering in the green.
ii) Long uniform optical paths over the uniform surface to assist focussing (some desert also satisfies this - but not always i) or [in the evenings] iii) ).
iii) Creation of temperature inversions: water warms up less than land, so temperature inversions are more common over water, particularly in the evening.

Desert cools down quite rapidly overnight, so I would expect "green flashes" over desert to be more common in the early morning (when the ground is cold - and any dust is more likely to have settled).

If you search NASA's website, Astronomy Picture of the Day, you will find explanations and pictures of this effect.

Interesting. The first time I ever heard about a green flash was in the latest "Pirates of the Caribbean" movie and I thought it was just made up. I definitely never have experienced this phenomenon. Maybe it is because I am a land-luber, Ha Ha.

I've previously answered what I believe causes a part (sometimes all) of the sun to appear green - as this is what is normally known as a "green flash". But the question actually states the sky goes green. This effect is more common, and can be (relatively) long-lasting.

Essentially, as the sun sets, it is not just the path to your eye that becomes longer, but also the path through the atmosphere to the sky. So Rayleigh scatter preferentially and progressively attenuates the bluer parts of the spectrum as the sun sinks - this applies both to the light reaching your eye directly, and (later) the light reaching the sky. The light scattered from a clear (cloud-free, etc) sky is also Rayleigh scattered. As the sun becomes lower in the sky the scattered levels (relative to the sun's output) gradually change; based on scattering alone**, the visual peak in the light scattered from a clear sky should move from blue (sun=high) through green to yellow and finally to red as the sun goes lower.

The situation is complicated by the effects of refractive dispersion, and also by high altitude absorption - but I think this shows the most significant part of the trend.

Fyz

**The relative level would be sky=1/exp(k/lambda⁴)/lambda⁴, with k representing the effective length of the scattering attenuation path, and so increasing from small (sun high in the sky) to larger values as the sun sets. The shift of the peak is clearly visible if you plot this using a spread sheet.
I checked the effect on a spread-sheet plot, using "sky" versus lambda for lambda=0.35...0.75 (no units in spread sheet, but actually wavelength in microns), and setting k successively to 0.01, 0.02, 0.03, 0.05, 0.1, 0.2, and 0.3

Another interesting phenomenon is the polarization effect. Take a clear warm day, no clouds or pollution in the ski, usually mid morning or late afternoon and notice the darker blue band in the ski which is at 90 degrees from the direction toward the sun. This is more noticable if you compare the ski looking through polarized sun glasses and not. The rest of the ski will darken with the polarizers except the blue band.

This seems to cover it http://mintaka.sdsu.edu/GF/

Al

you watch too many pirate movies.

When my parents retired to Florida, they lived in a condo on the beach on the Gulf coast. Residents would gather at the seawall at sundown, as the sunset vistas were always marvelous over the ocean, usually cocktail or other fine beverage in hand. When I would visit them, my parents always said to look for the green flash right where the sun disappeared on the horizon. They had seen it many times. Sorry to say, I never saw it there.

The only green flashes I ever saw were caused by excessive alcohol consumption long after the sun went down!

Good catch my copper-zinc primate friend (Answer #1)! Thanks for not boring us with a long, quasi-theoretical, pedantic monologue! Thanks also to the others who provided links to other web-pages with more info.

I've actually saw this when I took a sailboat cruise near Bon Aire. It only happens when you salute the sunset with your Heinikan bottle.

All manner of material on this subject on the Internet:

http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html

That's the clearest explanation I've seen why the top edge of a continuous setting sun should be yellower (or greener or bluer) than the bottom. Such an effect should last from the time it first appears until the green part steadily descends below the horizon - which doesn't quite correspond to any description I've seen of a green flash. Assuming the descriptions of the phenomenon to be accurate (short duration, etc), I fear we have to postulate a measure of additional focusing is required for the green to become visible.

the text does say that the initial presentation is an oversimplification...

The effect can be seen only in clear air and with a visible horizon (at sea, on a mountain or in a desert). How does nature produce this phenomenon?

I can only speculate as to how nature is involved, it is obvious that light can be split into it's individual wave lengths, so, how does nature take advantage of that?

As stated this only occurs at sea, on mountings and in a desert, where temperature inversions occur, as has been previously pointed out. Therefore I will try to expand on that.

In the case of the desert it is well documented that mirages occur, and temperature inversion is considered to be the cause. So what is it that nature has produced to cause this phenomenon? Not only is an image trance ported over a long distance, but I think it can also be magnified? So to speculate on that, I believe that a temperature inversion is an area where the air is split into two layers, hot and cold, and it is the height at which these layers meet that I wish to speculate about.

Warm air can carry more moisture than cold air, so went warm moist air meets colder air the warm air becomes saturated, and if the airs temperature is lowered enough it can cause precipitation. So I speculate that in a temperature inversion there is a moist saturated layer, where the refractory angle of the air layers above and below are different, thus causing the moist layer to act like an optic fibre? And I think it may be possible that the moisture in the air has an infinity for the green wave length, like when you are out in the ocean the water appears green, the green ocean? And further to the above as the layer has a curvature following the earth surface, it magnifies the light passing through it.

So I think that this maybe natures phenomenon that cause the so called green flash?

Regards JD.

I think there may be a problem with definitions here. Mirages are caused by the temperature gradient being rather high near ground level. That gives a temperature gradient in the same direction as the general trend in the atmosphere. A "temperature inversion" is when the temperature gradient is reversed - i.e. atmospheric temperature is cooler nearer the ground than higher up.

I now believe the requirement for a green flash (as opposed to a graded colouring across the sun**) includes a temperature inversion that starts (or at least has a peak gradient) some way above the ground, so that light that goes through this region of relatively lower scattering (therefore able to pass green) can be bent downwards towards the viewer - i.e. a sort of upside-down mirage.
**That is just a (possibly more extreme version of) a standard sunset(/sunrise). If you look at pictures of green flashes, you will almost always find the image of the sun is "broken up" in some way - even if it is just a separation between the "normal" setting sun and the flash.

I am unfamiliar with details regarding mirages and you are possibly correct, but I only put it forward as an example of natures involvement, if you get what I mean, a phenomenon of nature? and I also agree with your description of temperature inversion. But I do think that moister in the air may play a part in the scattering of the light wave, but as its only speculation I could be wrong. Thank you for your input.

Regards JD.

"phenomenon of nature" covers a great deal...

I agree that water vapour can play a part in modifying the refractive index, but at moderate temperatures (25-degrees**) I think the effects are somewhat small compared with pressure or temperature. Moreover, high moisture levels are associated with increased scattering; and as scattering preferentially removes the shorter wavelegths, a high moisture level would mean an increased preponderence of red.
However, not knowing the exact details of the conditions under which the effects occur, I could still be wrong.

**For example, near ground level a 70% change in relative humidity would correspond to a 0.3% change in pressure, a 1-degree change in temperature, or a height change in equilibrium atmosphere in the order of 150-feet.

"I now believe the requirement for a green flash (as opposed to a graded colouring across the sun**) includes a temperature inversion that starts (or at least has a peak gradient) some way above the ground, so that light that goes through this region of relatively lower scattering (therefore able to pass green) can be bent downwards towards the viewer - i.e. a sort of upside-down mirage.
**That is just a (possibly more extreme version of) a standard sunset(/sunrise). If you look at pictures of green flashes, you will almost always find the image of the sun is "broken up" in some way - even if it is just a separation between the "normal" setting sun and the flash."

Succinctly put, with only a hint of pedantry. Bravo!

Thank you - a Kudos from STL, albeit with a sting in the tail. Given your known view of my style, thats definitely one for the record.

BTW, I'm still far from certain that the challenge means a green flash to the sun rather than a general greening of the sky as stated at the beginning: i.e. "a green sky"??

The "green flash" has legendary, almost mystical, prominence among the Snow Birds of Gulf Coast Florida. You have not really "arrived" as a transplanted and (usually) retired Northerner until you have observed the "green flash" at least once at sunset, usually with cocktail or other beverage-of-choice in hand.

My guess it is much the same for West Coast beach dwellers. As described in the articles which earlier posters provide links for, the phenomena is much more rare and notable than a general "greening of the sky", which often accompanies or precedes severe weather.

If you re-read the question you will note that although he first refers to the effect as "green sky", he also says it is known as the "green flash" and notes it occurs primarily at sunset with clear skies, not stormy skies as is usually the case with the general "green sky" effect, which can also take place at most any time of day.

Here is a link to several responses for "What" is a "green sky", and here is a link to "Why" a "green sky" occurs.

I hope this clears up your confusion!

Yes, that leaves a question as to whether the ambiguity is deliberate, or the originator didn't know what was usually meant by a green flash, or had merely not audited his text.

Late sunset green skies can also happen under clear conditions, early morning or late evening. Because the required condition is out-scattering of blue from the light that reaches the sky, the refractive conditions for it are almost the reverse of those that bring green light down to the eye. So it's sunrise over ocean (perhaps not many people watch that?), or sunset over desert, or where hills block the lowest-level paths. Given my habits, it should be no surprise that I've most often seen this from inside a mountain valley, where you get a deep green sky with vivid orange clouds (all effectively lit from below). But I think this version is more common when the sun is no longer quite visible (or maybe is just visible along a mountain pass).

I'm begining to wonder if the Aurora would qualify under the questions wording.

This site seems to explain the green sky effect:

http://www.gi.alaska.edu/ScienceForum/ASF2/262.html.

I have seen an approaching storm system that had a green tint.

Incredible, late evening and the sun is almost overhead - and in Alaska too...

I found two more articles about green sky:

http://www.sciam.com/article.cfm?articleID=25F743DC-E7F2-99DF-3BB7C05144B603BD

http://www.newscientist.com/article/mg15420862.700-green-sky-at-night-is-chemists-delight.html

I doubt that the auroras account for the green flash or green sky because of they do not match the conditions given.

That's better...

My 2¢,

sorry, but the original question is *terribbly* written. I've never heard of a 'green sky' effect which could be all kinds of things.

The 'green flash' is a specific and very real physical phenomenon. Lots of good answers here which pretty much cover the mechanism, which can be found explained all over the internet and in many textbooks covering i.e. visual astronomy and/or atmostpheric phenonmenon.

The only time I've seen it was sitting on a (BIG) log with a friend watching for it near Copalis Beach on the Olympic Penninsula in Wash State. Having studied astronomy and worked in a planetarium, I knew what it was but it was still astonishingly beautiful to see. For me as magical is the aurora.
It's possible to photograph it (maybe even some of the mentioned links show pics, haven't checked).

nice day

rf_g

Yes, see Kris' post #3 above for the hyperlink on "Hmm." which takes you to the Wikipedia article "Green Flash" with several very good shots.

The Green Flash does not refer to the color of the sky, but to a brief optical phenomenon that occurs just as the sun's upper limb drops below the horizon. This is a rare occurence, and conditions must be just right for it to occur. Basically a mirage of the setting sun's last specular light.

http://en.wikipedia.org/wiki/Green_flash

Negative. If you look at the color spectrum, blue-green's, green's and green-yellows are in the middle of the visible spectrum hence green would not be the Primoral pronounced color but an interim phase.

No, this is obviously a psychological effect that has be introduced by the media hype on "green ecology" "Green Phase' or "Green Shift" is not a visible but cultural phenomenon produced by mass hysteria generated by mass media.

There have been no documented scientifically adjudicated instances of "green phase" or "green state" sightings that have ever been corroborated. Therefore, it must be deduced that the driving factors for being able to see such occurrence can be definitively narrowed down to a preposition to a "Green" state. Hence the entire phenomenon is a purely Freudal construct and can probably be related to an insecurity regarding your mother...She wasn't Green enough.

Real answer can be found here.