First of all, the green flash at sunset is a real phenomenon. It is not imaginary nor is it necessary to have consumed a glass of wine before sunset to see it. We have witnessed spectacular green flashes at sunset on three occasions in Naples. The first time was on board the Naples Princess for a sunset cruise, the second was from a beachfront home that we were showing real estate buyer clients and the third was from the north beach pavilion in Pelican Bay where we live. The common ingredients on these three occasions were that each day was crystal clear without a cloud in the sky and the water in the Gulf of Mexico was very calm. (Wine was available on only two of those occasions.) I wish that I could take credit for the above photo of the green flash but, I was never prepared properly and it is extremely difficult to photograph. If I had been smart enough to video just one more tropical sunset with my Flip video camera, I would have captured the elusive green flash. If you are equipped well enough and skilled enough as a photographer, I would recommend doing some internet research on the subject of photographing the green flash and then be patient…….very patient. Otherwise, just relax and enjoy every tropical sunset that you possibly can – don’t ever take a sunset for granted – and be satisfied that you were one of the priveleged to share this beautiful phenomenon.
Here is some of the science behind the green flash:
The atmosphere bends different wavelengths differently, just like a prism. This effect is called dispersion. If you look at a very bright star just above the horizon with binoculars, you may see it flicker with different colors because of dispersion. The sun emits the largest amount of light in the green part of the spectrum and less of the green light gets scattered in the open sky.
Light moves more slowly in the lower, denser air than in the thinner air above, so sunlight rays follow paths that curve slightly, in the same direction as the curvature of the earth. Higher frequency light (green/blue) curves more than lower frequency light (red/orange), so the green rays from the upper rim of the setting sun remain visible after the red rays are obstructed by the curvature of the earth.
If the sky is extremely clear at sunset, then the refraction of sunlight bends the red and yellow light down before it gets to your eye and the blue and violet light is scattered or absorbed before it reaches you. The last pinpoint of the setting sun may appear green for a second or two. (See the diagram below)
The green flash is rarely seen because it’s unusual for the horizon to be that clear and unobstructed. You need a very flat horizon, otherwise the sun won’t be low enough for dispersion to be effective. An ocean horizon (the Gulf of Mexico) is the best, but even on very clear days at sea there is enough haze on the horizon to redden the sun too much. The green flash is often seen in the Arctic and Antarctic because the cold air is very dry and because the sun sets at a grazing angle to the horizon, making the effect last longer.
The refraction angle for the green end of the light spectrum (the shorter wavelengths) is slightly larger than the red part. As a result, the spectrum of colors of the sun is spread over a small vertical distance. So, the top rim of the sun appears green and the lower rim appears red. When the sun sets, the green rim is the last to disappear. The effect can also be seen at sunrise, but this is a lot harder to ever see because you don’t know exactly where to look.
We hope that some of this scientific background helps but, if nothing else, it makes a believer out of you if you had any doubts or thought that the green flash was an urban legend. Most importantly, we hope that this article encourages you to appreciate every sunset and be thankful for every day. May your sunsets be tropical and here’s wishing you a green flash of your own.