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Sea Science: by Scott McDowell
When mariners share sea stories of encounters with gales and rogue waves, rarely does a session end without mention of the elusive green flash. With fine detail, they describe the tiny burst of green light seen above the upper rim of the sun at the exact moment the red orb sets on the clear horizon.
Some sailors are adamant they’ve seen the flash multiple times but the majority are naysayers who scoff, claiming it’s nonsense – just a myth.
The first reference to the green flash was in Jules Verne’s 1882 novel “The Green Ray” where the heroine was searching for this elusive phenomenon of remarkable beauty. “If there is a green in Paradise, it cannot be but of this shade, which most surely is the true green of Hope!” Present day romantics have said that once you’ve seen the flash, you’ll never again have troubles with matters of the heart. I wish this had been true.
Enter atmospheric physicists, successful myth busters who’ve proven that the green flash is real and a startling phenomenon when atmospheric conditions are optimum. Not only does the flash occur at sunsets but it also can erupt at sunrise; the latter is far more difficult to catch as the observer’s eye must be focused on the exact position on the horizon, immediately before the sun rises, which is a low probability in itself.
To understand the significance of the flash’s color green, one must first realize that the full spectrum of colors in sunlight is not visible at all times of the daylight cycle. When directly overhead, the sun’s bright rays appear white (containing all colors) but the setting sun shines only orange or crimson red. This is because green, blue and violet light have shorter wavelengths and, consequently, are totally dispersed from view. Incoming sun rays immediately above the horizon pass through roughly 40 times more Earth atmosphere than when sun rays descend from directly overhead. That’s why we don’t see a blue or purple sun as it sets.
Furthermore, midday sunlight is 10,000 times brighter than rays of the setting sun due to the increased amount of atmosphere and dust particles along a light path tangent to the Earth’s surface.
When the dominant red rays of the setting sun pass through the thick slice of the atmosphere, they are bent downward toward the earth by the process of refraction. (In the same manner that light rays are deflected and color-separated as they pass through water or a glass prism.) As the sun sets below the horizon, the bent red rays can reach the eye of an observer even when the top of the sun is no longer in sight.
Green and blue light, with shorter wavelengths than red light, are bent even more by refraction. Consequently, although the sun has set well below the horizon, green and blue light can reach the observer after the red rays are no longer visible. And because the bright red rays are not visible, the weaker greens and blues can be detected, but only for a short time – a flash of precisely 1.4 seconds, as computed by the physicists who know the index of refraction differs among the various colors of light.
When vertical gradients of atmospheric temperature change (e.g., warm air over cold air, or vice versa), so does light refraction and the creation of what we call optical mirages. Under certain atmospheric conditions, the mirage effect can cause green flashes to last up to 30 seconds. Lengthened flashes also occur at high latitudes, compared to typical flash durations of 2 seconds near the tropics.
Physicists have also determined that the intensity of the flash is actually quite weak – six times weaker than the human eye should be able to detect. For this reason, cameras are much more capable of detecting flashes than the naked eye. But under special atmospheric conditions, the mirage effect can amplify the flash such that a lucky observer can witness the spectacular event.
Even more rare is a blue flash. Because blue light is bent (refracted) even more than green light, it’s the last component of sunlight that glows on the horizon after the sun has set. This short-wavelength light is, however, easily scattered away from the line of sight and extremely weak to the observer’s eye. Therefore, only green flashes are visible in all but rare occasions.
Scott E. McDowell has a doctorate in ocean physics, is a licensed captain and author of Marinas: a Complete Guide, available at www.scottemcdowell.com. Comments are welcome below.