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The Green flash exposed |
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Osa Conservation |
 Although not uncommon, the green flash is one of the more elusive atmospheric phenomena. It is most commonly observed just as the sun completely sets in the evening and occasionally in the instant that it breaks the horizon at sunrise.
As the photograph below shows, the green flash consists of a halo of green perched like a beret on top of the setting sun. The green flash is very seldom observed across rugged terrain due to distortion of the light rays by the topography. The best place to observe the green flash is across the ocean’s horizon. P.J. townies can just forget about ever seeing one, but from the whole western coast of the peninsula the light show can be seen, particularly in the dry season when there are fewer late-afternoon clouds. The green flash is easy to understand if you have completed post-graduate studies in physics.
 The first ingredient of a green flash is refraction of white light into its seven constituent colors. This is achieved by the Earth’s atmosphere, which acts as a giant prism enveloping the planet. Because the sun’s light must pass through a much thicker portion of the atmosphere at dawn and dusk than, for instance, at noon, the refraction of light is greatly intensified at these times, which is why we have sunsets and sunrises of a beautifully shifting array of colors. Secondly, atmospheric water vapor in the sky absorbs light in the yellow-orange range of the spectrum and scatters light in the blue range of the spectrum. As a curious side note, this is why the sky is blue rather than chartreuse, mauve, or hot pink. As the sun sets, therefore, yellow and orange colors are absorbed by low-level humidity in the air, and blue light is dispersed. The only constituent colors of light reaching us are red and green. Now then, as every junior physics aficionado will know from his or her experiments with a prism, the red end of the spectrum is "bent" (refracted) at a smaller angle than the violet end of the spectrum. Across the distances involved, this different degree of "bending" causes two images of the sun to be visible, the predominant red sun located lower in the sky, and the blue-green sun, which appears to hover above the red one. When the red sun drops beneath the horizon, there is a brief interval in which the green sun remains visible above the horizon. It is at this moment that a vivid flash of green appears on the horizon. The green flash is not often seen because the phenomenon requires an absolutely clear line-of-sight view of the setting sun over an unbroken horizon. Distant clouds that may not even be visible are sufficient to disrupt the display. Headed for Carate? Don't forget to check out the horizon as the sun sets, and if you're lucky, or persistent--or both--you’ll get flashed sooner or later.
Thanks to Terry Conroy, owner of the Lookout Inn in Carate for raising questions about the nature of the green flash and for undertaking some of the preliminary research leading to this article.
Article courtesy of PD Collar, Courtesy of El Sol de Osa The Osa Peninsula’s Newspaper |
