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The Green Flash

Sunset/Sunrise Optical Phenomena

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The Green Flash is a phenomena that is sometimes observed at sunset or sunrise just as the sun slips completely below the horizon at sunset or just before it begins to appear at sunrise. A display of emerald green color appears just above the horizon only lasting for a second or two. On occasion, two or three flashes may be observed. For the Green Flash to occur the surface must be warmer than the air above. The event is more often observed when the horizon is free of clouds and when viewing across the ocean.

The Green Flash is the result of the combination of two physical processes, atmospheric dispersion and atmospheric extinction. Due to atmospheric dispersion, the image of sun is separated into discs of different colors when low on the horizon. The red disc is the brightest and swamps the visibility of the others when the sun is above the horizon. The discs are refracted at slightly different angles in the order of red, yellow, green, blue and violet. The red disc slips below the horizon first and without its dominance, the upper crests of the other overlapping discs may then become viewable as they follow the red disc below the horizon. This process would suggest that the trailing flash would be violet, however there is another process at work. Due to atmospheric extinction, air molecules scatter the shorter wavelengths more than the longer wavelengths. This factor, coupled with the longer trip that the light must take through the atmosphere at low viewing angles, is what causes the blue and violet discs to be dispersed and therefore they are not seen as discs near the setting/rising sun.

The Green Flash effect is further enhanced by a physiological process at the viewer level, at least for sunset flashes. When the observer views the bright red setting sun, the retina adjusts and becomes less sensitive to red light. With a still ample supply of red light available, the effect is not very noticeable by the viewer. So with reduced perception of red, the yellow disc that precedes the green is perceived as green.

So to see the Green Flash, situate yourself at a comfortable restaurant on a west-facing beach just before sunset with some friends--most any restaurant on Playa Zicatela will do. Order a cerveza or a coco frío and watch as the sun slips below the horizon. If there is no flash, the evening will still be a success. Or if you are one of those morning persons and know where the sun will make its first appearance, you can try for the sunrise version.

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