What comes to mind when you think of a green flash? A streak of green as a comic book super-hero blazes a trail to save a destitute citizen? The aftermath of a flashbulb? While those occurrences may well create a green flash in truth or in movies and comics, the optical phenomenon in question is the brilliant green flash seen hovering above the horizon as the sun plunges into the sea–or anywhere else, for that matter.
So what is the green flash?
From Scottish legends and beyond, confusion abounds concerning the phenomenon known as “The Green Flash.” In the past, reports Andrew T. Young, associated with San Diego State University, many theorized that the green flash was an afterimage, much like what you see when a flashbulb goes off in front of your eyes. But, since the green flash can also be observed occasionally at sunrise, this makes an afterimage impossible. The flash can also be photographed–another reason for ditching the afterimage theory.
One explanation for the green flash is discussed at length by Mark J. Coco, who, during his life, was an avid astronomical enthusiast and free-lance writer. His article “Stalking the Green Flash!” in the December 1996/January 1997 issue of Weatherwise, gives his observations. [The green flash] “is based on atmospheric refraction. When the sun is near the horizon, its light is refracted (bent) by the atmosphere in a manner similar to a prism…The result is the sun is seen with a red rim on the bottom, and a green, sometimes blue, rim on the top.”
The author goes on to explain that this is not the only explanation. Two other lines of reasoning also prevail. “First, on some occasions, the only light visible at the time is green (the rest of the sun being hidden) and therefore, the eye sees green…The second reason relies upon atmospheric behavior.” Given the right circumstances, the atmosphere can act as a lens or a prism. When it behaves as a lens it can produce an image called a mirage. This causes a green light to hover above the rim of the sun as it disappears into the sea. Viola! The green flash.
Typically, the flash occurs within one degree of the horizon and lasts about one second in temperate latitudes, longer at higher latitudes, and can be seen almost anywhere in the world. Reportedly, Admiral Byrd once observed a green flash that occurred off and on over a period of 35 minutes. One surfer in Hawai’i, while bobbing in the lineup, watched a green flash repeatedly, for several minutes as the swells rose again and again to meet the bottom of the setting sun.
How to observe a green flash
Whatever the explanation, here’s how to track down a green flash. Choose a beach or area with a low, uncluttered horizon. (Most of Hawai’i’s west-facing beaches are prime locations.) Stand or sit so that your view is not blocked and wear good sunglasses that won’t alter the natural color of the sun. Gray polarized lenses are particularly well suited. Go on a clear day when no haze, vog, smog, or low clouds are present. Just as the sun begins to dip into the ocean (or other suitable horizon), look aside, then back quickly. As the sun’s upper rim slips away, the green flash will “magically” appear. At last–your first elusive–green flash! And it certainly won’t be your last.
To see a graphic explanation, go to: www.hyperphysics.phy-astr.gsu.edu and for a more detailed, yet simple explanation for green flash hunters, visit: www.minataka.sdsu.edu/GF or Google “A Green Flash Page” by Andrew T. Young.