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January 2005 19 THE A T M O S P H E R I C R E S E R V O I R Examining the Atmosphere and Atmospheric Resource Management Have you seen the GREEN flash? By Aaron Gilstad bleaching effect on the red sensitive sun and is visible before the sun drops parts of the retina, so the initial flash over the horizon. It accompanies what No, itʼs not a new comic book that would be yellow will be seen as is called a mock mirage, which is character. If you havenʼt heard of it green, effectively intensifying the caused by a strong temperature inver- before you may not even realize you green vision and lengthening the time sion (warmer air above cold air) below had seen one. Donʼt feel bad if you that the flash is visibly green. eye level. The refraction distorts the havenʼt or at least shape of the sun, donʼt think you such that it ap- have seen one, pears to have the many have not. middle pinched. Green flashes can The effect is be easily missed, caused by look- some last only a ing down through couple seconds, the inversion then while others may back up through it last up to fifteen due to the curva- seconds. They can ture of the earth. be seen either at The atmospheric sunrise or sunset, conditions help although sunset extend the lifespan is far more com- of the green flash mon as it is much Photo: Thomas “Hank” Hogan up to as many as easier to locate the fifteen seconds. sun before it sets than before it rises. There are two common types of This type, especially, should be visible Either way, you must be looking green flashes that account for 99 per- in North Dakota if viewed from above directly at the sun to see them, so it is cent of all reports, each in response the inversion. best to use a filter. to rays of light bending (called Although, it is far more common to According to Andrew T. Young, refraction) over a steep temperature see the green flashes on the coasts, this an astronomy professor at San Diego gradient. The most common type, appears to be mainly due to topographi- State University, green flashes are called “the last glimpse” appears as a cal interference or limitation as op- atmospheric phenomena where the flat bottomed oval just above the ho- posed to anything to do with the water top edge of the sun changes color to rizon. It is most visible just as the sun itself. The flashes are artifacts of refrac- exhibit a greenish tint. Green flashes dips below the horizon as seen from tion not reflection, allowing at least the are quite variable phenomena, in fact a few yards above sea level. This possibility of seeing a green flash. But, green flashes are not even always green flash usually lasts about two to even if you donʼt see one from home as green, but can also be blue, yellow, three seconds. The inferior mirage, the sun sets, youʼll still be able to enjoy or violet. Why green flashes are often as seen over asphalt on a hot summer the North Dakota sunset; one of the seen as green, is not purely physical, day (hot air below cooler air), is the most beautiful youʼll ever find. but at least partly a physiological ef- responsible culprit. Light is refracted fect on the observer. The rate of light such that part of the sun nearest the Atmospheric Resource Board dispersion accounts for the physical horizon disappears until it is below North Dakota State Water Commission 900 East Boulevard, Bismarck, ND 58505 coloring, red light disperses fastest, the horizon and the reversed image (701) 328-2788 with yellow, blue, and violet dispers- flashes above the horizon. www.swc.state.nd.us/ARB/ ing more slowly. Additionally, when The second looks like a thin, ND Weather Modification Association looking toward the sunset, your eye is pointy strip of green which has been PO Box 2599, Bismarck, ND 58502 overloaded with red light which has a pinched off from the top edge of the (701) 223-4232 North Dakota Water ■ January 2005 19.
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