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The Green Flash
| Article
# : |
10128 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
4 / 1993 |
1,495 Words |
| Author
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Craig F. Bohren
Craig F. Bohren is Distinguished Professor of Meteorology at
Pennsylvania State University. More of his thoughts on clouds
and light can be found in his recently published What Light
Through Yonder Window Breaks?, a sequel to Clouds in a Glass
of Beer, for which he was the first recipient of the Louis J.
Battan Author's Award of the American Meteorological Society.
In 1988 Bohren was elected a fellow of the Optical Society of
America for "outstanding contributions in radiative transfer
and atmospheric optics." |
Visible at both sunrise and sunset, a green light quickly flashes on the horizon.
In the last few moments of sunlight, a pillar of green light some times juts from the upper rim of the disappearing sun. This is the green flash, mention of which usually evokes one of two responses: that it is a rare and exotic phenomenon seen only by those who can afford ship fare to Tahiti; or that it is a charming myth, on all fours with the tooth fairy or the Seven Cities of Cibola. It inspires the awe of the credulous and the scorn of the skeptic. Few atmospheric phenomena can evoke as much gush and nonsense.
But the green flash is not rare in either time or space. One does not have to wait for years, hidden in a blind or cloaked in camouflage to see it. All that is a required is a low horizon and a clear sky. An ocean or large lake provides such a horizon. Only ignorance and laziness are real impediments to seeing a green flash.
The U.S. West Coast is more favorable location for viewing green flashes then the East, nor because of different frequencies of occurrence but because of the ease with which the setting sun can be observed. To observe the rising sun, one must get up early and known where it will emerge above the horizon. But observe the sun at the end of day does not require heroic self-discipline. A California surfer can watch the sun descend bellow the western horizon all year long from the comfort of a warm and sandy beach whereas a New Englander must rise early and may have to stand shivering on a rocky shore waiting for the sun to appear from an uncertain direction.
The comparative ease with which the green flash can be seen over oceans, especially those lapping western shores, has engendered the notion that it is a uniquely maritime phenomenon. Yet to those who know what to look for, green flashes can be seen from the coast of California to the plains of Kansas to the mountains of New Hampshire. Indeed, they can be seen both at sunrise and sunset at the same location.
Binoculars or a telescope green flash. The dire warning about not looking directly at the sun are more applicable for times when the sun is overhead. When it skirts the horizon, attenuation of sunlight by atmospheric molecules and particles greatly reduces its intensity. Moreover, the most damaging part of sunlight, its ultraviolet component. During the fleeting moments when setting, watching it through binoculars is no more dangerous than looking at an
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