Green flash
Why does the sun turn green just before it sets? J.R., Albuquerque, New Mexico
 A rare look at the green flash as the sun sets on Mount Wilson, California. Copyright
1994. Lu Rarogiewicz. Used with permission.
The sun turns green just before it sets because the
atmosphere acts as a weak prism, bending the light,
say David Lynch and William Livingston in their outstanding book, "Color and Light in Nature".
The prism bends the blue light the most and the red light the least and all other
colors, somewhere in between. So the apparent sun you see at sunrise or sunset is a vertical stack,
a continuum, of sun images: each showing its own color and at its own location in the vertical
array.
Graphic: Courtesy of USATODAY.com
Since the atmosphere
prism bends the blue light
the most, the blue image
of the sun appears highest.
So, why doesn't the top
of the sun look blue
instead of green? Because
there's more than one phenomenon we're coping with. Blue light also interacts more with air
molecules than the other colors do and the interaction scatters the blue light over the sky (which,
by the way, is the reason the sky is blue). More green light than blue gets through the atmosphere
and that's why we see a green flash. In extremely clear air, the "green" flash is occasionally blue,
says Lu Rarogiewicz, weatherman on Mount Wilson and retired astronomer.
"Contrary to popular belief," say Lynch and Livingston, " the green flash is quite common,
especially over water." The difficulty is in observing it.
"There's a restaurant in San Diego named the Green Flash with a western view of the horizon
over the bay," says Rarogiewicz, "A few times a year folks in the restaurant will see the green
flash and they applaud spontaneously. 'WOW! Look at that! I don't believe that!'"
Further Reading:
An introduction to green flashes by Andrew T. Young of San Diego State University
(Answered December 2000) |
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