Green flash

Q: Why does the sun turn green just before it sets?
J.R., Albuquerque, New Mexico

A: 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.

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: http://mintaka.sdsu.edu/GF/

(Answered December 2000; updated April 24, 2008)