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Old 02-28-2002
Dan Dickison Dan Dickison is offline
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The Green Flash

I've heard a lot about a phenomenon called the "green flash," which evidently happens at sunset. Can you explain what it is?

Dan Dickison responds:
You're not alone in wondering about this interesting sunset phenomenon, which is often dismissed as a simple product of fiction. It's true that Jules Verne popularized the green flash in his work Le Rayon Vert, and much later Herman Wouk touched on it in his Caribbean novel Don't Stop the Carnival, but it does exist. I speak from experience, having witnessed it twice.

The scientific explanation is that if the atmosphere is sufficiently clear, as the sun drops below the horizon, its rays refract. The atmosphere refracts sunlight like a giant prism, and near the horizon, this effect is intensified because the sun's rays must travel the longest possible path through the atmosphere. What I've described thus far is something you know as the sunset, so no surprises there.

Now, scientists say that the violet and blue light is scattered, and atmospheric water vapor absorbs some of the yellow and orange sunlight. What's left is red and green light, which scientists say is mostly what reaches your eyes in the final seconds of the sunset. Prisms refract the higher wavelengths less, so red light is bent less than green. What that means is that the image that reaches your eyes is both red and blue-green. Because red light is refracted less, the red is closer to the horizon than the green. When the red drops below the horizon, the green is still momentarily visible, creating a distinct green flash before it also disappears.

Sailing folklore holds that seeing a green flash is a sign of impending good weather. There's even an old English proverb that supports this view:

                                 "Glimpse you ere the green ray,
                                   Count the morrow a fine day."

Here's hoping this information at least amuses you if it doesn't fully answer your question.