The Weather Notebook: The Green Flash

The Green Flash
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Dave Thurlow, Host

The last time you rode off into the sunset you may have been a little saddle sore, but you also may have been dazzled by a colorful western sky, which might even have included a rare burst of green. Hi, I'm Dave Thurlow, and this is The Weather Notebook.

Andrew T. Young Photo

The setting sun can create a mosaic of brilliant colors -- red, orange, pink, blue, violet. One color you don't see is green -- but it's there, if only for a brief, glorious moment. That moment is called the "green flash," and if you're ever lucky enough to see it, you'll never again error in matters of sentiment. Or so says an old Scotch legend. The green flash occurs at the final glimpse of the setting sun, and it lasts only for a second or two. To see it a clear, unobstructed, and low horizon is best, like the ocean, or distant hills.

As the sun sinks below the horizon several different effects come into play to create the green flash. First, refraction of sunlight in the atmosphere slightly separates its colors, like a prism. The red light from the sun sets first, and the blueish colors set last.

Next, our atmosphere weakly absorbs yellow light. At sunset the sun's light has to plow through much more atmosphere than when the sun is overhead, so the yellow light gets eaten up. And the atmosphere scatters away blue light (which, after all, is why the sky is blue), leaving a green image for the last instant as the sun slips below the horizon.

You can see the green flash at sunrise, too, if you're looking in exactly the right spot, which isn't easy. Richard Byrd, on one of his expeditions to the south pole, saw the green flash for an amazing 35 minutes, as the rising sun slid across the horizon at the end of the long, dark winter. Just imagine that, and see if you aren't green with envy. At least for second or two.

Today's show was written by David Appell. The Weather Notebook is a production of the Mount Washington Observatory. Underwriting is provided by Subaru with major support provided by the National Science Foundation.