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Boston doubles its luck with double rainbows

If a rainbow is a sign of good fortune, Bostonians have recently doubled their luck.

Last Wednesday, the city looked up to see two rainbows stretched across the sky. Then, improbably, another double rainbow sighting followed Monday night.

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Keith L. Seitter, executive director of the American Meteorological Society, says every rainbow has the potential to be a double. To produce a single rainbow, sunlight bends as it enters a raindrop, reflects off the back of the raindrop, and then bends once more as it leaves. The bending, or refraction, separates white light into the spectrum of colors recognizable in a rainbow.

So what has to happen to create a double rainbow? Some of the light traveling through a raindrop must reflect a second time before escaping, thus rainbow number two.

The second rainbow will be dimmer than the first, added Kim Buttrick, a meteorologist at the National Weather Service in Taunton. And, because the light is reflected twice in the raindrop, the colors will be inverted, red on the inside and violet on the outside. Seitter is not sure why Boston has seen two double rainbows in just one week. But he said that when clouds are dark and of uniform length, rainbows appear bright, and double rainbows are easier to spot.

But does a double rainbow mean two pots of gold? “Seeing the secondary rainbow in addition to the primary feels very much like a treasure to me,” he said.

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