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Moonbow
Wed Feb 23, 2005
Listen in RealAudio 
Although not as spectacular as solar rainbows, the rare lunar rainbow or "moonbow" is
as thrilling for the weather watcher as a birder spotting a Strix nebulosa. Hi, I'm Bryan
Yeaton for The Weather Notebook. That's a Great Grey Owl, by the way.
Moonbows form the same 42-degree arc as rainbows, but to the naked eye, they
appear a delicate white, or even black, rather than multi-hued. They've been described
as eerie and ghost-like, like moonlight that spawns them.
They form just as daylight rainbows do. When the moonlight strikes drops of falling
rain in the sky opposite the moon, it is reflected and refracted within the raindrop, which
acts like a prism and separates the moonlight into the rainbow spectrum.
Because moonlight is reflected sunlight, it has the same colour spectrum as sunlight.
But we see the moonbow as essentially colourless because our eyes don't perceive
colours well in low-light. A camera, however, can catch the colours and produce a
photo which looks just like a rainbow.
Since the moonlight is weaker than sunlight, about the only time you can see a
moonbow is when the moon is full or nearly full. And, you also need rain to make a
moonbow. Because they are so faint, moonbows are easily obscured by light and air
pollution present around most cities. For these reasons and since scattered showers
are more common during the day moonbows are a rare sight, indeed.
Two locations noted for their frequent moonbow observations are Kentucky's
Cumberland Falls and Africa's Victoria Falls. There, the bows form from drops in the
waterfall mist. Reportedly, Niagara Falls once had frequent moonbows, but they are
now overwhelmed by the surrounding lights.
Thanks to our contributing writer, meteorologist Keith Heidorn. Our show is produced
by the Mount Washington Observatory, with funding from Subaru, and the National
Science Foundation.
Today's Links
Moon Light Effects
http://home.hiwaay.net/~krcool/Astro/moon/moonring/
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