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Although not as spectacular as solar rainbows, seeing the rare lunar rainbow or moonbow is as thrilling for the weather watcher as a birder spotting a rare species.
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. Observers have described them as eerie and ghost-like, rather faint due to the moonlight which spawns them.
Moonbows formed as daylight rainbows do except they arise from moonlight. When the moonlight strikes drops of falling rain in the sky opposite the moon, they are 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. We see the moonbow as essentially colourless because human eyes do not perceive colours well at low-light levels. A camera, however, will catch the colours and produce a moonbow photo looking like a rainbow if properly exposed.
Since moonlight is less brilliant than sunlight, about the only time you might see a moonbow is when the moon is full or nearly full. And, of course, you need rain to make a moonbow. Because they are faint, moonbows are often obscured by air and light pollution present around most cities. For these reasons and since scattered showers are more common during the day moonbows are not often seen.
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 local urban lights.
[This essay, authored by Keith C. Heidorn, PhD, originally aired on The Weather Notebook on 28 July 2003.]
A spectacular pair of photos was posted recently on the Space Weather website showing a double moonbow in Hawaii taken by Ethan Tweedie. To view these photos, click here.
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