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A moonbow (also known as a lunar rainbow, black rainbow, white rainbow, lunar bow, or space rainbow) is a rainbow produced by light reflected off the surface of the moon (rather than from direct sunlight) refracting off of moisture in the air. Moonbows are relatively faint, due to the smaller amount of light reflected from the surface of the moon. They are always in the opposite part of the sky from the moon.
Because the light is usually too faint to excite the cone color receptors in human eyes, it is difficult for the human eye to discern colors in a moonbow. As a result, they often appear to be white. However, the colors in a moonbow do appear in long exposure photographs.
A true moonbow is lit by reflected light from the Moon, not directly by the Sun. In contrast, a colored rainbow (or a white fogbow) seen when the sun is rising or setting, or in twilight, is not a moonbow because it is still produced by sunlight.
A colored circle around the moon is not a moonbow—it is usually a 22° halo produced by refraction through hexagonal ice crystals in cirrus cloud. Colored rings close to the moon are a corona, a diffraction phenomenon produced by minuscule water droplets or ice crystals in clouds.
Moonbows are most easily viewed when the moon is near to full (when it is brightest). For true moonbows, other than those produced by waterfalls or sprays, the moon must be low in the sky (less than 42 degrees and preferably lower) and the sky must be very dark. Since the sky is still light on a rising full moon, this means they can only be observed 2 to 3 hours before sunrise, a time with few observers. And, of course, there must be rain falling opposite the moon. This combination of requirements makes moonbows much rarer than rainbows produced by the sun. Moonbows may also be visible when rain falls during full moonrise at extreme latitudes during the winter months, when the prevalence of the hours of darkness give more opportunity for the phenomenon to be observed.
Few places in the world frequently feature spray moonbows. Such sites in the United States include several waterfalls in Yosemite National Park, California and Cumberland Falls, near Corbin, Kentucky. Victoria Falls, in Africa on the border between Zambia and Zimbabwe, is also widely known for spray moonbows.
Moonbows are also seen with some regularity in the cloud forests of Costa Rica, in mountain towns like Monteverde and Santa Elena. These occur when clouds of mist are blown in from the Caribbean by the Christmas Winds. The Christmas Winds happen from the end of December through late January or early February. These clouds of mists create a streaming pattern of stripes giving rise to their popular name in Spanish of Pelo de Gato or Cat's Hair. Since these are wind-blown clouds of mist and not true rain that falls from the sky, the sun or moon are not obscured by clouds, and so moonbows happen in this part of Costa Rica almost every full moon in the months of December through February. The moonbows that are caused by Pelo de Gato are not limited to just before dawn but can happen after sunset too, but it does need a full or nearly full moon.