A moonbow (also known as a lunar rainbow or white rainbow), is a rainbow produced by moonlight rather than sunlight. It is always positioned in the opposite part of the sky from the moon relative to the observer.
According to BBC, moonbows are much fainter than solar rainbows, due to the smaller amount of light reflected from the surface of 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, a moonbow often appears to be white. However, the colors in a moonbow do appear in long exposure photographs.
Moonbows are most easily viewed when the moon is at or nearest to its brightest phase full moon. For moonbows to have the greatest prospect of appearing, the moon must be low in the sky (at an elevation of fewer than 42 degrees, preferably lower) and the night sky must be very dark. Since the sky is not completely dark on a rising/setting full moon, this means they can only be observed 2 to 3 hours before sunrise (a time with few observers), or 2 to 3 hours after sunset. And, of course, there must be rain falling opposite the moon.
2geton reports that 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.
Numerous places in the world feature sprays, fog- or mist-induced bows. And also they have occasionally been seen in various places around the globe in the night sky during a rain shower around the time of the full moon. Seeing them all the time, nonetheless, requires going by one of just two places on the planet: Victoria Falls on the Zambia-Zimbabwe border and Cumberland Falls close Corbin, Kentucky, in the US.
