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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 (as opposed to direct sunlight) refracting off of moisture laden clouds in the atmosphere. 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.
By definition, true moonbows are associated with atmospheric rain events and arise at a time when conditions are optimal in terms of levels of reflected solar light being bounced off of the Moon's albedo surface. All true moonbows are therefore short-lived & transitory in nature. It is important to note that true moonbows are independent of all terrestrially fixed geographical features such as waterfalls etc for their existence.
Moonbows are most easily viewed when the moon is at or nearest to its brightest phase full moon. For true moonbows to have the greatest prospect of appearing, the moon must be low in the sky (less than 42 degrees and 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. 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.
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.
Spray, mist or fog related bows are classified as false moon-bows. Such false bows (particularly waterfall spray bows) are dependent on geographical features. Moreover : since the same 'spray bow' is very often visible & reproducible in daylight hours, it means that such a bow cannot be linked exclusively to the effect of moonlight.
Numerous places in the world feature spray, fog or mist induced (false) bows. In the United States such false bows may be seen in relation to various waterfalls including 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.
False 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 false moonbows happen in this part of Costa Rica almost every full moon in the months of December through February. The bows 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. Moonbows are also found in Kauai, with the moon rising in the east, during light rain, but must be captured by a time-exposure photo, as they appear white to the naked eye of most people.