A moonbow is produced by the refraction of moonlight off moisture contained in the atmospheric air, and can only be seen when sunlight is reflected off the surface of the moon. A moonbow is relatively faint due to very small amounts of light that are reflected off the surface of the moon. Unlike colorful rainbows that are produced by sunlight on rainy days, moonbows often appear white due to a decrease in light brightness, preventing the activation of the eye color receptors. Moonbows are more easily seen during full moon nights, clear skies and against a background of falling rain or mist.

Formation of a moonbow

A moonbow is formed indirectly by sunlight via the sun reflected off the moon. Sunlight reflected this way is less intense, producing almost imperceptible moonbows; however, the scarce light that is reflected off the surface of the moon is able to produce a moonbow when interacting with the rising mist of a waterfall or the falling rain with the moon on the opposite side and the observer in between. The light that is reflected of the surface of the moon produces a faint moonbow; however, a colorful moonbow can sometimes be produced when the full moon appears in the sky on dry, cloudless nights, allowing sufficient amounts of light to be refracted off the rain or mist.

Best time to observe a moonbow

A moonbow can easily be observed during full moons on cloudless nights. To more easily observe a moonbow, the moon must be at or below 42 degrees in the sky and rain must be falling opposite the moon. Moonbows are also visible at very high latitudes where the night hours are longer during the winter. At these latitudes, a moonbow can be observed during a full moon rising. At other latitudes, the combination of elements required for a moonbow to be visible, added to cloudless, drier, and light rain, allows the visibility of a moonbow. All of these requirements for the visibility of moonbows make them rarer than rainbows, which are produced by sunlight.

False and true moonbows

A true moonbow is the one that is produced by sunlight reflected off the surface of the moon. A moonbow that is produced by the light reflected off the moon is hundreds of times less bright than a rainbow that is produced by sunlight, and often appears white. A false moonbow is a light phenomenon, that although, is produced by light coming off the moon, it’s not really a moonbow. Halos are colored circles that formed like a circle 22º around the moon and sun. A halo that forms around the moon is called a winter halo or moon ring. A colored corona that forms around the moon is produced by tiny water droplets or ice crystals contained within clouds.

Spray moonbow

Spray moonbows usually occur at the site of a waterfall. They’re produced when moonlight interacts with the water droplets contained in a waterfall. A few places in the world are known to produce spray moonbows. Some of the most renown include Yosemite National park in California, and Cumberland Falls in Kentucky. In Africa, Victoria Falls between Zambia and Zimbabwe is known for producing spray moonbows. Waimea, in Hawaii, is also known for the creation of spray moonbows. Frequent moonbow observations are produced at Kentucky Cumberland Falls and Victoria Falls. The most appropriate time to observe a moonbow at any of these locations is around a full moon.

A moonbow is also known by other names based on its characteristics, including white rainbow, space rainbow, lunar rainbow, lunar bow and spray bow. It’s important to remember that for a moonbow to become visible, there must be some rain, clear skies, and a full moon; however, a clear moonlit crescent night can be just enough to reflect sunlight off the surface of the moon and create a moonbow. A colorful moonbow can best be observed during the winter season, when the atmospheric air is drier. A moonbow is one of the most staggering natural phenomena on the planet.