Moonbows

Moonbows or lunar rainbows are rare natural optical phenomena that occur when the Moon’s light is reflected and refracted off water droplets in the air.

Faint moonbow seen over Yosemite Falls


Rarer than rainbows

Moonbows are similar to rainbows, but they are created by moonlight instead of direct sunlight. Moonbows are rarer than rainbows because a variety of weather and astronomical conditions have to be just right for them to be created. Firstly, the Moon has to be very low in the sky – no more than 42 degrees from the horizon. Secondly, the Moon has to be a full or a near full moon. Thirdly, the sky must be very dark for a moonbow to be observed – any bright light can obscure it. And finally, water droplets must be present in the air in the opposite direction of the moon.

Moonbows occur on the opposite side of the Moon and tend to look white to the human eye. This is because their colours are not bright enough to be perceived by the receptors in the human eye. It is possible, however, to view the colours in a moonbow using long exposure photography.



Locations

Moonbows are more frequent in some locations around the world. Most of these locations tend to have waterfalls, which create layers of mist in the air, and therefore become fertile grounds for observing moonbows. Some of these locations are the Yosemite National Park in California and Cumberland Falls State Resort Park in Kentucky, U.S.; Victoria Falls on the border of Zambia and Zimbabwe; and Waimea in Hawaii, U.S.