What is a Moonbow or Lunar Rainbow?

Examining the Phenomenon of Lunar Rainbows
Image Credit: Arne-kaiser

The image above shows an example of a moonbow that appeared over Maui, Hawaii in February 2016. Although moonbows are also commonly known as lunar rainbows, or white rainbows in many parts of the world, all are formed by moonlight.

Regular rainbows are formed when sunlight is refracted through tiny water droplets, such as might be present in the atmosphere over large waterfalls or after a rain shower. On the other hand, moonbows are formed when moonlight is refracted through water droplets under the same conditions. However, moonbows are always only visible at a point in the night sky that is exactly opposite the Moon in relation to the position of the observer.

As a practical matter, moonbows are as faint as they are because the light that forms them is so much less intense than sunlight. For this reason, moonbows are not bright enough to activate the color receptors in human eyes, which is why they always appear to be white, or various shades of light grey. In fact, while the colors that occur in regular rainbows are present in moonbows, they are so de-saturated that they only become visible in long-exposure photographs.

When to View Moonbows

Since moonlight is so faint when compared to sunlight, moonbows can be most easily seen when the Moon is at, or close to full, and ideally when the Moon is lower than about 42 degrees above the horizon under very dark skies, and with sufficient moisture in the atmosphere. Since the sky can never be fully dark during a rising or setting full moon, though, moonbows can only be observed for about two or three hours prior to sunrise, or for about two to three hours after sunset.

In practice, this set of required circumstances makes the formation (and observation) of lunar rainbows much rarer than regular rainbows that are formed by sunlight. Nonetheless, at extreme latitudes, moonbows can be observed more frequently during wet or rainy conditions in winter, since the higher number of hours of darkness offer longer observing times.

Where to See Moonbows

Although moonbows can appear almost anywhere on Earth if the required conditions are met, many places in the world are known for the regularity with which the phenomenon occurs. Some of the best locations to view moonbows include Yosemite National Park in California and the Cumberland Falls near the city of Corbin, in Kentucky, where waterfalls blast huge volumes of spray mist into the lower atmosphere. Other good locations include the Victoria Falls in Southern Africa and the Plitvice Lakes in Croatia, where spray-induced moonbows are also common.

One other good location to see moonbows is over some high-altitude parts of the rain forests of Costa Rica, where they appear as the result of the Christmas Winds (that blow from late December to early February) introducing large volumes of water mist from the Caribbean into the local micro-climate. During this period, moonbows occur at almost every full Moon and are often visible both before dawn and after sunset on the same day.

The islands of Hawaii also offer some good opportunities to view moonbows, especially on Hawaii Island. While moonbows are relatively common there, they are almost always very faint, meaning that their true nature can only be revealed in long-exposure photographs. Less favorable locations include the Big Island of Hawaii, and Velasco Lemery Iloilo, in the Philippines.

Moonbows in History

Moonbows are a natural phenomenon that have existed for as long as there has been liquid water on Earth. For instance, moonbows are mentioned in Aristotle’s seminal work, Meteorology, which was published in about 350BC. In fact, moonbows and various explanations for how they form can be found in literally thousands of books and pamphlets dating from ancient Greek and Roman times, right through the Middle Ages, and into the modern Age of Enlightenment that marked the late Victorian era.

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