Image source: Stefan Lee Goodwin – A 10pm blood red night rainbow over Scotland

Rainbows are a familiar natural phenomenon, but have you ever heard of moonbows? Moonbows, also known as lunar rainbows, are a unique variation of rainbows that are illuminated by the moon instead of the sun.

Unlike the traditional rainbow we are accustomed to, moonbows are created by the reflection of moonlight on water droplets in the air. The process of formation is similar to that of a rainbow, but the light source is different. Interestingly, moonbows are usually positioned opposite the Moon relative to the observer.

Image source: Arne-kaiser – Lunar rainbow over Kihei, Maui, Hawaii, US

Moonbows have been mentioned as early as Aristotle’s Meteorology in around 350 BC. However, they are much fainter compared to daytime rainbows because the moon reflects a smaller amount of light. Consequently, discerning the colors of a moonbow can be challenging for the human eye. The reflected light is too dim to activate the color receptors in our eyes, making moonbows appear white. Nevertheless, through long exposure photography, the colors of moonbows can be captured and appreciated.

The best conditions for observing moonbows are during a full moon when it is at its brightest phase and not obscured by clouds. Additionally, the moon should be low in the sky, with an elevation of less than 42 degrees at least. For optimal viewing, the night sky should be completely dark. Although the sky may not appear entirely dark during a full moon, moonbows can still be seen for a couple of hours after sunset.

Image source: Garry – A night rainbow on Fraser Island, Queensland, Australia

These specific requirements contribute to the rarity of moonbows compared to regular rainbows. Moonbows occur less frequently, with a probability of less than 10% of that for rainbows. In some cases, moonbows can be witnessed during the full moonrise in winter months when rain falls at extreme latitudes. However, the vividness of colors in a moonbow depends on the size of the water droplets. Smaller moisture drops result in less vibrant colors.

Moonbows can be induced by various atmospheric conditions such as fog, spray, mist, and rain. They are often observed near waterfalls in the United States, including Niagara Falls in New York, Yosemite National Park in California, and Cumberland Falls near Corbin, Kentucky. Victoria Falls, located on the border between Zimbabwe and Zambia, is also renowned for its spray moonbows.

As mentioned earlier, moonbows can be seen for approximately three hours around the full moon, during the end of evening twilight or before sunrise. Summer is considered the ideal time for longer-lasting moonbows since the moon spends more time in the lower sky. In other seasons, moonbows may last only up to one hour.

If you’re keen to spot a faint moonbow during showery weather, try capturing it with a camera mounted on a tripod. While the naked eye may not perceive many colors, you can witness the true colors through photography.

Moonbows offer a captivating and enchanting sight, showcasing the beauty and wonder of nature’s nocturnal palette. So, keep your eyes open and your camera ready for the mesmerizing spectacle of a moonbow, a night rainbow illuminated by the gentle glow of the moon.