What Is a Moonbow? Learn All About This Rare Phenomenon

We’re all familiar with the phenomenon of a rainbow. A rainbow is caused by refraction, reflection, and dispersion of light in water droplets, which causes a reflection of light and results in a multicolored arc in the sky. However, contrary to popular belief, not all rainbows appear during the day. This reflection of light can also occur differently and result in something called a moonbow.

Rainbow lovers should rejoice to learn that moonbows exist to offer additional multicolored arcs in the sky. But what is a moonbow? How is it different from a rainbow? How often do moonbows happen? In this article, you’ll learn the answer to these questions and more to understand what a moonbow is.


What Is a Moonbow?

Let’s jump right into the central question: What is a moonbow?

A moonbow, also known as a moon rainbow or a lunar rainbow, is an optical phenomenon caused when the light from the moon is refracted through the water droplets in the air. In other words, it’s a rainbow produced by moonlight rather than sunlight.

Does this sound sort of like a rainbow? It makes sense that it would. However, there is a slight difference between the two. 

Moonbow vs. Rainbow

Rainbows and moonbows both result in circular arcs and derive from light, but there is a difference between the two.

While rainbows result from direct sunlight hitting water droplets in the air, moonbows are caused when water droplets in the sky refract sunlight reflecting off the moon. 

In other words, the only true difference between a rainbow and a moonbow is the source of light. A rainbow’s light source is direct sunlight, while a moonbow’s light is sunlight refracted from the moon’s surface. As a result, the more substantial difference you’ll notice between a moonbow and a rainbow is that moonbows only happen at night.

Let’s take a closer look at how exactly moonbows are formed.

Moonbow in cloudy sky at night

How Do Moonbows Happen?

As mentioned, a moonbow requires moonlight to be refracted and reflected by water droplets at a particular angle to create a rainbow. 

A few things need to happen for a moonbow to form and be visible to the human eye. Firstly, the moon must be near to or at its fullest phase and low in the sky, and the night sky must be very dark. There also needs to be rain falling opposite the moon or another source of water droplets, such as a waterfall. Finally, to see a moonbow, the observer has to have the moon behind them.

One of the main issues with moonbows, however, is that, even though the familiar colors of the rainbow are there, they appear much fainter than regular rainbows due to the smaller amount of light reflected from the surface of the moon. Rather than seeing the full spectrum of colors, moonbows often appear white to the human eye. However, it’s possible to discern colors of a moonbow using long-exposure photographs.

How Often Do Moonbows Happen?

Rainbows can typically be seen anytime and in all corners of the world under the right circumstances. Chances are that you’ve seen a rainbow many times without trying. However, this is unfortunately not the case when it comes to moonbows, where you’ll have to be more strategic to spot one.

Moonbows are quite a rare phenomenon – much rarer than a typical rainbow due to the more complicated process of making a moonbow. If all of the steps described above do not happen together and simultaneously, then a moonbow will not appear. Even if the events do all happen, that also doesn’t guarantee that a moonbow will appear.

Your best bet for spotting a moonbow is to find a waterfall that produces a lot of mist. Most waterfalls don’t make enough mist for a visible moonbow – the waterfall needs to be quite large for a moonbow to form and be visible.

If you are very determined to catch a glimpse of a moonbow, the best time to see moonbows is a couple of hours after sunset or before sunrise near a large waterfall. If you’re up for a road trip, Yosemite National Park is one of the best places to see a moonbow.

Your best chance of seeing a moonbow at Yosemite is during the spring or summer during a full moon on a clear night, where a moonbow may be visible from the park’s waterfalls. 

Colorful moonbow at bottom of waterfall in Yosemite

Other places where you can see a moonbow include Hawaii, given its waterfalls and tropical rain, and Cumberland Falls in Kentucky. You might also see moonbows in Niagara Falls from the Canadian side. If you’re looking to spot a moonbow internationally, Victoria Falls, at the border of Zambia and Zimbabwe, is famous for spotting moonbows. 

Conclusion: Look Out for Moonbows

Hopefully, you’ve learned how exactly moonbows form and what you can do to try and catch a glimpse of one.

A moonbow is a beautiful but rare sight. If you’re hoping to spot a moonbow, your best bet is to check the calendar to mark the days with a full moon during the month.

Remember, the best time to spot a moonbow is a couple of hours after sunset, and you must position yourself to ensure that the moon is behind you. You’ll also want to have your camera ready for long-exposure photographs so that you can always glance back at the rare occurrence and reminisce.

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