Here’s the Best Place on Earth to Catch an Ultra-Rare Moonbow

An elusive phenomenon is just regularly scheduled programming.

At Cumberland Falls, moonbows are regularly scheduled programming. | Jim Vallee/Shutterstock
At Cumberland Falls, moonbows are regularly scheduled programming. | Jim Vallee/Shutterstock

Maybe—if you’re the lucky type—you’ve caught a moonbow arcing over the base of Yosemite Falls at night. Perhaps you've spied one hanging in Costa Rica’s cloud forests, or hovering in the rolling mist of Victoria Falls. But there’s only one place in the Western Hemisphere where the elusive phenomenon occurs predictably and regularly: southern Kentucky’s Cumberland Falls.

Even if you’ve seen a moonbow—an arc of ghostly white light varying from imperceptible to undeniable—you might not have known what it was. Sometimes called a lunar rainbow or a white rainbow, moonbows are a rare phenomenon requiring very specific conditions to appear. At Cumberland Falls, they’re part of the park’s regularly scheduled programming.

Seeing a moonbow at Yosemite Falls requires dumb luck. | Scott Kunath / shutterstock

Bret Smitley, Park Naturalist at Cumberland Falls State Resort Park, walks us through the science behind the phenomenon. 

“As a moonbeam enters a droplet from the falls,” he writes, “the higher density of the water slows down and refracts the light. The inside surface of the droplet acts as a mirror, reflecting the light back to the observer.” 

Essentially—just like a rainbow—a moonbow is “a result of light passing through the cumulative effect of millions of droplets.”

For moonbow potential, two things are needed: a full moon and a body of water. That's not exactly a unique combination, but it also leaves a lot to chance. For surefire moonbows, the stars aligned at Cumberland: the water source is in front of you and the moonlight behind. The gorge walls are low and wide enough that they don’t block the moonlight but high enough they block the wind. Even better, Smitley adds, “Most waterfalls do not produce enough mist. Cumberland produces a lot.” 

Cumberland Falls, it’s worth noting, is the “Niagara of the South,” a 68-foot-tall, 125-foot-wide waterfall cascading into a rocky gorge at 3,600 cubic feet of water per second. And sitting in south-central Kentucky's Daniel Boone National Forest, the area is remarkably free from light pollution, the last necessary ingredient in the moonbow “recipe.” 
(The actual Niagara Falls, it turns out, used to have a consistent moonbow, but light pollution has made that a phenomenon of the past.)

The only other place on the planet to have reliable moonbows? Victoria Falls, on the border between Zambia and Zimbabwe.

The moonbow’s spectrum can only be captured by camera. | Patrick Jennings / shutterstock

Cumberland Falls’ moonbows are so reliable there’s a calendar for them—dates surround the full moon, with two or three days on either side. To catch the brightest ‘bows, hope for a clear night on the full moon, and keep an eye on sunset and moonrise times.

“The moonbow can usually be seen about an hour after dark, after the moon has cleared the top of the ridge,” explains Smitley. Each subsequent night, though, the moon rises later and later—expect to add 30 minutes for each successive night in the moonbow’s five- or six-day tour.

And if you’re already planning that Instagram shot, be prepared to see with your eyes something totally different than what pops up on your camera.

“To most people, it looks like an arc of white light, with some color fading in and out,” Smitley notes. “Photographs sort of cheat, because long exposures will pick up the color.” (Note: Cell phone cameras can pick up the phenomenon, but DSLRs—and a tripod—usually warrant the best results.)

As for where and when to go, though the moonbow can technically be seen from the Lower Overlook, the Upper Overlook on top of the falls is the best viewing spot, according to Smitley. Winter is a great season to check off the quest, as the crowds have thinned and the moon rises swiftly, as early as 6pm.

And while little remains in the lunar dregs of 2020, a moonbow is expected on December 31st until 11pm—should you want to ring in the New Year with moonshine and moonbows, you found your chance.

Jacqueline Kehoe is a writer, photographer, and geology geek. See her work on Instagram at @j.kehoe.
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