A lunar rainbow, commonly called a moonbow, is a rainbow produced by moonlight rather than sunlight. It forms through the exact same physics as a daytime rainbow, but because moonlight is so much dimmer, the arc typically appears white or faintly colored to the naked eye. They’re rare enough that most people will never see one without specifically seeking one out.
How a Moonbow Forms
Moonlight is reflected sunlight, so it contains the full visible spectrum of colors. When that light enters a water droplet, it bends (refracts), bounces off the back wall of the droplet, and bends again as it exits. This double refraction separates the white light into its component wavelengths: red and orange at longer wavelengths, blue and violet at shorter ones. The process is identical to what creates a daytime rainbow.
The key difference is intensity. The moon reflects only a small fraction of the sunlight that hits it, so the light entering each raindrop is dramatically weaker. Your eyes have two types of light receptors: cones, which detect color, and rods, which detect brightness in low light but are essentially colorblind. In the dim conditions that produce a moonbow, your rod cells do most of the work, which is why the arc looks white or pale gray in person. A camera set to a long exposure, however, gathers enough light to reveal the full spectrum in vivid detail. Many of the striking moonbow photos you’ll find online are long-exposure shots that capture colors invisible to the photographer standing there.
If the water droplets are very small, as in fog or light mist, the colors can’t separate cleanly. Instead of a colorful arc, you get a fogbow: a broad, white or grayish band. Double moonbows are theoretically possible too. When light bounces twice inside a raindrop, it exits at a slightly higher angle with the color order reversed. But each bounce loses light, so a second moonbow is extremely faint, and a third or fourth is virtually undetectable.
Conditions You Need to See One
Three things have to line up at once, which is why moonbows are so uncommon.
- A bright moon, low in the sky. A full or nearly full moon provides the most light. But the moon also needs to be low, generally within about 42 degrees of the horizon. A few hours after sunset or a few hours before dawn on a full moon night is the sweet spot. When the full moon is high overhead, it can actually overpower the effect.
- Water droplets in the air opposite the moon. Rain, waterfall mist, or heavy spray needs to be falling in the part of the sky directly opposite the moon from your perspective. You stand with the moon at your back and look toward the rain.
- Dark skies. Light pollution washes out the already faint arc. Rural locations or parks far from city lights give you the best chance.
That combination, a bright low moon, rain or mist in the right direction, and dark skies, simply doesn’t happen very often in most places. Daytime rainbows are common because the sun is intensely bright and rain showers are frequent. Moonbows require the same geometry but with far less light to work with, so even when conditions are close, the result may be too faint to notice.
Where Moonbows Appear Reliably
Waterfalls are the most dependable moonbow producers because they generate continuous mist regardless of weather. The two most famous locations are Cumberland Falls in Kentucky and Victoria Falls on the border of Zambia and Zimbabwe.
Cumberland Falls is the only place in the United States where moonbows occur predictably enough to plan a trip around. The waterfall drops 68 feet and spans 125 feet across, earning the nickname “Niagara of the South.” That width creates a large, persistent mist cloud that refracts moonlight into a visible arc. Moonbows appear there roughly 2 to 3 nights around each full moon, adding up to about 60 viewing nights per year. The state park publishes a moonbow schedule so visitors can time their trips.
Victoria Falls, roughly twice the height and five times the width of Cumberland Falls, produces even more spray and is the other reliably documented moonbow site in the Western Hemisphere. Both locations draw photographers who set up long exposures to capture the colors the eye can’t quite resolve.
Spotting One in the Wild
If you’re not near a famous waterfall, your best odds come on clear full moon nights when a rain shower is passing through. Position yourself so the moon is behind you and look toward the rain. Give your eyes at least 10 to 15 minutes to adapt to the darkness, and avoid looking at your phone screen, which resets your night vision.
What you’ll likely see is a pale, ghostly arc rather than a vivid band of color. It can be easy to mistake for a cloud formation or dismiss as a trick of the light. If you want to confirm the colors are really there, try photographing it. Even a smartphone on night mode with a few seconds of exposure can sometimes pull out faint reds and blues that your eyes couldn’t detect. A DSLR or mirrorless camera on a tripod with a 15 to 30 second exposure will reveal the full rainbow spectrum clearly.
Moonbows are one of those phenomena that reward patience and a bit of planning. They aren’t supernatural or even especially rare in the right conditions. They’re just ordinary physics working with a dimmer light source, producing something that looks quietly extraordinary.