A rainbow forms when sunlight strikes airborne water droplets and is separated into its component spectral colors by characteristics of the water. The separated light is reflected back to our eyes when it strikes the backside of the droplets: Voila—a rainbow!
There’s nothing random about a rainbow—despite their seemingly random advent and location in the sky, rainbows follow very specific rules of nature. Draw an imaginary line from the sun, through the back of your head and exiting between your eyes—when there are airborne water droplets to catch that light, a will rainbow form a full circle at 42 degrees surrounding that line (this won’t be on the test). Normally, because the horizon (almost always) gets in the way, we see no more than half of the rainbow’s circle (otherwise it might be called a “raincircle”). The lower the sun is, the more of the rainbow’s circle we see and the higher in the sky the rainbow extends; when the sun is higher than 42 degrees (assuming a flat horizon), we don’t see the rainbow at all unless we’re at a vantage point that allows us to look down (for example, looking into the Grand Canyon from the rim).
Read more about rainbows on my Photo Tips Rainbows Demystified page.
Moonlight is nothing more than reflected sunlight—like all reflections, moonlight is a dimmer version its source (the sun). So it stands to reason that moonlight would cause a less bright rainbow under the same conditions that sunlight causes a rainbow. So why have so few people heard of lunar rainbows (a.k.a., moonbows)? I thought you’d never ask.
Color vision isn’t nearly as important to human survival in the wild as our ability to see shapes, so we evolved to bias shape over color in low-light conditions. In other words, colorful moonbows have been there all along, we just haven’t be able to see them because they’re not bright enough. But cameras, with their ability to dial up sensitivity to light (high ISO) and accumulate light (long exposures), “see” much better in low light than you and I do.
While it’s entirely possible for a moonbow to form when moonlight strikes rain, the vast majority of moonbow photographs are waterfall-based. I suspect that’s because waterfall moonbows are so predictable—unlike a sunlight rainbow, which doesn’t require any special photo gear (a smartphone snap will do it), capturing a lunar rainbow requires at the very least enough foresight to carry a tripod, and enough knowledge to know where to look.
Nevertheless, even though we can’t see a moonbow’s color with the unaided eye, it’s not completely invisible. In fact, even without color, there’s nothing at all subtle about a bright moonbow—it may not jump out at you the way a sunlight rainbow does, but if you know where to look, you can’t miss a moonbow’s shimmering silvery band arcing across the water source.
Despite frequent claims to the contrary, moonbows can be seen on many, many waterfalls. Among the more heralded moonbow waterfalls are Victoria Falls in Africa, Cumberland Fall in Kentucky, and (of course) Yosemite Falls in Yosemite National Park.
Yosemite Falls is separated into three connected components: Upper Yosemite Fall plummets about 1400 feet from the north rim of Yosemite Valley; the middle section is a series of cascades dropping more than 600 feet to connect the upper and lower falls; Lower Yosemite Fall drops over 300 feet to the valley floor. While there are many locations from which to photograph the moonbow on Upper Yosemite Fall, the most popular spot to photograph it is from the bridge at the base of Lower Yosemite Fall.
The Lower Yosemite Fall moonbow is not a secret. Arrive at the bridge shortly after sunset on a full moon night in April, May, and (often, if the fall is still going strong) June, and you’ll find yourself in an atmosphere of tailgate-party-like reverie. By all means come with your camera and tripod, but leave your photography expectations at home, or risk appreciating the majesty of this natural wonder. In springs following a typical winter, the mist and wind (the fall generates its own wind) on and near the bridge will drench revelers and cameras alike. After a particularly wet winter, the airborne water and long exposures can completely obscure your lens’s view during the necessarily long exposures. And if the wet conditions aren’t enough, if you can find a suitable vantage point, expect to find yourself constantly jostled by a densely packed contingent of photographers and gawkers stumbling about in limited light. Oh yeah, and then there are the frequent flashes and flashlights that will inevitably intrude upon your long exposures.
If, knowing all that, you still have visions of a moonbow image, it’s best to come prepared:
I’d taken my May workshop group to Glacier Point on this night, so we didn’t arrive at Yosemite Falls until nearly an hour after the moonbow started. This late arrival was intentional because California’s severe drought has severely curtailed the mist at the base of the lower fall. In a normal year the mist rises so high that the moonbow starts when the moon is quite low (remember, the lower the sun or moon, the higher the bow); this year, I knew that the best moonbow wouldn’t appear until the moon rose and the bow dropped into the heaviest mist. Not only that, the later it gets, the few people there are to deal with.
I’d given the group a talk on moonlight photography that afternoon, but we stopped at the top of the trail to practice for about 20 minutes, using the exquisite, tree-framed view of the entire fall. When everyone had had success, we took the short walk up to the bridge and got to work.
We found conditions that night were remarkably manageable—by the time we arrived at the bridge, at around 9:45, the crowd had thinned, and our dry winter meant virtually no mist on the bridge to contend with. I started with couple of frames to get more precise exposure values to share with the group (moonlight exposures can vary by a stop or so, based on the fullness of the moon, its size that month, and atmospheric conditions), then spent most of my time assisting and negotiating locations for my group to shoot (basically, wedging my tripod into an opening then inviting someone in the group to take my spot).
This image is one of my early test exposures—I went just wide enough to include the Big Dipper (just because it’s a test doesn’t mean I’ll ignore my composition). In wetter years I’ve captured move vivid double moonbows and complete arcs that stretch all the way across the frame, but I kind of like the simplicity of this image, and the fact that I was able to include the Big Dipper, which appears to be pouring in the the fall.