Posted on May 12, 2020
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.
Posted on April 21, 2019
There are many (many!) beautiful sights in Yosemite, but when most people think about Yosemite, they think about waterfalls and granite. The granite is forever (virtually), but Yosemite’s waterfalls come and go with the season: exploding from the granite walls in spring, most of Yosemite’s waterfalls are bone dry by summer’s end. And some years are better than others—three springs ago, Bridalveil and Yosemite Falls were barely a trickle, too dry to photograph (unprecedented in my lifetime). The next spring the deafening roar of waterfalls was back, echoing throughout Yosemite Valley.
Moonbow, April 18, 2019
I just returned from my annual Yosemite Moonbow and Dogwood photo workshop on Friday night (technically, it was early Saturday morning). The dogwood are just starting to pop, but the waterfalls are going strong, with enough snow in the high Sierra bank to keep them roaring through summer.
My group photographed more waterfall rainbows than I could count, on both Bridalveil and Yosemite Falls, but the highlight was Thursday night’s lunar rainbow (moonbow) shoot on the bridge beneath Lower Yosemite Fall. Nothing compares to the first time seeing a moonbow. A shimmering silver arc, a moonbow is clearly visible to the naked eye—proper exposure in a camera reveals the moonbow’s vivid color.
A “practice” moonlight shoot the previous night helped prepare everyone for the difficulties of photographing in the dark. And while my group came prepared for moonlight photography, the crowds and mist make things difficult even for the seasoned veteran. The crowds weren’t too bad this year, but while lots of water in the fall means a better moonbow, it also means a wetter photographer.
I feared that the thin cloud cover that had delivered a spectacular sunset just as the full moon rose just an hour or so earlier, would douse the moonlight necessary for a moonbow, but that turned out to be a non-factor. One problem was contrails, more than I’ve ever seen. Some chose to leave the sky (or most of the sky) out of their frame; I opted to include the sky, then carefully execute a contrailecotmy in Photoshop.
Because most of my time on the bridge is spent assisting the group, I only got to click a handful of frames. I started on the (drier) paved open area before the bridge, but after working with a workshop participant on the bridge, I decided the view there was worth getting wet.
I went wider with this year’s images than previous years, using my Sony 12-24 G lens on my Sony a7RIII camera. I focused on the moon, then turned around and set up my composition. Concerned about too much water on my front lens element, I bumped my ISO to 1600 to keep my shutter speed at 10 seconds or faster. When I was ready to click, I wiped down the front of my lens with a towel that I lifted just as my shutter clicked.
I just scheduled my 2020 Yosemite Spring photo workshops, April 5-8 and May 4-7. Both are timed for the full moon to maximize our moonbow chances. And of course it’s not all about waterfalls and rainbows—this year’s spring workshops included some spectacular clearing storms, beautiful moonrises, and brilliant poppies. In addition to great photography, you’ll improve your photo skills with daily training and image reviews. You’ll also have lots of fun.
Click an image for a closer look and to view slide show.
Category: Bridalveil Fall, Cathedral Rocks, full moon, Merced River, Merced River Canyon, Moon, Moonbow, Moonlight, Sony 100-400 GM, Sony 12-24 f4 G, Sony 2X teleconverter, Sony a6300, Sony a7R III, Yosemite Tagged: Lower Yosemite Fall, moonbow, moonlight, night photography, Yosemite, Yosemite Falls
Posted on April 6, 2018
Even though your spellcheck says it doesn’t exist, I promise you that a moonbow is a very real thing indeed (and I have the pictures to prove it). Some argue that “lunar rainbow” is more the technically correct designation, but since that moniker just doesn’t convey the visual magic, I’m sticking with moonbow.
This won’t be on the test
Because a moonbow is a rainbow, all the natural laws governing a rainbow apply. But all the moonbow’s physics can be summarized to:
1) Your shadow always points toward the center of the moonbow (put your back to the moon and note the direction your shadow points)
2) The higher the moon, the lower the moonbow and the less of it you’ll see
3) When the moon is above 42 degrees (assuming flat terrain), the moonbow disappears below the horizon
Each spring, Sierra snowmelt surges into Yosemite Creek, racing downhill and plunging 2,500 feet in three mist-churning steps as Yosemite Falls. Shortly after sunset on spring full moon nights, light from the rising moon catches the mist, which separates and bends it into a shimmering arc. John Muir called this phenomenon a “mist bow,” but it’s more commonly known today as a moonbow.
While a bright moonbow is visible to the naked eye as a shimmering silver band, it isn’t bright enough for the human eye to register color. But thanks to camera’s ability to accumulate light, the moonbow’s vivid color shines in a photograph.
I just returned from the first of two moonbow workshops scheduled for this spring, but haven’t had time to process this year’s moonbow images. The above image was captured a few years ago near the bridge at the base of Lower Yosemite Fall. Not only was it crowded (the moonbow is no longer much of a secret), wind and mist made the necessary 20- to 30-second exposures an exercise in persistence. Not only was I able to capture the moonbow, as you can see, I now have photographic proof that the Big Dipper is the true source of Yosemite Falls.
Click an image for a closer look and slide show. Refresh the window to reorder the display.