I’m fortunate to have a ringside seat for many of Mother Nature’s most exquisite phenomena, but few excite me more than the shimmering arc of Yosemite’s moonbow. A “moonbow”? I thought you’d never ask….
As you may have figured, a moonbow is a rainbow caused by moonlight. (Don’t be fooled by the fact that your spellcheck doesn’t recognize “moonbow”–it’s a very real thing indeed, and the more technically correct “lunar rainbow” designation just doesn’t seem to convey the magic.) Because a moonbow is a rainbow, all the natural laws governing a rainbow apply. But all this physics isn’t as important as simply understanding that your shadow always points toward the center of the rainbow/moonbow; the rainbow/moonbow will only appear when the sun/moon is 42 or fewer degrees above the horizon (assuming a flat horizon)–the higher the moon/sun, the lower the rainbow. When the moon or sun is above 42 degrees, the rainbow disappears below the horizon.
Each spring, High Sierra snowmelt surges into Yosemite Creek, racing downhill and plunging into Yosemite Valley below. A Yosemite icon, Yosemite Falls drops 2,500 feet in three magnificent, mist-churning steps. On spring full moon nights, light from the rising moon catches the mist, which 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 (breathtaking) silver band, revealing the bow’s color requires the camera’s ability to accumulate light. The above image, from a couple of years ago, was captured 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. To include the Big Dipper in this frame (I love the way it appears to be the source of the fall), I composed vertical and wide (19mm). This was a 30 second exposure at f4 and ISO 400.
Understanding the basic physics of a rainbow makes it possible to photograph a moonbow from other, less crowded locations in Yosemite Valley. In the image on the left, the moon had climbed so high that the moonbow had almost dropped from view. And it was so small at this point that I couldn’t see it at all with my unaided eyes. But I knew it would be there, so I exposed the scene enough to make it nearly daylight bright, again orienting the composition vertical and wide to include the Big Dipper.