Eloquent Nature by Gary Hart

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A sunset myth

If your goal is a colorful sunset/sunrise and you have to choose between pristine or polluted air, which would you choose? If you said clean air, you’re in the minority. You’re also right. But despite some pretty obvious evidence to the contrary, it seems that the myth that a colorful sunset requires lots of particles in the air persists. If particles in the air were necessary for sunset color, Los Angeles would be known for its incredible sunsets and Hawaii would only be known for its beaches.

But what is the secret to a great sunset? Granted, a cool breeze, warm surf, and a Mai Tai are a good start, but I’m thinking more photographically than recreationally (sorry). I look for a mix of clouds (to catch the color) and open sky (to pass the sunlight), with a particular emphasis on a clear western horizon (or eastern horizon for sunrise) to give the sunlight a clear path. But even with a nice mix of clouds and sky, sometimes the color fizzles. Often the missing ingredient, contrary to common belief, is clean air—the cleaner the better.

Light and color

Understanding sunset color starts with understanding how sunlight and the atmosphere interact to color the sky. As you may know, visible light reaches our eyes in waves of varying length; we perceive each wavelength as a discrete color. Starting with the shortest wavelengths and moving toward the longest, visible light goes from violet, indigo, blue, green, yellow, orange, and red. (These color names are arbitrary labels we’ve assigned to the colors we perceive at various wavelength points along the visible portion of the electromagnetic spectrum—there are an infinite number of wavelength-depenedent colors between each of these colors.) When a beam of sunlight passes through a vacuum (such as space), it moves in a straight line, without interference, so all its wavelengths reach our eyes simultaneously and we perceive the light as white.

Color my world

White light is changed by interaction with a foreign object, such as our atmosphere. In an atmosphere without impurities (such as smoke and dust), light interacts only with air molecules. Air molecules are so small that they scatter (reflect) only a very narrow range of wavelengths. This atmospheric scattering acts like a filter that catches the violet and blue wavelengths first, allowing the longer wavelengths to pass through. When our sunlight has traveled through a relatively small amount of atmosphere, the wavelengths that reach our eyes are the just-scattered violet and blue wavelengths, and our sky looks blue (the sky appears more blue than violet because our eyes are more sensitive to blue light).

On the other hand, because the longer orange and red wavelengths are less easily scattered, they penetrate a much greater distance through the atmosphere. When the sun is on the horizon, its light has passed through much more atmosphere than it did when it was directly overhead—the only light reaching our eyes has been stripped of its shorter, blue and violet, wavelengths, leaving only the orange and red wavelengths to color our sky. Sunset! (Or sunrise.)

Pollution dampens the filtering process. Rather than only reflecting specific colors, light that encounters a molecule larger than its wavelength is more completely reflected—in other words, instead of reflecting only the blue and violet wavelengths, polluted air catches some orange and reds too. Anyone who has blended a smoothie consisting of a variety of brightly colored ingredients (such as strawberries, blueberries, and cantalope—yum) knows the smoothie’s color won’t be nearly as vivid as any of its ingredients, not even close. Instead you’ll end up with a brownish or grayish muck that might at best be slightly tinted with the color of the predominant ingredient. Midday light that interacts with large particles in the atmosphere is similarly muddied, while polluted sunrise and sunset light has already had much of its red stripped out.

Test this for yourself the next time a storm clears as the sun sets, and compare the color you see to the color on a hot, summer city evening.

For example: Sentinel Dome, Yosemite

Sentinel Dome in Yosemite provides a 360 degree view of Yosemite and surrounding Sierra peaks. Among the many reasons it’s such a great sunset spot is that from atop Sentinel Dome you can see what’s happening on the western horizon and plan your shoot long before the sunset happens. On this summer evening I was up there shortly after an afternoon rain shower. Lots of clouds remained, but there was an opening on the western horizon for the sun to slip through just before disappearing for the night.

Rather than settle for a more standard Half Dome composition, I wandered around a bit in search of an interesting foreground. I ended up targeting this group of dead pines on Sentinel’s northeast slope, a couple of hundred feet down from the summit. It was no coincidence that sunset that night, one of the most vivid I’ve ever seen, came shortly after an afternoon rain had cleansed the atmosphere. Not only did the clouds fire up, the color was so intense that its reflection colored the granite, trees, and pretty much every other exposed surface.

For example: South Tufa, Mono Lake

Sunset, South Tufa, Mono Lake

The air on Sierra’s east side is much cleaner than air on the more populated west side, and the clouds formed as the prevailing westerly wind descends the Sierra’s precipitous east side are both unique and dramatic. Mono Lake makes a particularly nice subject for the Eastern Sierra’s brilliant sunrise/sunset shows. Not only does it benefit from the clean air and photogenic clouds, Mono Lake’s tufa formations and (frequently) reflective surface make a wonderful foreground. The openness of the terrain surrounding Mono Lake allows you to watch the entire sunrise or sunset unfold. Many times over the course of a sunrise or sunset I’ve photographed in every direction.

The image here was captured near the end of a particularly vivid sunset. The air was clean, with just the right mix of clouds and clear sky; perfectly calm air allowed the lake’s surface to smooth to glass. I find that the more I can anticipate skies like this, the better prepared I am when something spectacular happens. In this case I was at the lake well before the color started, but because it looked like all the sunset stars were aligning, I was able to plan my composition and settings well before the color started.