Sunsets are Red Because the Sky is Blue

Gary Hart Photography: California Sunset, El Dorado Hills, Sierra Foothills

California Sunset, El Dorado Hills, Sierra Foothills
Sony a7R
Sony 70-200 f/4 G
ISO 100
f/8
1/200 second

From time to time I’ll edit one of the many articles in the Photo Tips section of my blog, tweaking and clarifying a few things just to keep it fresh. But every once in a while I do a complete rewrite. Here’s my latest such effort, a brand new article explaining how the interaction of sunlight with our atmosphere gives us blue skies and red sunsets. Spoiler alert: Sunsets are red because the sky is blue.

(And of course everything below applies to sunrise, only in reverse.)

Sunset Color

Nature photography isn’t rocket science, but a basic understanding of nature’s processes can make the difference between success and failure. That applies even to something as fundamental as sunsets and sunrises, from the way sunlight interacts with the atmosphere, to the conditions necessary for vivid sunset color.

Light and color

Sunlight reaches Earth in energy waves of varying length. The total of sunlight’s visible wavelengths combine give us light that appears white. The colors we perceive when specific wavelengths within the visible spectrum are absorbed or scattered, with each wavelength creating a distinct color. While the visible portion of the sun’s energy generates a virtually infinite number of colors, we’ve all become familiar with the arbitrary color name labels assigned to wavelength points throughout the spectrum.

Moving from the longest visible wavelength to the shortest, visible light breaks down into some shade of red, orange, yellow, green, blue, indigo, or violet. Not coincidentally, these are also the colors of the rainbow we see when the white light of the sun, separated by refraction in airborne water droplets, is reflected back to our eyes. Maybe you remember from your college physics days the mnemonic acronym for the rainbow colors and their order (from the outermost to the innermost color): ROY G BIV.

When a beam of sunlight passes through a vacuum (such as space), all of its wavelengths reach our eyes simultaneously and we perceive the visible portion as white. When sunlight encounters something (like a tree, a rock, air molecules, or whatever), some of its light will either be absorbed or scattered, depending on the wavelength and the properties of the interfering medium. So, unless we’re in space, the light that eventually reaches our eyes has either been reflected or stripped of certain wavelengths by whatever it encountered on its journey.

For example, a patch of fresh snow reflects all of the sun’s visible wavelengths uniformly and appears white to our eyes. A piece of coal uniformly absorbs most of the sunlight that strikes it, so we see coal as black. And when sunlight strikes a leaf, all of its wavelengths except those that we perceive as green are absorbed, while the green wavelengths bounce to our eyes.

Color in the sky

Since our atmosphere is not a vacuum, sunlight is changed simply by passing through the air. In an atmosphere without impurities (like dust, smoke, and water vapor), light interacts only with air molecules. In very simple terms, an air molecule will scatter any wavelength that’s smaller than it is, so the shortest wavelengths are most easily scattered. This scattering of incoming solar energy by atmospheric molecules becomes a filter that catches the violet and blue wavelengths first, allowing the longer wavelengths to pass through and continue their journey to more distant eyes.

When the sun is overhead, sunlight travels through a relatively small amount of atmosphere. The wavelengths that reach our eyes are the first to be scattered, the short violet and blue wavelengths, making the sky blue (the sky appears more blue than violet because our eyes are more sensitive to blue light).

On the other hand, when the sun is on the horizon, the light that reaches our eyes has passed through much more atmosphere than it did when the sun was directly overhead. The shorter violet and blue wavelengths are long gone, bluing-up the sky for others on their way, and the only remaining wavelengths are the longer, less easily scattered, orange and red wavelengths. It’s sunset! (Or sunrise.)

Airborne impurities dampen the atmosphere’s filtering process, so contrary to popular belief, a vivid sunset requires clean, unpolluted air. That’s because smoke, dust, and water molecules are much larger than air molecules. Rather than only scattering specific colors the way tiny air molecules do, larger molecules scatter much more completely—in other words, instead of scattering only the blue and violet wavelengths, polluted air catches lots of orange and red too (and everything in between).

Anyone who has blended a smoothie consisting of a variety of brightly colored ingredients (such as strawberries, blueberries, cantaloupe, and kale—uh, 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 at best might be slightly tinted with the color of the predominant ingredient.

Verify this yourself: The next time a storm clears, check the color in the sky—whether it’s midday blue, or sunset red, it’s easy to see how much more vivid the color is when the air is clean. And what’s better known for its sunsets, Hawaii, where it rains almost daily, or Los Angeles, with its urban sprawl and exhaust-spewing vehicles?

Another source of color at sunset has become all too familiar to anyone in or near wildfire-prone regions is red-rubber-ball sunsets when a fire is nearby. A vivid sunset requires intense sunlight, the more intense the better. In a typical brilliant sunset, while the rest of the sky is filled with color, the sun itself is far too bright to photograph as anything but a white disk (without rendering the rest of the scene much too dark). But when sunlight has to battle its way through smoke particles, the total amount of light is significantly reduced and there’s not enough scattered light of any wavelength to color the sky. But look straight at the sun—it’s so inherently bright that some of its longest wavelengths have battled their way to your retinas, turning the sun red while the rest of the sky is a murky brownish-gray.

Getting the most from your sunset images

Any time rain has cleared the atmosphere and the remaining clouds are mixed with sunlight, there’s a good chance for a vivid sunrise or sunset. I have a few go-to locations near home, and at my frequently visited photo locations (Yosemite, Grand Canyon, Death Valley, Hawaii, and so on) that I beeline to when the conditions for color in the sky look promising.

Wherever I am, as I prepare my shot shortly before the sunset show begins, I look for clouds receiving direct sunlight. This is the light that will most likely color-up at sunset, starting with an amber glow that transitions to pink, and red. Conversely, if no clouds are getting light shortly before sunset, that may be an indication that the sunset will fizzle. But don’t give up, because Nature is full of surprises.

A couple of mistakes inexperienced photographers often make is giving up on sunset too soon, and forgetting to check the sky behind them. Some of the best sunsets I’ve ever seen have happened when the sun slipped through an unseen hole in the clouds just below the horizon. And shortly after the sun sets, the pink vestiges of the longest wavelengths still color the eastern horizon. As this color deepens, the steely blue of the Earth’s shadow starts to descend. This combination of rich color and soft, shadowless light creates some of the best color and light for photography. Even when the scene appears too dark to your eye, don’t forget that your camera can accumulate light and bring out color and detail lost to your eye.

Maximizing color in the high dynamic range conditions of a sunset requires careful exposure. Rather than trusting the preview image on your scenes with extreme contrast, it’s essential to trust your histogram. If the histogram for a high dynamic range sunset scene looks good (highlights and shadows not clipped), it’s likely that on the LCD the highlights will look too bright, and the shadows too dark. Resist the urge to fix one or the other in the field, and instead trust that you’ll be able to recover both in processing later. If you’re not sure (or just don’t trust your ability to read the histogram), backet your exposures by a stop or two around what you think is best.

And don’t forget to check your RGB histogram—even if the luminosity histogram looks good, it’s possible that the red channel is clipped and you’ll need to reduce your exposure a little.

About this image

It was January 2015 and I had only recently made the switch from Canon DSLR to Sony Alpha mirrorless. With fewer than 1000 frames shot on my new Sony a7R, I’d already been completely blown away by its dynamic range compared to my Canon 5DIII, and was anxious for opportunities to reap these newfound benefits.

On this winter evening I ended up in the foothills south and east of Sacramento, my go-to photo location closest to home. I have a number of spots here, each highlighted by one or more oak trees atop a west-facing hill that gives me great silhouette opportunities from the back side.

Rather than return to one of my tried and true spots, I wanted to find something new. I was alarmed at all the development underway in this once pastoral area, but I was able to get to this spot because a new road had been carved into the hills behind it. Construction had already begun and while it’s illegal in California to cut down our beautiful oaks, it seems that they can develop right up to them and it wouldn’t be long before this trio was completely surrounded by homes. It was pretty clear that this shot wouldn’t be possible if I were to return in even just a few weeks.

As you can see here, I once again confirmed the dynamic range of my new Sony sensor. But of course no camera can shoot directly into a completely unfiltered sun, but this evening I benefited from the moisture-laden haze that hovers over the Central Valley in winter.

One other thing I remember exciting me about my new camera was its extra resolution. My longest lens at the time was the Sony 70-200 f/4 and I wanted to shoot this tighter. I just shot it at 200mm, but found that I had plenty of resolution to crop it down to what you see here.