The Science of Sunsets
Everyone at one time or another has marveled at a strikingly colored sunrise or sunset. Colorful sunrises and sunsets have, in fact, inspired imagination for centuries. Although brilliant low-sun colors can appear everywhere, some parts of the world are especially known for their twilight hues; the deserts and tropical oceans quickly come to mind. For example, rarely does an issue of Arizona Highways not include an eye-catching sunset image; sunsets also often provide the backdrop for Caribbean and Hawaiian postcard views. Likewise, even casual observation reveals that colorful sunrises and sunsets favor certain seasons. For example, in the mid-latitudes, including much of North America and Europe, fall and winter most often produce spectacular low-sun hues.
Why do striking sunsets appear in some parts of the world more than others, and why are they most often seen during certain months? What atmospheric conditions create truly memorable sunrises and sunsets? These and other twilight phenomena are explored in the paragraphs that follow.
What dust and pollution don't do
It is often stated that natural and anthropogenic dust and pollution cause colorful sunrises and sunsets. In fact, the brilliant twilight "afterglows" that follow major volcanic eruptions do owe their existence to the injection of small particles high into the upper atmosphere (will be said on this later). If, however, it were strictly true that an abundance of atmospheric aerosols, especially in the lower part of the atmosphere, were responsible for brilliant sunsets, large urban areas would be celebrated for their twilight hues. In fact, aerosols of all kinds --- when present in abundance in the lower troposphere as they often are over urban and continental regions --- do not enhance sky colors --- they subdue them. Relatively clean air in the lower levels is, in fact, the primary ingredient common to brightly colored sunrises and sunsets.
To understand why this is so, one need only recall how typical sky colors are produced. The familiar blue of the daytime sky is the result of the selective scattering of sunlight by air molecules. Scattering is the re-direction of light by small particles. Such scattering by dust or by water droplets is responsible for the shafts of light (“crepuscular rays”) that appear when the sun is partly blocked by clouds --- or partly illuminates a smoky room or misty forest. Selective scattering, meanwhile, is used to describe scattering that varies with the wavelength of the incident light. Particles are good selective scatterers when they are very small compared to the wavelength of the light.
Ordinary sunlight is composed of a spectrum of colors that grade from violets and blues at one end to oranges and reds on the other. The wavelengths in this spectrum range from .47 um for violet to .64 um for red. Air molecules are much smaller than this --- about a thousand times smaller. Thus, air is a good selective scatterer. But because air molecules are slightly closer in size to the wavelength of violet light than to that of red light, pure air scatters violet light three to four times more effectively than it does the longer wavelengths. In fact, were it not for the fact that the human eyes is more sensitive to blue light than to violet, the clear daytime sky would appear violet instead of blue!
At sunrise or sunset, sunlight takes a much longer path through the atmosphere than during the middle part of the day. Because this lengthened path results in an increased amount of violet and blue light being scattered out of the beam by the nearly infinite number of scattering "events" that occur along the way (a process collectively known as multiple scattering), the light that reaches an observer at the surface early or late in the day is noticeably reddened.
The effect just described is demonstrated vividly in Figure 1. In the image, the anvil cloud of an approaching evening thunderstorm is blocking low-level sunlight and casting crepuscular rays over the middle and right parts of the view, while unblocked sunlight continues to illuminate the entire depth of the atmosphere at the left. As the lower left part of the scene is dominated by sunlight that has taken a long path through the lower troposphere, that part of the sky appears notably orange and red. In contrast, in the shadow of the cloud, where the sky is viewed primarily by scattering from less-reddened sunlight topping the cloud, the sky is comparatively blue.
Because of the substantial difference in the path length of sunlight between midday and sunrise or sunset, it can be said that sunrises and sunsets are red because the daytime sky is blue. This notion is perhaps best illustrated diagrammatically: A beam of sunlight that at a given moment helps produce a red sunset over the Appalachians at the same time contributes to the deep blue of the late afternoon sky over the Rockies (Figure 2).
Now what happens when airborne dust and haze enter the view? Typical pollution droplets such as those found in urban smog or summertime haze are on the order of .5 to 1 um in diameter. Particles this large are not good selective scatterers as they are comparable in size to the wavelength of visible light. If the particles are of uniform size, they might impart a reddish or bluish cast to the sky or result in an odd-colored sun or moon; it is this effect that accounts for the infrequent observation of "blue suns" or "blue moons" near erupting volcanoes. Because pollution aerosols normally exist in a wide range of sizes, however, the overall scattering they produce is not strongly wavelength-dependent. As a result, hazy daytime skies, instead of being bright blue, appear bluish-gray or even white. Similarly, the vibrant oranges and reds of "clean" sunsets give way to pale yellows and pinks when dust and haze fill the air.
But airborne pollutants do more than soften sky colors. They also enhance the attenuation of both direct and scattered light, especially when the sun is low in the sky. This reduces the total amount of light that reaches the ground, robbing sunrises and sunsets of brilliance and intensity. Thus, twilight colors at the surface on dusty or hazy days tend to be muted and subdued, even though purer oranges and reds persist in the cleaner air aloft. This effect is most noticeable when viewed from an airplane, shortly after take-off on a hazy evening. A seemingly bland sunset at the ground gives way to vivid color aloft as soon as the plane ascends beyond the hazy boundary layer. When the haze layer is shallow, a similar effect sometimes is evident at the surface, as shown by the extended sunset sequence in Figure 3. The photographs show a billowed altocumulus wave cloud formation in the lee of Virginia’s Blue Ridge Mountains that erupts into a blaze of fiery oranges and reds once the sun has dropped far enough below the horizon that it no longer directly illuminates the thin veil of surface-based haze present below the clouds. The haze layer appears as a dark band just above the horizon in the last (enlarged) view.