The Science of Sunsets

Stephen F. Corfidi

December 2020

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.[1] 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.

Figure 1

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).

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[2]. 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[3]. 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.

Figure 3

Because air circulation is more sluggish during the summer, and because the photochemical reactions that result in the formation of smog and haze proceed most rapidly at that time of the year, late fall and winter are the most favored times for sunrise and sunset viewing in most parts of the world. Pollution climatology also largely explains why the deserts and tropics are noted for their twilight hues: air pollution in these regions is, by comparison, minimal.

The role of clouds

The twilight sky can inspire awe even when devoid of clouds --- as shown, for example, in Figure 4 --- with the crescent moon and crepuscular rays (produced by clouds below the horizon) on a crisp, fall evening in central Pennsylvania. But the most memorable sunsets tend to be those graced by at least a few clouds. Clouds can catch the last red-orange glow of the setting sun and the first rays of dawn, reflecting that light to the ground. But certain types of clouds are more closely associated with eye-catching sunsets than others. Why?

Figure 4

To produce vivid sunset colors, a cloud must be high enough to intercept "unadulterated" sunlight...i.e., light that has not suffered attenuation and/or color loss by passing through the comparatively dirty boundary layer. This largely explains why spectacular shades of scarlet, orange, and red most often grace cirrus and altocumulus layers, but only rarely low clouds such as stratus or stratocumulus. When low clouds do take on vivid hues, as they most often do over tropical oceans and sometimes in fresh, polar air streams (a polar-air example, with orange-tinged stratocumulus near Tulsa, Oklahoma, is shown in Figure 5), it is a clue that the lower atmosphere is very clean and, therefore, more transparent than usual.

Figure 5

Some of the most beautiful sunrises and sunsets feature solid decks of middle or high clouds that cover the entire sky except for a narrow clear strip near the horizon. A five-minute, wide-angle photo sequence of such a sunset over Baltimore, Maryland is shown in Figure 6. In the middle latitudes, skies like these most often are associated with progressive jet-stream disturbances in the westerlies; i.e., they mark zones of transition between west-to-east moving regions of atmospheric ascent (cloud cover) and descent (clear skies). When viewed at sunrise, a sky of this type implies that the weather is likely to deteriorate as the mid- and upper-level moisture and ascent continue eastward. At sunset, of course, the opposite is true, hence the saying "Red sky at night, sailor's delight; Red sky in morning, sailor take warning."

A large expanse of cloud-free sky (at least several hundred km in width) must exist beyond the visible horizon to yield relatively long-lasting, low-sun sequences like the ones shown in Figures 3 and 6. Narrower clear zones are accompanied by shorter periods of color as shadowing by upstream clouds limits the amount of time and space available for the production of vivid hues. This is another reason why particularly memorable sunsets so often are accompanied by fair weather the following day; long-lasting sunset colors can only occur when the area of clear skies extends far upstream from the observer.

Sunsets like the one in Figure 6 are perhaps most notable for the "bathed in red" effect that they produce. The entire landscape takes on a surreal crimson or saffron hue as the clouds reflect the fading sun's red and orange glow, allowing very little blue (scattered) light from the upper levels of the atmosphere to reach the ground. In those parts of the world well removed from the westerlies, the conditions that create these types of sunrises and sunsets most often occur in conjunction with tropical cyclones. In fact, in the West Indies, such sunrises and sunsets have long been recognized as harbingers of hurricanes.

An often-overlooked factor involved in the production of full-sky coverage of vivid low-sun colors (e.g., Figure 6) is that the cloud systems associated with both hurricanes and mid-latitude disturbances typically are sloped. The cloud bases, in particular, generally tilt upward on both the forward and rearward parts of the disturbances. The favorably sloped cloud sheets effectively catch a maximum possible amount of incident sunlight and, much like a vast, gently tilted theatre screen, are optimally oriented to reflect the reddened light down to the ground.

Figure 6’s sunset sequence also illustrates how large particles --- in this case raindrops falling from the departing upper-level cloud deck in the top view (a) --- tend to mute sunset colors. The overall coloration at this point is a dusky orange. Several minutes later, once the rain has cleared the area, more vibrant shades of red and orange overspread the scene (image c).

Figure 6

Certain cloud forms also characteristically assume shapes and textures that add interest. For example, altocumulus clouds often arise in areas where the wind changes speed and/or direction with height. This change in the wind (known as wind shear) can give rise to wave- or roller-like motions that are manifested as "ripples" or "billows" in the clouds. The grazing illumination of the low sun on such formations can create spectacular cloudscapes that vary over time as the angle of illumination changes with the rising or setting sun (e.g., Figure 3). Elevated cumuliform clouds such as those in Figure 6 also add interest, as the vertical extent of the clouds and their trailing precipitation cascades (fallstreaks) result in gradations of lighting that are not as easily realized with more uniform cloud decks.

A more subtle reason that some cloud types more commonly are associated with memorable sunrises and sunsets is related to the mode of formation of the cloud --- in particular, to the processes responsible for the cloud’s lower side. Middle and upper-level tropospheric clouds, such as many varieties of altocumulus and altostratus, typically arise in quasi-horizontal sheets that mark vertical zones of transition between two air streams of different origin --- with the clouds occupying a moist (saturated) layer that has surmounted a drier, unsaturated one. In contrast, lower clouds forms such as cumulus and stratus, more often are associated with generalized uplift through the cloud bases. This uplift leads to a gradual increase in relative humidity below the cloud bases and, via hydroscopic processes, to an increase in the size of natural and man-made pollutants in the sub-cloud layer. These enlarged particles diminish the intensity and spectral purity of incoming sunlight below the clouds.

Twilight hues from volcanoes

Tropospheric clouds are not the only ones that can enhance the beauty of the twilight sky. As already mentioned, particles in the upper atmosphere also can produce colorful sunrises and sunsets. Stratospheric particles derived from strong volcanic eruptions can exist as thin veils of dust or sulfuric acid droplets at altitudes of twelve to eighteen miles. Like the stars and planets, these aerosols usually are invisible during the day because they are obscured by the scattered sunlight (blue sky) of the troposphere. About 15 minutes after sunset, however, with the troposphere in shadow and the stratosphere still illuminated by sunlight passing through the lower atmosphere well to the west, these high-level clouds come into view. Since their colors achieve greatest intensity after the sun has set at the surface, volcanic twilights are known as "afterglows."

Three different twilight afterglows are shown in Figure 7. All three were observed over the eastern United States in September 1991 following the massive eruption of the Philippines volcano Mount Pinatubo in June of that year. As the photographs show, afterglows vary markedly in appearance depending upon the depth and height of the stratospheric clouds in the observer's vicinity. Color and intensity also are affected by the amount of haze and tropospheric cloudiness along the path of light reaching the stratosphere.

Figure 7

The first picture (a) shows a lilac afterglow high above the fading light of a brilliant, early-fall sunset. The cirrus streaks in the foreground have long since become shaded, but in the center of the view, a distant tropospheric cloud below the horizon is casting a dark shadow across the afterglow. Blue light scattered downward through the thin cloud producing the afterglow, mixed with the red light which illuminates it, is responsible for the lilac hue.

The middle example (b) shows a red-orange afterglow produced by a thicker aerosol cloud. The nearer parts are being illuminated by light that has passed through the troposphere and is therefore strongly reddened. More direct sunlight illuminates the brighter region close to the horizon. A similar cloud, viewed through a hazier lower atmosphere, appears in the last photo (c). Because of the haze, there is increased attenuation (especially along the horizon), and the intense colors of the previous example have been replaced by paler shades of pink and white.

Note that it is only when small volcanic particles have been lofted well into the stratosphere that colorful sunset afterglows appear. Volcanic particles that remain suspended in the troposphere after an eruption are comparatively large in size and number. As a result, they attenuate sunlight and otherwise subdue twilight hues, just like man-made dust and haze. Viewed through a veil of tropospheric volcanic ash, a sunset is dusky and dull.

Mount Pinatubo's sunset afterglows persisted to varying degrees for about eighteen months after the initial explosion. In more recent years (especially 1998, 2003, and 2019), sunset colors in many areas have been subdued by the introduction of large smoke particles into the lower atmosphere by forest fires across the western United States, Canada, China, and Australia.

Wildfire smoke

Depending on prevailing weather patterns, smoke from large and persistent wildfires can overspread vast areas well downstream from the conflagrations. As with haze aerosols, smoke particles are relatively large compared to the wavelength of sunlight. Selective scattering by smoke is therefore minimal; there is not substantial preferential scattering of short-wavelength (blue) light. Also, more light than usual is absorbed (attenuated) while on the way to the ground when the sky is smoke-filled. As a result, smoky skies are dusky --- with brightness and color saturation both subdued (Figure 8). Some readers no doubt will pause here, recalling the “red” skies seen over California when wildfires ravage parts of the West. Because shorter wavelengths are more readily attenuated by smoke than longer wavelengths, smoke layers, if sufficiently deep, can impart a reddish or orangish cast to the sky and landscape. While such displays can be awe-spiring and even foreboding, the colorations involved are subtle; it is the reduced intensity of the sunlight that makes the color shifts involved noticeable.

Figure 8

Further reading

The preceding paragraphs have provided only an introduction to the physics and meteorology of the twilight sky. Regular observation will, on occasion, reveal low-sun colors and behaviors different from those presented here. Such skies nevertheless reflect only variations on the basic scattering and absorption processes that have been discussed. The following references are suggested for further reading:

Bohren, C. F., and A. B. Fraser, 1985: Colors of the sky. The Physics Teacher, 23, 267-272 (May).

Lynch, D. K., and William Livingston, 1995: Color and Light in Nature. Cambridge, 277 pp.

Meinel, A. B., and M. Meinel, 1983: Sunsets, Twilights, and Evening Skies. Cambridge, 173 pp.

Minnaert, M., 1954: The Nature of Light and Colour in the Open Air. Dover, 362 pp.    [An updated (1995) version, with color photographs, is Light and Color in the Outdoors. Springer, 424 pp].

Naylor, J., 2002: Out of the Blue. Cambridge, 372 pp.        

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[1] Selective scattering is also known as “Rayleigh” scattering for the nineteenth and early twentieth century English physicist John William Strutt, more often known as Lord Rayleigh (1842-1919).

[2] Pollution particles (such as sulfate aerosols from industrial processes) are hydroscopic; their size therefore typically varies with the concentration of water vapor in their vicinity.

[3] The atmospheric boundary layer is the layer near the surface that contains most of the atmosphere's dust and haze. These aerosols are most concentrated in the boundary layer when a temperature discontinuity (inversion) exists at the top of the boundary layer, inhibiting mixing of the particles through a deeper layer.