It is commonly stated that vivid sunrises and sunsets are caused by both natural and man-made dust and pollution. Clean air is, in fact, the primary component shared by vividly colored sunrises and sunsets. To see why this is so, consider how common sky hues are created. The color of clouds, for example, depends on their composition: blue for ice or nonglacierized snow, gray for old snow or salt water, white or pale yellow/orange for new snow or dry land, red for smoke or volcanic ash, and brown or black for soot or organic material from plants or animals.
The color of soil also reflects its composition. Black soil with a high content of metal oxide particles is dark because much of the light is reflected back out to space. Brown soil with a large amount of decomposed organic matter is dark because it absorbs light at all wavelengths including those that would otherwise reach the eye. White soil with a high concentration of calcium carbonate is bright because it reflects most of the light that falls on it. Gray soil with an equal amount of sand and clay is half white and half black, reflecting some light and absorbing another part.
Soil is made up of tiny particles called pigment grains. When these particles are very small--less than 0.05 mm (0.002 inches) across--they are invisible to the naked eye. Only the effects of overall size and shape can be seen.
Air pollution, in addition to atmospheric gases, water droplets, and dust particles, influence the color of the sky during sunrise and sunset. Sunlight is scattered into a range of colors by aerosols hanging in the air. More sunlight is dispersed when there are more aerosols or smog, resulting in purple or pink sunsets.
Sunset colors vary depending on the type of pollution present in the atmosphere at that time of day. Greenish colors indicate relatively low levels of nitrogen oxides and sulfur dioxide, while yellowish tones mean high levels of these pollutants. Purple hues signal the presence of particulate matter such as soot or mineral dust from sources such as forest fires or desert sandstorms.
At any time of day, but especially during sunrises and sunsets, viewers can see traces of other colors in the sky not caused by natural elements. These colors result from man-made materials such as smoke from factories or vehicles, or dust blown from farms or construction sites. They can also be seen in urban areas where coal or oil smoke is mixed with air pollution from cars and trucks.
The color of the sky during twilight periods depends on the amount of dust in the atmosphere and its composition. If it's clean, then the sky will appear blue; if there's too much dust in the air, then it will appear gray.
The authors argue that during dusk, the sky is full with pollution and wind-borne particles. More dust and pollution, on the other hand, might disperse light across a bigger region of the sky, giving a larger drape of hues, whereas dawn colors tend to be more centered around the sun.
Also, at sunset, the sun is below the horizon, so all its light has to travel through more of the atmosphere to reach us. This allows it to take on colors that would be obscured by clouds at sunrise.
Sunrise and sunset colors in the sky are also influenced by the composition of the landscape below. If there are large amounts of white material, such as salt flats or snow, then they will affect what you see in the sky at both times of day. Dark green vegetation, on the other hand, will only influence what's up in the air at sunset when combined with a blue sky.
Last but not least, the angle at which you view the scene influences how much color you see. If you look directly at the sun, for example, you will see very little color in the sky because of how bright it is. But if you gaze slightly to the side, then you can see more detail about what's going on in the sky during sunset.
Overall, sunsets are beautiful because they provide an opportunity to see details in the sky that wouldn't be apparent at sunrise.