by Elizabeth Waddell
Sunrise. Sunset. Gradually changing from blue to orange, to fiery red; the clouds seem to glow. The science behind the show is just as impressive.
When light travels through the atmosphere, it comes into contact with materials that absorb, scatter, and reflect parts of the spectrum. Atoms of oxygen and nitrogen scatter the blue end of the spectrum making the sky appear blue and causing the sun to appear yellow. At sunrise and sunset, the angle of the sun to the earth increases, and light from the sun travels greater distances through the atmosphere. As more and more of the blue light is removed, only the red end of the spectrum remains visible.
The most memorable sunsets tend to be those with at least a few clouds. Clouds catch the last red-orange rays of the setting sun and the first light of the dawn. Middle and high clouds can be particularly spectacular because they catch the last rays of sunlight passing through the clean, upper atmosphere. When low clouds do take on vivid hues, it is a clue that the lower atmosphere is very clean and therefore more transparent than usual.
In general, air pollution mutes sunset hues just as it degrades daytime views. But sometimes dust particles in the atmosphere contribute to colorful sunrises and sunsets. Certainly the brilliant sunsets that follow major volcanic eruptions owe their existence to the ejection of dust and other particles high into the stratosphere. When the stratosphere is illuminated by sunlight passing through the lower atmosphere to the west, these high-level clouds of dust particles come into view. Since their colors achieve greatest intensity after the sun has set at the surface, volcanic twilights are known as "afterglows."