Question submitted by Chris St. Louis
Here's the background to the effect:
When the sun is high in the sky, its light travels a relatively short path through the atmosphere to reach your eye, the viewing point of the light. In that sunlight are all of the wavelengths of visible light, and each wavelength is viewed as a different color. The molecules in Earth's atmosphere, mostly nitrogen and oxygen molecules, scatter some of those wavelengths, but not others. Because those molecules are small compared with the wavelengths of visible light, they only end up scattering the shorter wavelengths, sending those light beams in all directions, out of the direct path of the light on its way to your eye. This selective scattering of wavelengths, or colors, is called Rayleigh scattering. The sky is blue during the daytime because the wavelengths for violet and blue are the shortest in the spectrum. They are scattered more than any other colors in the light, and as these wavelengths scatter across the sky, that sky turns blue.
As light travels through the atmosphere, hitting those molecules along the way, more and more of the shorter wavelengths are scattered. By the time the light reaches your eye, all the blue and violet has been scattered out, leaving only the longer wavelengths in the sky for you to see. That's why a setting sun turns the sky red, orange, yellow and all shades in between. All of that scattered blue and violet is busy creating a blue daytime sky somewhere else in a different time zone.
This is all happening in a clear sky, with mostly nitrogen and oxygen doing the scattering. If you add other molecules into the mix -- molecules in smog or fire smoke, for instance -- the picture starts to change.
Smog is simply a combination of the words "smoke" and "fog," and it comes from lots of different sources, some totally natural. It's composed mostly of aerosols, solid or liquid particles suspended in the sky. Volcanic eruptions produce heavy smog when they send tons of volcanic ash into the sky. Forest fires do the same when all of that burning matter billows upward, turning the sky nearly opaque.
Here's where it gets a little complicated. Whereas volcanic smog and forest-fire smog are composed of fairly uniform aerosols, man-made smog is composed of countless different molecules and different types of matter, and all of these aerosol particles vary in size. Size is the deciding factor in whether aerosols enhance a sunset's colors or hinder them.
When something like a volcanic eruption or a large forest fire sends particles into the air, it produces smog that blocks only certain wavelength of light. As sunlight moves through this atmosphere, it encounters nitrogen and oxygen, which scatter the blue end of the spectrum, and volcanic or fire ash, which is larger and scatters a good amount of the longer yellow wavelengths, too. What results is an incredibly red sunset until the ash clears.
Even the late afternoon ambient light will appear "redder" than normal. Photographers love this light.. it's considered "soft".