There’s science that explains sunsets. And if you think winter along Minnesota's northshore produces some of the most beautiful – you’re right. More vivid sunsets occur when there’s a clean lower atmosphere.
Many folks think smoke from summer wildfires drifting in from Canada contributes to the most colorful sunsets along Superior's shore, but Stephen Corfidi of the NOAA said there’s a lot of confusion about that theory.
Some particles in the atmosphere are required to produce orange and red low-sun colors. But when the lower atmosphere — where we live — has an abundance of dust, pollen, or small water droplets, those particles actually reduce the brightness of the sunlight.
“Thus, a brilliant red-orange beam of light created by passage through a long expanse of clear skies will be robbed of its brilliance and color by the large lower atmospheric particles that tend to be found in wildfire ashe or dust... especially when if those particles build up over time.”
Rather, it’s clean air that is the main ingredient common to brightly colored sunrises and sunsets, he said. And in winter, weather patterns usually result in a clearer lower atmosphere, creating a better chance for a vivid sunset.
“Because air circulation is more sluggish during the summer, and because the photochemical reactions which result in the formation of pollens and dust build up in summer, winter is the most favored time for sunrise and sunset viewing over most of the United States,” Cordi wrote.
Sunset colors are created by a phenomenon called Rayleigh scattering. It’s the same phenomenon that makes the sky appear blue during the day. Sunlight contains all the colors of the rainbow. But not all the colors reach the ground in the same concentration. Nitrogen and oxygen molecules in our atmosphere act as little mirrors for blue and violet light, in particular. That means not as much blue or violet light reaches the ground. Instead, it bounces around in our atmosphere, creating the blue dome of sky we’re all so familiar with.
At sunset, light has to travel through a greater distance of atmosphere to reach our eyes — so even more blue light, and even some green and yellow light, gets filtered out. That leaves us with the warmer hues of the visible light, the reds and oranges, and it’s why many sunsets look like fire.
The air is typically filled with these tiny particles called aerosols, which are produced by things like trees but also by industrial activities. Aerosols attract water vapor and enlarge, essentially filtering the light we see in the sky and scattering it. But in the winter when the air is colder and dryer, there are less of these color-filtering aerosols in the air, meaning we see colors in all their intensity. That’s why the skies above Arctic, [and Grand Marais, Minnesota], tend to be so incredible at sunset—in the clean cold air of the Arctic where there are no trees or industrial activities, aerosols are not present to dampen the intensity of the sunset.
(Portions of this post originally from Vox, Mercury News, and the NOAA)