What makes the vivid colors of sunrises and sunsets? Area residents were treated to a spectacular sunrise this week.
In an article in National Geographic, written by Amanda Fiegel, Stephen Corfidi, a National Oceanic and Atmospheric Administration (NOAA) meteorologist who’s written about the science of colorful sunsets, said colorful sunsets and sunrises are the result of something called "scattering."
Corfidi said, "When a beam of sunlight strikes a molecule in the atmosphere, what’s called "scattering" occurs, sending some of the light’s wavelengths off in different directions. This happens millions of times before that beam gets to your eyeball at sunset.
"The two main molecules in air, oxygen and nitrogen, are very small compared to the wavelengths of the incoming sunlight—about a thousand times smaller. That means that they preferentially scatter the shortest wavelengths, which are the blues and purples. Basically, that’s why the daytime sky is blue. The daytime sky would actually look purple to humans were it not for the fact that the sensitivity of our eyes peaks in the middle [green] part of the spectrum—that is, closer to blue than to purple.
"But at sunset, the light takes a much longer path through the atmosphere to your eye than it did at noon, when the sun was right overhead. And that is enough to make a big difference as far as our human eyes are concerned. It means that much of the blue has scattered out long before the light reaches us. The blues could be somewhere over the West Coast, leaving a disproportionate amount of oranges and reds as that beam of light hits the East Coast."
He also noted that "there’s a good sunset every night; we just can’t always see it from the ground. You may have noticed this if you’ve ever taken off in an airplane at sunset. It might not look like anything special from the ground, just a whitish-pink sky, because you’re still within the atmosphere’s ‘boundary layer.’ That’s where all the large particles are trapped, things like dust and pollution. But as the plane gets above the boundary layer, into cleaner air, suddenly the sunset looks very vivid. It’s all a matter of perspective."
Steven Ackerman, professor of meteorology at University of Wisconsin-Madison, wrote in Science Daily that scattering affects the color of light coming from the sky, but "the details are determined by the wavelength of the light and the size of the particle. The short-wavelength blue and violet are scattered by molecules in the air much more than other colors of the spectrum. This is why blue and violet light reaches our eyes from all directions on a clear day. But because people can’t see violet very well, the sky appears blue.
"Because the sun is low on the horizon, sunlight passes through more air at sunset and sunrise than during the day, when the sun is higher in the sky. More atmosphere means more molecules to scatter the violet and blue light away from your eyes. If the path is long enough, all of the blue and violet light scatters out of your line of sight. The other colors continue on their way to your eyes. This is why sunsets are often yellow, orange, and red."