What Causes Color In A Sunset?

Many evenings I would be at a beach, Cape Meares Lighthouse, or Netarts Bay to watch the sun go down and get sunset pictures, but there would be no colorful sunset. So what makes a beautifully, vibrant sunset? To find the answer to this question, here are three resources with easy to understand explanations.

1.
The Physics of Sunsets, Posted by Ethan on February 13, 2013
http://scienceblogs.com/startswithabang/2013/02/13/the-physics-of-sunsets/

Ethan Siegel did a really good job explaining the colors we see in sunsets and its not too technical. Some sites I visited made the explanation hard to understand, so I recommend visiting his blog to read the whole thing because he uses charts and pictures for better understanding. Here are some statements from his blog, The Physics of Sunsets:

"The first and most obvious is the change in coloration of the Sun, as well as a severe drop in the Sun’s brightness. On an airless world like the Moon, the Sun at sunset would look no different than at any other time. But it’s the Earth’s atmosphere that makes sunsets so special.

When the Sun appears progressively lower and lower on the horizon, its light needs to pass through more and more of the atmosphere to reach our eyes. You might not think of the atmosphere as being a very good prism, but when you pass through around 1000 miles of it just before the Sun dips below the horizon, it starts to add up.

The bluer wavelengths of light get scattered away, leaving only the reddest wavelengths that reach your eye. As the sun drops towards the horizon, it progressively loses violets and blues, then greens and yellows, and finally even the oranges, leaving only the reds behind.

Also, despite its red appearance, there really still is blue and green light coming from the Sun, of course, while this is going on. But these shorter (i.e., bluer) wavelengths refract slightly more than the lower frequency ones, meaning that the reds come in at a different, shallower angle than the greens and blues, that come in at a slightly steeper angle.

Given a clear path to the horizon — such as over the ocean — this means that there’s a slight region of space just above the reddened Sun where only the shorter wavelength light is visible!

And when that happens, in addition to the normal color gradient that comes with a sunset, you can also get a small, separate region above the disk of the Sun that appears yellow, green, or even blue!"

2.
Here is a paragraph from an article by Amanda Fiegl, National Geographic, PUBLISHED October 29, 2013
http://news.nationalgeographic.com/news/2013/10/131027-sunset-sky-change-color-red-clouds-science/

"So really, 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."

3.
November 15, 2007, Source: University of Wisconsin - Madison
https://www.sciencedaily.com/releases/2007/11/071108135522.htm

"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 we can't see violet very well, the sky appears blue.

Scattering also explains the colors of the sunrise and sunset, Ackerman says.

“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.”

And because red has the longest wavelength of any visible light, the sun is red when it’s on the horizon, where its extremely long path through the atmosphere blocks all other colors."

So on the clear evenings here at the Pacific Northwest with no vibrant sunsets, I'm assuming it's because more of the blue lights are still getting through the atmosphere instead of the yellow, orange, and red when the sun is setting. Once in a while there is slash burning smoke between Tillamook and Netarts creating spectacular sunsets. No matter what your evening sunset looks like, keep enjoying life and looking forward to the beautiful sunsets in your future.