A sunset in Knoxville.

Just below this beautiful sky, an ultimate frisbee game was taking place.

On the sideline, my teammate and I had completely forgotten the game and were mesmerized by the beauty above, and we were not the only ones furiously snapping pictures with our phones. I promise you, no post-processing filters were used on this beautiful picture!

I told my teammate that, in all the places I’ve lived, I’ve never seen as many spectacularly vibrant sunsets as I have in Knoxville. She shared that she had heard beautiful sunset colors were created from pollutants in the atmosphere and wondered if the beautiful Knoxville sunsets were due to pollution from the local coal plants.

Being the scientist that I am, I decided to follow up on this theory and figure out why Knoxville sunsets are so beautiful.

First, we must ask what makes sunsets beautiful; what is responsible for the bright orange and red colors? Perhaps the best place to start is with the colors of light, or the rainbow.

Different colors of light are associated with different wavelengths, red having the longest wavelength and violet having the shortest wavelength. Another way to think about this is how many “waves” occur within a set period, or the frequency. Red light has the lowest frequency and least amount of waves; violet light has the highest frequency and the most waves. (Psst: if you’re having trouble visualizing wavelengths of light, think of white light as a flat lake, yellow light as a calm ocean day and blue light as an ocean with a bunch of white caps). 

In the middle of the day, the sun is white, and the sky is blue. The sun appears white because white light contains all the different colored wavelengths. Since the sun is directly above us, the light doesn’t have to travel far through the atmosphere and doesn’t get scattered, allowing all the wavelengths to reach our eyes.

When you look at the rest of the sky, it looks blue because the light is being scattered by all the molecules in the earth’s atmosphere. Since violet light has more waves, it can be scattered in more directions and hit our eyes more frequently. Technically, the sky is violet because of all these scattered light waves, but because of how our retina processes the light, our brain “sees” the sky as blue. 

At sunset, the sun is at an angle – as opposed to directly above us – so the sunlight must travel horizontally through the troposphere to reach our eyes. Troposphere? That’s the layer of dense air andwater vapor lying closest to earth.

Through this route, the sunlight is scattered even more, in part due to dust and other particulates. The light reaching our eyes is no longer white, but yellow or red. This is because the blue light is now scattered so much it can’t reach our eye.

The light that does make it through the troposphere is the red, orange and yellow light of sunsets (think gentle ocean swells). When we see a red sunset it’s because all the other colors of light have been scattered so much that red is the only color that canreach our eyes. 

Does this mean that air pollution is better for sunsets? It turns out this is a myth. Large particles from pollution can absorb the light, or cause inconsistent light scattering. The colors become muddled together and are less vibrant. This makes sense because smog-ridden cities are not known for beautiful sunsets.

Clouds also help to create spectacular sunsets, but they are not the source of color. Colors actually become muted when there are large clouds in the horizon because the sunlight cannot pass through the clouds. Lower clouds, like stratocumulus, will scatter or absorb the sunlight. However, when the clouds are high in the sky, such as with cirrus and altocumulus clouds, the clouds will reflect the sunlight. This creates spectacularly colored clouds, like the pink clouds in the picture above. 

The time of year will also impact the dramatic effect of the sunset. In humid climates (like Knoxville) fall and winter are the best sunset seasons, in part due to the dry air. As it turns out, this picture was taken in the fall. Summer does not make for good sunsets because the hot temperatures give way to evaporation, and the resulting water vapor is stored in the air as humidity.

Water vapor increases the density of the air, so it becomes more stagnant and moves slower. Smog is created when the dense air mixes with photochemical reactionsto create the all-too-familiar summer haze. You might glimpse a spectacular summer sunset just after a local thunderstorm. This is because the rain and wind clear up smog and remove big particles from the air.

So, what makes spectacular Knoxville sunsets so beautiful? We can conclude that the local coal plants or pollution is not the source, but what is? One theory is based on the amount of rain Knoxville receives annually. The state of Tennessee has the 6thhighest average rainfall in the U.S. at 54.2 inches average total yearly precipitation (This data was compiled between 1971- 2000 by the NOAA National Climatic Data Center).

The average rainfalls in the southeast range from 50-60 inches per year. When the sun is not blocked by clouds, we may see more beautiful sunsets because the rain has knocked large particles down from the sky. Plus, sunsets with clouds are oftentimes more breathtaking, credit given to the reflection of the light creating a dramatic affect.

However, this is just a theory, as Tennessee air quality ranks 30th in the country. Tennesseans experience an exposure of 8.2 to particulate matter of 2.5 microns or less

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Abigail Barnes and Brooke Dulka, PhD students in the Department of Psychology, also contributed to this article.

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