What is it about smoke that makes a sunset more intense?

Western fires as seen from space

Wildfire smoke and flames from satellite imagery over Idaho and Montana on Sept. 3, 2017.
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Wildfire smoke and flames from satellite imagery over Idaho and Montana on Sept. 3, 2017.

It’s fire season in the West, which also means it’s the season for some of the most spectacular sunsets of the year. But why? What is it about smoke that makes a sunset more intense?

Interestingly, it has a lot to do with what makes the sky blue.

Everyone is familiar with a rainbow or a prism. This illustrates something important about the seemingly white light from the Sun: it is in fact made up of many different colors of light. We can see those colors, from red to violet, when the Sun’s light is separated into its parts passing through a material like glass or water, a process called refraction.

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A rainbow over downtown Boise after a rainstorm shows the spectrum of colors that make up the Sun’s white light. Kyle Green

The sky separates the colors of light in a different way. Imagine the Sun’s rays traveling towards your eyes. The gases in the air, primarily oxygen and nitrogen, are just the right size to scatter the blue part of the spectrum. Since this happens all over the sky, you see blue light traveling from all directions, giving the sky it’s color. The only part of the Sun’s rays that get to your eyes are the yellows and oranges, giving the Sun its characteristic warm glow. If you were to look at the Sun in space, it would appear white, not yellow.

During a sunset, this effect is intensified. When the Sun is close to the horizon, its rays must travel through much more of the atmosphere. This means that even more blue gets scattered and the Sun and the surrounding sky takes on a darker red and orange color.

Boise’s skyline at sunset. When the Sun is low, more of the blue is scattered away and the sky appears red. Pete Zimowsky Special to the Idaho Statesman

So what does smoke do for how we see the sky? Unlike the air, smoke doesn’t just scatter blue light, it scatters all colors. This means that the smoke will reflect whatever color is around it. During the day, smoke takes on a blueish-white hue and reduces visibility. Clouds have a similar reflective property — reflecting all colors — that gives them their fluffy white appearance.

When the fire is close, the smoke hangs low in the sky, reflecting light in all directions, washing out color. So, low smoke will actually make sunsets less brilliant. The haze of pollution works in the same way.

If the fire is far away, such as in California or Canada, the smoke can get high into the atmosphere, high above an otherwise clear sky. As the sun sets, the smoke, along with high, wispy clouds, will reflect the red and orange light back to the ground, intensifying the sunset and the twilight sky.

In Idaho, the beginning and the end of fire season will yield the best sunsets, since the sky will be relatively clear of low-lying smoke. The intense sunrises and sunsets seen in the last few weeks is likely from smoke that has traveled all the way from fires burning in Siberia. Now that there are more fires in the Treasure Valley, the sunsets are likely to become more muted until things clear out. So, enjoy the view while it lasts.

Kevin Davenport is an experimental physicist at the University of Utah and a 2018 AAAS Mass Media Science and Engineering Fellow with the Idaho Statesman: 208-377-6411, @tropnevaDniveK.