Why Does the Sky Sometimes Turn Purple?

The right conditions plus the perfect timing can create a beautiful purple sky.

Scenic View Of Silhouette Mountains Against Sky During Sunset
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A blue sky is a good sky. It's a reassuring sight that promises clear weather and bright days. But what does it mean when the sky turns purple?

While it's not uncommon to spot purple skies during sunsets or sunrises, we can't help but wonder what causes them. Here, we discuss why our eyes see different colors in the sky and which factors impact those colors.

How Light Waves Travel

Sunny day and a blue sky
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To understand why the sky is sometimes purple, it's helpful to first understand how light travels.

The light that is produced from the Sun is white. However, when you put it through a prism, you see all the different colored light waves in the spectrum: red, orange, yellow, green, blue, indigo, and violet.

Light travels in waves—sometimes in short, dippy ones and other times in long lines with lots of peaks. Typically, light travels in a straight line unless something gets in its way, like a prism or molecules in the atmosphere.

Gases and particles in the atmosphere scatter light, and since blue travels on shorter, smaller waves, it causes charged particles to move faster, scattering more light. So we see more blue than we do red because blue gets particles worked up more than the other colors. Our eyes are also a bit more sensitive to blue light.

Violet is always there, as well, but our eyes detect it less so than blue. So, the right conditions have to be met for purple, or violet, to be visible.

The Role of Angles

Watching sunset on Beacon Hill
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In some cases, it's a matter of where the sun is coming in at a certain angle. Some of those colors that get blocked out by blue, like yellow, red, and orange, appear a lot during sunrises and sunsets for this reason.

"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," explained Steven Ackerman, professor of meteorology at the University of Wisconsin–Madison. "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."

Other Factors That Cause a Purple Sky

Full Moon in the sunset cloud in the purple sky
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But other factors can come into play that can jumble up the light waves and the particles even more.

According to Sarah Keith-Lucas from BBC Weather, "dust, pollution, water droplets, and cloud formations" can influence the colors of the sky, too. Occasionally, pink and purple will appear more often than red and orange. This is partially due to "the optical illusion of the pink wavelengths lighting up the base of the cloud (due to the low angle of the sun's rays), and these pink clouds superimposed on a dark blue sky. The combination of pink and dark blue can make the sky appear a deep purple."

In the case of Hurricane Michael and other hurricanes, water droplets, a setting sun, and low cloud cover played a part in creating a purple sky after the storms have passed. Getting those purple hues is about having just the right conditions happen at the right time.