A blue sky is a good sky. It's a reassuring sight that promises clear weather and bright days.

But what in the world does a purple sky mean? It's not uncommon to spot purple skies during sunsets or sunrises. Sometimes they even appear after hurricanes, as they did in Florida following Hurricane Michael.

But what causes them?

Seeing blue

A blue sky with a white cloud and a canyon wall against it
That's certainly a good view. (Photo: Juanedc/Wikimedia Commons)

To understand why the sky is sometimes purple, it's helpful to first understand why the sky is blue.

Light is white, but 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.

Light will travel 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, which helps.

All about the angles

A red and orange sunset at Osaka Kansai International Airport
Even the landing lights of Osaka Kansai International Airport can't detract from that red-orange sunset. (Photo: Ken H/Wikimedia Commons)

Violet is always there, but we don't detect it as well as we do blue, so blue wins the light wave battle for our eyes. The right conditions have to be met for purple, or violet, to be visible.

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."

Purple skies

Electricity pylons can be seen during sunset in front of a purple sky in Ludwigsburg, Germany
That's some purple power in Ludwigsburg, Germany. (Photo: Sascha Schuermann/AFP/Getty Images)

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 due in part 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, water droplets, a setting sun and low cloud cover played a part in creating a purple sky after Michael had passed.

In the end, getting those purple hues is about having just the right conditions happen at the right time.