Why wildfire smoke makes the air look orange

Hundreds of large-scale Canadian wildfires near Quebec recently swept smoke across the border into the U.S. Last week, this caused a large swath of the eastern U.S. skies to be covered in a smoky haze, making the air quality potentially hazardous.

At the peak of the smoky air last week, the skies in specific areas of the northeast, like Philadelphia and New York, took on an eerie orange cast. While the storm system that brought the smoke down from Canada has moved on for now, some residual haze lingers in certain areas.

In 2020, wildfires in the western U.S. blanketed San Francisco in an orange hue, too.

So why does wildfire smoke make the air look a different color?

It’s due to the way atmospheric scattering works. In airless places, the sky is black. But because Earth has air in the atmosphere, light gets scattered when the sun’s rays bounce off air particles in a scientific phenomenon called Rayleigh scattering.

Blue is on the shorter-wavelength end of the color spectrum, so it spreads more quickly and is the dominant color in the sky on clear days. However, smoke particles are larger and thus scatter longer wavelength colors like red and orange.

The orange effect caused by smoke is similar to what happens at sunset. Light has to travel through more air to you than it does during the middle of the day, so longer wavelengths are more apparent.

“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,” Steven Ackerman, professor of meteorology at the University of Wisconsin, Madison, explained in a news release. “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.”

Residents of New York posted images and videos of the smoky orange skies caused by the wildfires. In these from @isardasorenson, the orange colors are very dark.

This video taken by @michellestenzel shows the scene in Brooklyn that same day.

In the post, resident Michelle Stenzel references climate change, which scientists believe is increasing the frequency and intensity of wildfires; this could lead to us seeing more orange skies in the future.

If you’re in an area faced with these kinds of conditions, take precautions. It’s best to only go out with a good N95 mask — yes, the same masks many coveted during the COVID-19 pandemic are also good at filtering wildfire smoke particulates. Hopefully, you still have some around, but if not, you can always purchase them at Amazon and other retailers; thanks to their popularity during and after the pandemic, you can easily find a large selection of types in different colors. Also, use an air purifier to keep your indoor air clean. Stay safe!