Why Does Smoke Turn the Sky Orange?

If you are one of the tens of millions of people who’ve been affected by smoke from the millions of acres of Canadian forest that are currently aflame, then you know what it’s like to live under a murky brown sky, with the sun a sullen reddish orb glaring menacingly down at you.

There’s very little good news to squeeze from this story. Burning a forest doesn’t just spew planet-warming carbon dioxide into the atmosphere; it also devastates entire ecosystems that would otherwise scrub some CO₂ from the air. Then of course there’s the more immediate problem that excess smoke makes for dangerously unhealthy air quality. And as predicted by climatologists, human-caused climate change is creating drier conditions that set the stage for more huge fires.

Still, there’s some interesting science here and a weird connection with not only astronomy but our view of our location in the universe itself.

The explanation is a little scattered but still absorbing.

Canada hosts one of Earth’s largest forests; more of a third of the country’s land is covered by trees. The trees’ wood and bark contains a lot of cellulose, a large biomolecule made up of the elements carbon, hydrogen and oxygen. Burning wood breaks these molecules apart and releases their constituent elements into the air (plus a lot of heat). Once freed, those elements bond with atmospheric oxygen and recombine to form different molecules. If the process is efficient, the only molecules at the end will be CO₂ and water.

A wildfire is not a high-efficiency wood stove, however—meaning it makes a wider variety of molecular by-products. Some are pure carbon and clump together to form tiny particles called black carbon, or soot. This happens at high temperatures, such as those created when wood burns. At lower temperatures, such as those of smoldering grassfires, the process creates complex molecules called brown carbon. Wildfires can also release surprisingly large amounts of water vapor—let’s just call it “steam”—by liberating moisture from burning wood.

Soot looks black because it absorbs light in the visible part of the spectrum—that is, the kind of light we see. If smoke is dense enough, all the visible light hitting it is absorbed, so it appears very dark. Steam, on the other hand, is an excellent reflector of light, so its plumes appear white.

Where things get interesting, though, is when the smoke isn’t so dense. Some light gets through so that it interacts with more of the particles in the smoke. Light behaves as a wave, and when it hits particles, it refracts, or bends, around them. The specifics are very dependent on both the size of the particles and the light’s wavelength, but generally the smoke will bend blue light much more than red light. This scatters the blue light, sending it off in somewhat random directions.

This basic reaction between light and matter occurs throughout our atmosphere, not just in smoke plumes. In fact, it’s why the sky is blue! Sunlight hits tiny nitrogen and oxygen molecules in the air, and the light’s blue component gets scattered off. These molecules are everywhere overhead, and many of them will scatter that blue light toward us on the ground. The result is that we see blue light coming from everywhere in the sky.

This is also why sunsets are so ruddy. As you look toward the horizon, you see through a thicker column of air, giving not just blue but also green and even yellow light more chances to scatter away. What passes through is the orange and red, which can sometimes make the sun appear dramatically vermillion.

A similar effect happens with smoke. A narrow plume hiding the sun will look dark in its center as all light is absorbed. The smoke thins out, approaching the plume’s outer limits, and colors it red and orange as those wavelengths pass through relatively unscathed. Meanwhile, at the edges themselves, some blue light can be scattered directly toward you, making the plume’s outline appear bluish.

A thick pall of smoke from horizon to horizon, though, can absorb all the infalling blue light, casting the entire sky a disturbing shade of orange, red or even brown. This is what happened in New York City last week, giving its sky a sickly, soiled appearance.

The effect of smoke on astronomy is obvious; it’s hard to observe faint objects when the sky above is largely opaque. But there are more subtle effects as well.

In the late 18th century William and Caroline Herschel, a brother-and-sister team of German astronomers, pondered our location in space. They reasoned that if the cosmos was finite and the sun was near its boundary, there would be fewer stars in one direction and more in the opposite direction. So they believed that by counting stars in different parts of the sky, they could figure out the sun’s relative position in the universe. After observing numerous patches of sky and meticulously adding up all the stars, they theorized we were located very near the cosmic center.

Of course, modern cosmology shows we hold no such privileged place in space. But at the time—well before the discovery of other galaxies or of cosmic expansion caused by the big bang—for the Herschels and everyone else, the Milky Way was essentially the entirety of the “observed” universe. But even if the universe was finite, even if the Milky Way was all there is, they still would’ve gotten their estimate wrong. That’s because they were also unaware of the existence of interstellar dust—small carbon-based molecules similar to soot that are created when massive stars die. The space between stars is so rife with this dust that no matter where your location is in the Milky Way, it seems like you’re in the center—because, much like smoke, the surrounding dust absorbs starlight to prevent a clear view of the galaxy’s overall shape.

Incidentally, we see the same scattering and absorption of blue light from clouds of interstellar dust as we do from wildfire smoke plumes. The nebula Barnard 68 is an exemplar of this. In the center, it’s black as pitch, with no stars visible through it at all. Near the edges, however, where the nebula’s dust is thinner, stars behind it are faint but visible and highly reddened because their blue light is scattered away. This makes the nebula appear like a hole in space with fuzzy, vaguely unsettling crimson edges.

Disturbingly, astronomers call this reddening effect “the extinction of starlight.” It’s helpful scientifically—it can be used to measure the distribution of dust within our galaxy, for example—but is all too reminiscent of our increasingly wildfire-polluted skies.

It’s striking that the same optical physics describing starlight and dust also explains far more local phenomena. If there’s any silver lining to be found in all this, perhaps it’s that the smoky air can be yet another reminder of the predictive power of science. Climatologists have said for decades that wildfires—and their region-choking plumes of smoke—would be more frequent and disruptive in a rapidly warming world. The smoke will dissipate, but the message should endure: when reputable scientists offer warnings about something, we should probably heed them.