Why Wildfire Smoke Turns the Sky Yellow

If you were standing on the streets of New York City any time this week, you could be forgiven for thinking you’d somehow been transported to Mars. The local skies were a foggy yellow-orange and the sun was partly obscured behind the haze. Photographs taken by the Perseverance and Curiosity rovers reveal strikingly similar conditions in the Martian atmosphere. On Mars, things are that way all the time. On Earth, the effect has been due to smoke from more than 400 active wildfires raging across eastern Canada. But what is it about wildfire smoke that turns the sky yellow instead of, say gray or even black? The answer has to do with atmospheric particles.

Sunlight streaming to Earth comes at us as a pure white beam, one that contains the seven wavelengths of electromagnetic energy that produce the colors of the visible spectrum: red, orange, yellow, green, blue, indigo, and violet. The closer to red the color is, the longer its wavelength; the closer to blue or indigo or violet, the shorter.

The molecules and particles that make up Earth’s atmosphere do not treat all of those wavelengths equally. Most of the colors are absorbed before the sunlight reaches our eyes. The exception is blue, whose wavelength is just the right size to dodge the molecular vacuum cleaner and get scattered down to us. The result: a blue sky—most of the time.

Read more: Why Wildfire Smoke Travels So Far and How Long It Will Last

When wildfires burn, things are different. Particles of smoke are larger than the other particles and molecules that typically make up the air, and that changes the filtration properties of the atmosphere, causing it to absorb the green, blue, indigo and violet, and scatter down the red, and especially orange and yellow, to us. “We [also] see muted red sunrises and sunsets under heavy smoke conditions,” Ryan Stauffer, an atmospheric scientist at the Goddard Space Flight Center told NASA. “In extreme cases like this week, the sun may become obscured entirely.”

Atmospheric filtration also explains why, even when the skies are clear, sunrises and sunsets are red, and dim enough to be looked at directly. When sunlight is traveling to us from the horizon, it has to pass through a thicker blanket of air than it does when it’s shining down on us from directly overhead; that is enough of a difference to screen out blue wavelengths and scatter down the red.

Read more: Even As Smoke Engulfs Us, We Can’t Wrap Our Heads Around Climate Change

For as long as the wildfires burn, NASA will be keeping an eye on them, with the Terra, Aqua, and Aura Earth-observing satellites monitoring them from above, making multiple observations, including how much sunlight the atmosphere absorbs or reflects away.

Oh, and as for our planetary cousin Mars—where fires can’t burn in a tenuous atmosphere that is 95% carbon dioxide—the orange skies are once again due to atmospheric particles. In this case the particles are iron oxide dust, kicked up by the Martian winds, which absorb blue light and scatter the orange and red down to the surface. Mars will permanently live in that gloom. On Earth, thankfully, it will lift when the wildfires are extinguished—and blue skies return.