A crimson sun suspended in a matte of smoky gray. It’s an ominous sight, captivating, to say the least. If you’ve ever wondered about the science behind the red sun during wildfires, you’re certainly not alone.

Two days ago, on my evening drive home from Alexandria, MN, something remarkable caught my attention. I gazed out of my car window, transfixed by a red sun. It hung like a warning above the horizon, casting its ethereal glow across a serene lake adorned with lily pads.

Mesmerizing as it was, it carried a deeper significance—a symbol of chaos and faraway destruction. It served as a distress call, a reminder of the devastating wildfires raging in Canada whose smoke was filling the skies across the Northern US. The sight, while otherworldly, seemingly magical, can be explained by the very same fascinating physics that give us our blue skies on a peaceful summer afternoon.

The Dance of Light and Smoke

To comprehend why smoke from distant wildfires tinges the sun with a crimson hue, we must first explore the intricate interplay between light and our atmosphere on any normal day. Sunlight encompasses a spectrum of colors, each composed of different wavelengths. It’s a marvel to consider that within a single beam of visible light, every color of the rainbow resides, with its own unique frequency. Various surfaces absorb or reflect light in distinct ways, revealing the vibrant array of colors. Prisms, for instance, manipulate light by bending it, allowing each frequency to separate and become visible—a process that also underlies the phenomenon of rainbows. If you’re keen to learn more about this topic, I explored it extensively in a post a few months ago about the illusion of color.

Particles in the Spotlight

While virtually invisible to our naked eyes, the skies are full of particles. When sunlight encounters these particles, it scatters, causing the blue and green wavelengths to disperse in different directions. This scattering phenomenon, known as Rayleigh Scattering, is responsible for the stunning blue skies we marvel at during daylight hours.

Sunsets, in general, exude a fiery appearance, intensified after evening storms or when clouds cloak the horizon. As the sun descends towards the horizon, its light traverses a greater distance and encounters a thicker portion of the Earth’s atmosphere, altering the spectacle we witness. During this journey, the shorter wavelengths of light have already been filtered out, leaving only the amber-hued, longer wavelengths. When combined with smoke, this effect becomes even more pronounced.

Wildfire smoke contains an assortment of minuscule particles, including ash, soot, and other organic matter. Collectively known as aerosols, these particles disperse throughout the atmosphere, interacting with sunlight in fascinating ways. As sunlight pierces through the smoky veil, some of the smoke particles have the ability to absorb light while others scatter it. This means the blue and green light is largely filtered out, leaving behind a captivating palette of crimson and amber tones.

Factors Influencing the Intensity

The are several factors that influence the intensity of the redness during a wildfire sunset.

The concentration and composition of the smoke particles, the proximity of the fire to your location, and the prevailing weather conditions all play pivotal roles. Regions closer to the wildfire source, where the concentration of particles is highest, will of course see the most exaggerated results. The position of the sun in the sky also plays a role. When directly overhead at midday, you’ll likely perceive the sun much as you usually would—white hot with an aura of yellow. Even still, the particles in the sky act as a filter, diffusing the light and diminishing its intensity. That’s why if you look directly at it (which you still shouldn’t, by the way), you see the sun less as a fire in the sky and more as a circular spotlight from above.