When wildfires rage or pollution levels climb, the sun often appears as an orange or deep red disk instead of its typical bright yellow. This shift is a straightforward physical reaction governed by how sunlight interacts with the various particles suspended in Earth’s atmosphere. The colors we see are simply the colors that successfully complete the journey from the sun, through the atmosphere, and to our eyes.
Understanding Light Scattering in a Clear Sky
The natural color of the sky and the sun is established by Rayleigh scattering. This process involves the atmosphere’s smallest components, primarily nitrogen and oxygen gas molecules, which are much smaller than the wavelength of visible light.
When sunlight enters the atmosphere, these small molecules most efficiently scatter shorter wavelengths, such as blue and violet. This scattered blue light reaches our eyes from all directions, making a clear sky appear blue.
Longer wavelengths, like red and orange, are less affected and pass relatively straight through the atmosphere. The light that travels directly to the observer appears yellow or white because most of the blue light has been scattered away from the direct path.
The Role of Smoke Particles in Altering Light
The introduction of smoke, ash, or other aerosols fundamentally changes how light is processed. Smoke is composed of particulate matter significantly larger than nitrogen and oxygen gas molecules. This presence of larger particles shifts the dominant light interaction from Rayleigh scattering to Mie scattering.
Mie scattering occurs when particle size is comparable to the wavelength of light. Unlike gas molecules, these larger smoke particles scatter all visible wavelengths of light—red, green, and blue—more uniformly.
This uniform scattering removes a large portion of the light from the direct path between the sun and the observer. The overall effect is a hazy, sometimes grayish or whitish appearance to the sky, as all colors are broadly diffused.
Why the Sun Appears Red During Smoke Events
The orange or red color of the sun during a smoke event is the direct result of this filtering action. As sunlight travels toward the ground, the dense layer of smoke particles effectively scatters away the shorter and mid-range wavelengths, including blue, violet, green, and yellow light. These colors are scattered out of the line of sight and diffused throughout the sky.
Only the longest wavelengths of light—the orange and red parts of the spectrum—can penetrate the thick layer of smoke without being completely scattered. These are the last colors remaining on the direct path to the observer’s eye.
The sun’s light is therefore severely attenuated, or dimmed, and the remaining light is dominated by the longest wavelengths. The intensity of the final color depends directly on the concentration of particulate matter in the air. A light haze results in an orange sun, while an extremely dense plume of smoke will filter out nearly everything but the deepest reds.
Comparing Smoke-Induced Redness with Normal Sunsets
The red sun seen through smoke is a different physical phenomenon from the red glow of a normal sunset. A typical sunset’s colors are created because the sun is low on the horizon, forcing its light to travel through a much greater thickness of the atmosphere than at midday. This extended path length increases the effect of Rayleigh scattering by gas molecules, filtering out the blue light.
In contrast, the red sun caused by smoke can occur at any time of day, even when the sun is high in the sky. This coloring is not dependent on the sun’s angle or the path length through the atmosphere’s standard composition. Instead, it is caused by the addition of large, non-gaseous particles like soot and ash, which are present throughout the air column. The smoke particles accelerate the light-filtering process, producing a much deeper and dimmer red color than a typical sunset, often giving the sun a muted, featureless appearance.