ASK A MET: How wildfire smoke turns the sky an eerie red
Ever wonder why the sky looks red during wildfires? The Weather Network explains how smoke in the air changes the way sunlight reaches us, casting a Mars-like filter on the landscape.
Ever wonder why the sky is blue on a clear day, but red when there are pollutants such as smoke in the air?
Wildfire smoke fills the air with tiny particles. The particles, such as soot, ash, and other aerosols, are much larger than the gas molecules in the atmosphere.
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In simple terms, the skies turn red from wildfire smoke due to how the air particles in the smoke scatter sunlight. That is due to two processes called Rayleigh scattering and Mie scattering.
Rayleigh scattering happens when light hits very small particles, much smaller than the wavelength of light (like oxygen or nitrogen molecules in the air). This process scatters shorter wavelengths more like blue and purple, much longer than red. That is why the sky looks blue.
Mie scattering occurs when light encounters larger particles, like smoke, dust, pollen or water droplets. These particles are about the same size or larger than the wavelength of light.
This process scatters all wavelengths more equally, not favouring blue like Rayleigh does. This leads to whiter or greyer appearance in the sky, especially when it’s hazy. It can also make the sun look milky or red, especially during wildfire smoke events. Light hitting smoke or fog gets muddled, everything's scattered evenly, and you lose the vibrant blues––leaving warmer tones like red and orange.
Under normal conditions, our sky appears blue because tiny air molecules scatter shorter wavelengths of light—like blue—more effectively than longer ones. But when wildfires fill the atmosphere with smoke, the story changes.
Smoke particles are much larger than air molecules, so they scatter light differently. Instead of favouring short blue wavelengths, they scatter all wavelengths more evenly. However, they also filter out much of the blue and green light. What remains are the longer wavelengths––reds and oranges––that pass through more easily, giving the sky its eerie, fiery glow.
It's even more dramatic at sunrise or sunset! The sun is low on the horizon, so the light has to pass through more of the Earth’s atmosphere. When wildfire smoke is present, the filtering is even stronger, and you can get a deep red or even a blood-orange sky.
It can also look apocalyptic, if the smoke is especially thick. The sun can appear dimmed or even blotted out, casting an eerie sepia or orange hue over everything. That is sometimes called a “smoke eclipse."
When wildfire smoke fills the sky, air quality drops. Try to stay indoors or wear a well-fitted and properly worn, respirator-type mask (such as a NIOSH-certified N95 or equivalent respirator) if you head outside. Also, use an air purifier to keep the air clean.