Sun-thing Strange: Fire in the Sky

Posted on May 19, 2023

Did you happen to notice any abnormally red, hazy sunrises or sunsets this week? What's going on? Why does Michigan suddenly look like a scene from Star Wars?

These eerie spectacles are the result of wildfire smoke drifting over from Canada. An unusual, early-season heatwave recently struck our northern neighbor, resulting in a string of wildfires in Alberta, British Columbia, and Saskatchewan. Over 90 fires were recorded in Alberta alone, with 27 of those fires categorized as “out of control.”

While Canada is relatively close as far as countries go, it’s not exactly walking distance … so, why is the smoke having such a dramatic effect on our view of the sun all the way down here?

Wildfires generate intense heat, driving smoke and very fine suspended particles and gases high up into the upper atmosphere. This smoke and the associated fine particles stay up there until they cool down and start to descend. If there are upper-level winds, that smoky-particle mix will be blown around, sometimes traveling a rather considerable distance.

Okay, but why does that make the sun look like it’s on fire? Basically, as the sun’s light passes through the smoky layer of air, the smoke particles filter out the shorter wavelengths of light (which appear blue, green, and violet) while longer wavelengths of light (which appear red and orange) still pass through. Under normal, non-smoky circumstances, tiny particles in the air scatter short wavelengths of light 10 – 15 times more than long wavelengths (this is known as “Rayleigh scattering” and helps answer the age-old question, “Why is the sky blue?”). During a wildfire, when the particles in the air are larger than they normally are, those short wavelengths are scattered less than usual, allowing the long wavelengths to become dominant.

There you have it! While the sun might look more fiery than usual this week, it's still just the same old giant ball of gas.