Breakdown: Why wildfires create red suns and moons

Published: Sep. 20, 2021 at 2:00 PM CDT|Updated: Oct. 3, 2021 at 5:47 AM CDT
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MEMPHIS, Tenn. (WMC) - Wildfires raging in western North America in 2021 have affected air quality and the look of the sky in places thousands of miles away.

At the time of this writing (September 20, 2021), more than 3.2 million acres have burned in 73 large fires and complexes in 12 states. Nearly 18,000 wildland firefighters, incident management teams, pilots, and support personnel are assigned to incidents. (National Fire News that is updated daily can be viewed here.)

2020 had significant wildfire season, too. You may recall seeing images showing the entire sky in San Francisco turned an orange hue that looked like it was straight out of a sci-fi movie.

You may have noticed hazy skies overhead here in the Mid-South, or even seen recent sunsets featuring a hot-pink to electric-orange orb. But what makes the sun and moon turn red?

The red hues are due to wildfire smoke blowing into the state, which has caused hazy skies and a red tint to the moon and sun.

Last year, Les Cowley answered the question about these red skies. Cowley publishes the great website Atmospheric Optics and is surely the world’s best-known living master of the physics of sky phenomena. Here’s the explanation he provided in 2020. It holds true again this year:

The color of our skies is a matter of the sizes of the particles making up our air. It’s also a function of the number of particles per unit volume in air, and to a much lesser extent – during wildfire season – the color of soot itself.

Particles smaller than visible light wavelengths scatter short wavelengths (e.g. blue light) much more strongly than long wavelengths (red). This is known as Rayleigh scattering, named for Lord Rayleigh in the 19th century, who derived the small particle limit. Lord Rayleigh determined that the scattering goes as the inverse fourth power of the wavelength.

Hence, blue light is scattered some 10 to 15 times more than red light. Air molecules scattering in this manner are what generate our blue skies.

Note that the light of even glorious red sunsets still has some transmitted blue. Not all is scattered away!

As particles get bigger they still scatter blue more than red, but the wavelength dependence weakens from the Rayleigh limit of the fourth power. Particles several times larger than light wavelengths scatter all wavelengths more or less equally.

Fresh smoke is an intermediate case. Look at a campfire sideways-on to the sunlight direction, and you’ll see its smoke is blue. If you are unfortunate enough to be downwind and in the smoke, the sun is reddened,

The wildfire smoke over the U.S. West [in 2020 was] largely in this regime. It scatters away more blue, and the sun’s transmitted light is reddened (but not completely denuded of blues).

All this holds for single scattering where a sun ray is scattered by only one particle before reaching the eye. Where the smoke clouds are dense, there is significant multiple scattering. In the limit of an optically thick cloud, the light inside the cloud (or sky) becomes a uniform color: that of the incident light before significant multiple scattering. Thus, clouds are white inside, and a clear blue sky gets milky white toward the horizon. Multiple scattering will modify the sky colors in San Francisco, for example, to an almost uniform orange-red. It is orange-red because the sunlight reaching the dense smoke has already been reddened by less dense smoke.

Sky colors with multiple scattering get complicated and need mathematical modeling to make predictions.

Bottom line: The color of our skies – and the objects in our skies – can change, depending on the sizes of the particles making up our air.

More particles per unit volume in air can also cause a color change.

Plus – during wildfire season – the color of soot itself influences sky color and the colors of visible suns and moons.

Each year, thousands of fires in the United States are caused by people. The major causes of these fires are from loss of control of debris burning; unattended and improperly extinguished or not extinguished campfires; and sparks or heat transfer from the use of vehicles and equipment like chain saws or recreational vehicles including trailers. Become a part of the solution. Learn more on how you can safely play and work around our natural resources on our public lands.

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