Red suns and moons are from wildfires

Red suns and moons: Dark yellow sun with big orange to red streaks across it, on black background.
View at EarthSky Community Photos. | Lou Musacchio of Montreal, Quebec, Canada, captured this image on June 5, 2023, and wrote: “The sun cloaked due to clouds of smoke from the wildfires sweeping across Quebec and other parts of Canada. I used the spot meter on my D7200 so I could expose more of the sun then rather the whole scene.” Thank you, Lou! If you’ve captured photos of red suns and moons, or strangely colored sky due to smoke from wildfires, please submit to EarthSky Community Photos.

Read more: Canadian wildfire smoke turns US skies orange

Wildfires, and red suns and moons

Wildfires raging across Canada have sent smoke careening across North American skies. The northeastern portion of the United States has heavy smoke and an air quality alert has been issued. Some plane flights were even halted in that region. In other parts of North America, many people have noticed hazy skies and red suns and moons. But what makes the sun and moon turn red?

North America nearly covered in areas marked with gray shades.
Wildfire season is off to an explosive start in North America in 2023, with ferocious blazes beginning first in Canada. This map shows smoke and fire as of June 7, 2023, with darker areas meaning heavier smoke in the area. People in the areas affected by smoke might get redder-than-usual sunsets. Image via

What causes the red suns and moons?

Les 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 his explanation for red suns and moons during wildfire season:

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!

Les Cowley continues:

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.

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 modified the sky colors in San Francisco in the year 2020, for example, to an almost uniform orange-red because the sunlight reaching the dense smoke had already been reddened by less dense smoke.

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

Smoke from Canada’s 2023 wildfires

Many of Canada’s provinces are battling large wildfires in June 2023, and that smoke poured across the border in the United States. A gray sky with an oddly dampened and reddish sun was often the result.

Yellow-orange, opaque sky above trees at the edge of a grassy area.
View at EarthSky Community Photos. | Lisa Ann Fanning of Monmouth County, New Jersey, captured this image on June 7, 2023, and wrote: “This is absolutely surreal. Smoke from wildfires in Quebec making their way to New Jersey and supposedly, the worst is yet to come.” Thank you, Lisa Ann.
Pinkish sky with reddish sun behind lacy leafy branches of a stand of trees.
View at EarthSky Community Photos. | Sudhir Sharma of Stormville, New York, captured this image on June 7, 2023, and wrote: “Canadian wildfires are causing tremendous pollution in USA, specially in New York State. This photo shows the orange sun in hazy skies here, about 1 1/2 hours after sunrise. The AQI index was about 153 compared to the 30-50 normally seen in our area. One can smell smoke in the air.” Thank you, Sudhir!

If you’ve captured photos of a strange colored moon, sun or sky due to smoke from wildfires, share it with EarthSky at EarthSky Community Photos!

More smoke conditions

Bottom line: Wildfire smoke is already drifting across North America in 2023, creating redder-than-usual sunsets. Here’s why wildfires cause red suns and moons.

June 9, 2023

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