THE MOON MAY LOOK BEAUTIFUL LATELY WITH ITS ORANGE GLOW BUT THE REASON IS A WILD ONE

 

CNF

6/4/25

Have you gazed into the sky lately and noticed a red sun or an orange moon and wonder how that could be since we are not in a lunar eclipse stage this month. Those of you that glanced at the sky during the full orange moon in March of 2025 witnessed an actual Blood moon in all its glory. The reason for the orange appearance of the moon of late differs considerably.

The Canadian wildfires have turned the moon orange in Tennessee due to wildfire smoke that rises into the upper atmosphere. This smoke causes both the moon and the sun to exhibit a reddish hue, which is a common phenomenon during such events.

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?

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.