It happens every summer. Forest fires in Canada pump out vast quantities of smoke which are carried by winds to the south and east. Arriving days later over the northern Great Plains and Midwest, the blue sky soon turns a pallid gray.
The smoke spreads in subtle ripples and bands and dims sun and stars alike. Technically, the sky is clear, and that’s what you’ll hear from the weather service, but the smoky haze creates an overcast of its own. Sunlight is less intense, while the solar disk glows pale yellow-orange compared to its normal white-yellow. It may even disappear from view well before sunset, fading away in the fiery haze.
Early this morning, under faux clear skies, I noticed an unusual pale blue disk or aureole around the sun about four fists (40 degrees) wide. Beyond that lay a wide, darker ‘ring’ tinted a pale gray-brown. Forest fires release gobs of minute smoke particles and oil droplets into the atmosphere which, like the ash from volcanic eruptions, can occasionally color the sun or moon blue.
It works like this. Particles that are about 1 micron across (1/1000 of a millimeter) are the same size as the wavelength of red light. The sun pours out all colors of light, but when the red portion strikes the ash or smoke, it’s scattered about the sky. The shorter wavelength blue light isn’t affected and continues to pass directly to our eyes, coloring the sun a pale blue. In effect, the particles act like a blue filter.
I’ve seen no blue moons or suns yet, but I wonder if the blue aureole might be the result of smoke particles. It resembles a phenomenon called Bishop’s Ring seen around the sun during volcanic eruptions and created by ash and sulfur droplets. Notice though the ball of the sun remains red-orange, indicating that the smoke particles are not the right size to create a blue sun. At least not yet.
If you live where the sky is affected by the smoke of distant fires, keep an eye on the sun, moon and sky for unusual colors, disks and rings. We’d love to hear what you’re seeing.
My blue disk this morning also reminded me of the blue aureole around the rising sun on Mars taken by the Spirit Rover. Dust in the Martian atmosphere scatters red light like much like ash and fire smoke do on Earth. Blue sunrises and sunsets there are probably fairly common.