Red-orange skies and wildfires

[fog37]

TL;DR

Reason why skies are red-orange due to the recent wildfires

Hello and happy Sunday.

I am trying to fully understand what causes the red-orange sky color during the recent and tragic wildfires on the west coast.

In general, sunlight encounters airborne particles in the normal sky (assuming no clouds or smoke). The air particles (I guess they are truly molecules, not really particles) are smaller in size than any of the wavelengths of sunlight in the visible spectrum. Given that particle size < wavelength, the smaller blue wavelengths are scattered more than the red wavelengths (Rayleigh scattering, inverse 4th power wavelength dependence). The extra scattering suffered by short wavelengths essentially removes more blue radiation from the starting sunlight beam that red radiation. This means that particles in normal sky (no fog or smoke) make the sky appear blue (short radiation) and the sunlight beam contain less blue radiation. Removing from the forward direction is scattering. Question: in which direction should point a spectrometer to verify that sunlight has less of blue wavelength component? Should we point it straight toward the sun? That would probably damage the device though...

When sunlight instead interacts with particles having size > wavelengths of visible light, scattering becomes wavelength-independent and all wavelengths are equally scattered.This is what happens with clouds which are white. So clouds are an example of wavelength-independent multiple scattering. Question: what if we had Rayleigh (wavelength dependent) multiple scattering? Would the overall effect be a more pronounced Rayleigh scattering?

Orange-red skies: in the case of the orange skies caused by the recent wildfires, the sky looks orange-red because (I believe):
a) sunlight is first "reddened" by Rayleigh scattering caused light being scattered by smoke particles with size< lambda. These smoke particles remove a larger portion of the blue energy from the beam. The beam has mostly red light.
b) The now reddened sunlight beam then collides with an area of smoke particles with sizes both smaller and larger than the visible wavelengths of light. Overall, wavelength-independent multiple scattering takes place and the sky becomes red-orange.

Is my understanding correct? Thank you as usual.

 

[Leo Liu]

https://www.nasa.gov/image-feature/...northwest-caused-by-smoke-from-siberian-fires

I searched your question on google and this is what I found.

"The smoke particles from the fires allow sunlight's longer wavelength colors like red and orange to get through while blocking the shorter wavelengths of yellow, blue and green. Those longer wavelengths give the sky a red or orange tinted appearance. Similarly, during sunrise and sunset times when the sun is near the horizon, sunlight has to travel through more of Earth's atmosphere to get to you. The additional atmosphere filters out the shorter wavelengths and allows the longer wavelengths to get through, providing reds and oranges during those times. "
P.S. Particulate matter in wood smoke has a size range near the wavelength of visible light (0.4 – 0.7 micrometers). So it makes sense that the visible light having a longer wavelength can circumvent the smoke particles and therefore be detected by your eyes.