Someone asked me this the other day. I said “probably”, but on second thought, I should have said “maybe”.

The color of a sunset is a manifestation of the same mechanism that causes the sky to be blue. The atmosphere is made up of gases. The most abundent of these gases is nitrogen, which has a diameter of about 3 Ångströms. Since the Ångström unit isn’t SI, let’s convert for our international colleagues. 1 Ångströms is equal to 10E-10 meters. That’s pretty small. Wikipedia gives a table of the size of some objects relative to each other. For comparison, the width of an average human hair is about 80 µm (8E-5 meter).

Objects this small don’t react with light (radiation) in the same way large objects do. A mirror, for instance, “reflects” the light such that the angle of incidence equals the angle of reflectance. This is called geometric optics. For extremely small objects (relative to the wavelength of light), geometric optics fails. The radiation doesn’t just bounce off the nitrogen particle like it would off of the mirror. Instead it undergoes Rayleigh scattering - names after Lord Rayleigh.

From the image below, the intensity of scattered light due to Rayleigh scattering is inversely proportional to the wavelength to the fourth power. The wavelength of blue light is shorter than the wavelength of red light. This means that blue light will be scattered more that red light. This is the reason why the sky is blue; it is also the reason that sunsets are red.

(Image taken from Department of Physics and Astronomy, Georgia State University

(Please do not make fun of my drawing below. I have no artistic ability, and it was made in Microsoft Paint. Not a good combination.)

This cartoon attempts to show Rayleigh scattering. Sunlight is actually made up of individual “color” of light. Each color of light has a different wavelength. When the white light hits a particle, such as nitrogen, the different wavelengths of light react differently. The blue light, shorter wavelength, gets scattered a lot. Yellow light, mid-wavelength, gets scattered a little. And red light, long wavelength, gets scattered the least. Now imagine it’s a sunset that you’re looking at. (Don’t look directly at the Sun!) The blue light will get scattered out of your line of sight, but the red light won’t. This causes the sunset to look red.

But why doesn’t the sun look red when it’s high in the sky? That has to do with how much scattering there is in the atmosphere. When the sun is high in the sky, around local solar noon, there is little atmosphere for the sunlight to travel through. At sunset and sunrise, there is a lot more atmosphere for the sun to travel through. The more atmosphere, the more scattering. Thus the sun appears redder at sunset.

When aerosols are added to the atmosphere, such as from the recent California fires, it provides more particles from which the incoming radiation can be scattered. Now the smoke particles aren’t as small as nitrogen molecules, but they still scatter light. When there is a lot of smoke in the atmosphere, the sun appears hazy and diffuse. That’s the same scattering, but there is so much scattering that little of the light is passing directly through the atmosphere. After the smoke dissipates, there is less scattering, but if it’s red sunsets you’re after that’s good. You want the happy medium between too little scattering and too much scattering.

Therefore, the fires in California may produce redder sunsets. It’s hard to say if the will produce redder sunsets. That will be determined by several meteorological variables. Which way is the wind blowing? Right now, there are westerly winds, meaning the smoke aerosols will be transported over the United States. This is good for those wishing for red sunsets. Another thing that could affect the color of the sunsets is the residence time of the aerosols. Luckily that’s easy to predict. The residence time of aerosols is highly correlated to the residence time of water in the atmosphere because the mechanism that removes most aerosols (of the smoke size) is precipitation. There is currently a high pressure system sitting over much of the Western United States, which means there will probably not be much precipitation in the near future. This means the aerosols will stay in the atmosphere longer. And there may be some pretty sunsets.

The redness of sunrises will probably not be affected because the sun “rises” in the east. Only areas west of the fires would see redder sunrises. So if you’re in a yacht in the Pacific somewhere, you may see some red sunrises. But if so, you’re probably not reading this blog anyway.

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