Roark: The blue skies of autumn
Published 2:30 pm Monday, October 23, 2023
By Steve Roark
I look forward to the skies of September and October and they are intense blue yearly. Autumn skies typically have a lot of big puffy cumulus clouds as well, highlighting the blue even more. Scanning the sky from overhead to the horizon will show that the brilliant blue overhead fades to a lighter blue near the horizon. Let us delve into some sky science and see what’s going on.
Email newsletter signup
Visible light from the sun is a combination of all the colors in varying wavelengths that we can observe. When this light enters the atmosphere, the different wavelengths get scattered. The reduced air humidity and lower sun position that occur this time of year scatter more blue light and less red and green, allowing more blue to reach our eyes, so the sky appears to have a more brilliant blue than summer. The sciencey name for this type of light scattering is Rayleigh scattering.
When looking straight up, you observe only around eight miles of atmosphere, so the sky has a more pure, darker blue. As you scan towards the horizon, the sky will appear a paler blue because the light must pass through more and more atmosphere before reaching your eyes. This allows for a greater mixture of color wavelengths to be scattered, and so the blue is not as pure and appears lighter.
There is often a narrow band of white sky at the horizon. This is because here, you are literally looking at the bottom of the atmosphere, which contains a higher concentration of heavier molecules such as pollutants, water vapor, dust, or smoke. Here, the reflected light wavelengths are scattered equally in all directions, and so the sky appears white. The heavy water vapor in clouds is also why clouds appear to be white. Large particles’ scattering of light in all directions is called mie scattering.
So there you have it. As you get out and enjoy the tree colors of Fall, pay attention to the sky colors of fall as well.
Steve Roark is a volunteer at Cumberland Gap National Historical Park in Tennessee.