Neon Clouds

This is a sunset sequence from last week, with images taken at about 3 minute intervals. I could see before it started that this would be one of classic Arizona sunsets. We’d gotten some rain, the clouds were dissipating, and the air was cleared of dust. What was striking to me me was the brilliant, almost neon-orange, linings that formed around some clouds.

I guess one could simply describe this as “clouds with red linings” since the mechanism is the same as for silver linings or gold linings, but the contrast was so strong “red linings” just didn’t do it justice. To see why that was, let’s take a closer look at just how a cloud lining (of any color) forms.

First, it’s important to note that you only see linings when the Sun is behind the cloud. We’ll use red light, as seen in the slideshow. Light of any wavelength will penetrate into a cloud only to a certain depth (PD), which depends on factors like cloud droplet size and volume density (droplets per cubic meter). When the cloud size is near PD, the edges of said cloud will transmit some of that light directly as well as by forward scattering. The center of said cloud is darker by comparison, as very little light has made it that far.

Next, the path of a photon through a cloud is really haphazard, with light at any depth being scattered in all directions. Photons that don’t hit a water droplet continue on till they do get scattered. Take a look at the graphic in my post about cloud edge effects (and scroll down) to see how that works. So the size of the cloud and its PD play a factor in the intensity of any lining.

Finally, and this was especially pronounced in the slideshow images, the darkness of the background and surrounding sky have a huge effect on the contrast of any linings present. This is just a matter of luck and geometry. In this case the Sun had set a few minutes before the first image, so the clear sky brightness was already dropping. Further, the cloud linings were superposed over the dark sides of more distant clouds. It was the “perfect storm” for neon linings.

Next Week in Sky Lights ⇒ Rime Ice vs. Hoar Frost

Afocal Astrophotography Experiment
Q&A: Rime Ice vs Hoar Frost