Black Clouds

Black clouds are not rare. On a moonless night, far enough away from any light domes, all the clouds are black. Clouds can only show colors when they’re illuminated by light, and since the Sun emits “white light” clouds are (mostly) also white.

But when the sky is totally overcast, or near sunrise or sunset, clouds can take on almost every color in the rainbow. See my Nov 11, 2014 post for a beautiful example.

Those “black” clouds in the photo might look black at first glance, but the strong background contrast misleads your perception. Look closely and you’ll see shades of gray and blue. The blue, of course, is scattered light from the sky. The gray is from a process called extinction — the gradual weakening of light as it passes through a scattering medium.

Clouds are great at scattering light because they’re composed of water droplets much larger than the wavelengths of visible light. They act like tiny spherical mirrors and scatter those wavelengths, so the colors mix to form “white” in the eye-brain system. Depending on the direction of sunlight, and the orientation and size of the cloud, the sunlight might not fully penetrate the cloud. The part facing the Sun would look white if the Sun were behind you, but the backs of the clouds (as seen in the photo) wouldn’t receive enough sunlight to scatter:

Exactly how far sunlight penetrates into a cloud depends on a parameter called optical depth. When you’re driving and encounter a fog bank, how far you can see into it is a function of optical depth. And there are many types of fog with differing densities. Fog is just a cloud at ground level. So for clouds of sufficient size, there’s always a limiting depth at which extinction leads to darkness. In that case, part of the cloud is in its own shadow.

For more on fog and visibility limits, see me April 29, 2013 post: The Meaning of Visibility.

It’s often said that “black” is the absence of color, and “white” is the presence of all colors. That’s true for pure light, and for pigments on a surface. The presence of colors in clouds, or lack thereof, is yet another example of this optical principle.

Next Week in Sky Lights ⇒ Moonrise, Moonset, and Phase

Anti-Sunsets
Q&A: Moonrise, Moonset, and Phase
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