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I was traveling in the day time from Saint-Petersburg to Sochi and was watching various clouds passing by. After some time I noticed that even though some clouds are very close to each other, they have very different brightness.

See the image below. Here there's a tall white (cumulonimbus?) cloud, intersecting a flat high-altitude (altostratus?) cloud. But in spite of the fact that the latter is much flatter, it appears to reflect much less intense light, and looks dark on the background of the large white cloud.

photo of the clouds

What is the reason for this difference in brightness? I'd expect both clouds to be of similar color, just the flat one maybe more translucent, but not darker.

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  • $\begingroup$ @TheDarkSide could you explain the change in the question body? I don't see how replacing one word with six, exactly preserving the meaning, is an improvement. $\endgroup$ – Ruslan Jul 29 '17 at 15:05
  • $\begingroup$ 1. The editing was for the tags, just noticed that wrong use of "despite", and edited in a correction. 2. If you think it is not an improvement, you can always roll back the edit. Cheers :) $\endgroup$ – 299792458 Jul 29 '17 at 15:09
  • $\begingroup$ @TheDarkSide could you point me to the reference explaining how that use of despite is wrong? (English is not my native language.) $\endgroup$ – Ruslan Jul 29 '17 at 15:18
  • $\begingroup$ Not mine either, but I'll look hard for a source & post if I find it, my naive logic is as follows: The sentence formation is different with "despite", versus "in spite of". e.g. your original was: "But despite the latter is much flatter, it appears to ...". If one had to use "despite" here, one would say "But despite being much flatter, it appears ...". If one doesn't want to alter the sentence formation much, i.e. preserve the fragment "latter is much flatter" as it is, it is more appropriate to use "in spite of", as I've edited in. But "despite the latter is much flatter" isn't fine. $\endgroup$ – 299792458 Jul 29 '17 at 15:27
  • $\begingroup$ Ruslan, I found something on another stack: Please look at ell.stackexchange.com/questions/506/…. Also, look at this one too: english.stackexchange.com/questions/5634/… $\endgroup$ – 299792458 Jul 29 '17 at 15:30
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Those flatter, thinner clouds are less opaque.

In general, shadows from the cloud itself or other clouds explain most of the variation in brightness. But in this case I believe you give the answer yourself when you mention that the flatter clouds should be more translucent:

  • they let more of the sunlight through, i.e., they reflect less light and are thus less bright, especially in contrast to the thicker clouds;
  • you see them from a shallow angle, with the sun behind you, so most of the light that does get reflected, might be so away from your eyes$^1$;
  • also, you're seeing them from above, and they might let you see more of the darker surface underneath.

$^1$Notice how the brightest parts of the clouds seem to be those facing you. Edit: As can be seen in the picture below, especially in the highlighted selection, the farther away you look from the line of sight to the sun, the less of its light is reflected toward your eyes.

source: https://cauldronsandcupcakes.com/2012/02/15/wanderlust-and-the-universal-ordering-system/ enter image description here source: https://cauldronsandcupcakes.com/2012/02/15/wanderlust-and-the-universal-ordering-system/

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  • $\begingroup$ The sun is not much behind me: notice the positions of shadows from cumulous clouds near the bottom of the image as well as the shadows on the wing's anti-shock bodies. $\endgroup$ – Ruslan Jul 29 '17 at 11:00
  • $\begingroup$ I did notice that. I meant that if you classify the sun as either behind or in front, it's behind: you can't see it without turning back. And the point here, as you can see with a ray simulation, ( scienceprimer.com/specular-diffuse-reflection ), is that in this configuration, very little of the light reflected diffusively by a horizontal cloud will be directed "back" at you. $\endgroup$ – stafusa Jul 29 '17 at 11:16
  • $\begingroup$ OK, I think this is convincing enough. $\endgroup$ – Ruslan Aug 11 '17 at 9:39
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I guess it is based on the size of water droplets.

If they are big, they will reflect bright white due to multiple reflections and refractions (think of soap foam); if the droplets are small enough then it will do light scattering insted (think of steam fog).

That's why there are only two shades of white, quite different.

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  • $\begingroup$ Why would result of scattering differ from "deferred scattering" after multiple refractions? The net light output should be the same if the droplets don't absorb light. $\endgroup$ – Ruslan Jul 25 '17 at 7:25
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I think you can find the answer to your question here (it is from the NOAA website, so I consider it reliable):

Sometimes, under direct sunlight, clouds will appear grey or dark grey against a blue sky or larger backdrop of white clouds. There are usually two reasons for this effect:

  1. The clouds may be semi-transparent which allows the background blue sky to be seen through the cloud. Thereby giving it a darker appearance.
  2. A more common reason is the contrast between the background (blue sky or additional clouds) and foreground cloud overwhelms our vision. In essence, our eyes are tricked with our perception of foreground clouds appearing dark relative to the overwhelming brightness of the background.

So, point 1 is basically saying that from those thinner clouds you have some white light from Mie scattering, but this is not much and it is mixed from blue-ish light from Rayleigh scattering coming from behind (notice that even if in the picture "behind" part of the gray cloud you see a white cloud, there is still air between the two, and that air scatters blue-ish light).

I have to say that I am not really convinced by point 2. I mean, it is clearly not some kind of optical illusion: the two kinds of clouds you show in the picture really have different colors. What is true, though, is that if you were to look at the thinner clouds from another place/angle you would probably perceive them as white.

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