Why are nearby clouds so different in brightness? 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.

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.
 A: 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/
A: 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.
A: 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:
  
  
*
  
*The clouds may be semi-transparent which allows the background blue sky to be seen through the cloud. Thereby giving it a darker
  appearance.
  
*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.
