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I saw these clouds on the horizon, behind a ridge (apologies I couldn't get more pixels):

enter image description here

Why is the front cloud darker than the cloud behind? There were no other clouds that I saw which could've been casting a shadow on the front cloud. What would cause a cloud to reflect less light?

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A possible reason the cloud reflects less light is that it has a lower density of microscopic water droplets in it as it has more air spaces in between that cloud. Notice how water droplets have almost no preference of scattering so it scatters almost all wavelengths of light so it appears that they reflect all light wavelengths, hence the ordinary white color of clouds.

However, Air scatters light near the blue end of the spectrum and hence more air spaces mean more blue light scattering so the cloud appears more blue in between. Hence the lack of water vapor (which is mainly responsible for all wavelengths of light instead to scatter giving a white-like image), that cloud appears bluish.

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  • $\begingroup$ But given that the far cloud appears more white despite having more air between me and it than the near cloud, would the amount of scattering taking place between individual droplets really be big enough to have such a dramatic effect? $\endgroup$
    – gandalf3
    Commented May 25, 2018 at 6:47
  • $\begingroup$ It’s not about the air between you and the cloud but the density of air and water inside the clouds. It’s like can you say that an iron block situated further away is less dense than a feather block situated closer? As light from the sun interacts with the atoms arrangement in the clouds and then comes to your eyes. The difference in distance between them is outweighed by the difference in air and water density of the clouds. $\endgroup$
    – Tausif Hossain
    Commented May 25, 2018 at 6:51
  • $\begingroup$ Yes, but it seems light must travel through a lot of air for the effects of scattering to be very significant. I'm not convinced the light paths inside the cloud would be long enough to compare with, say, the distance between me and the far cloud. $\endgroup$
    – gandalf3
    Commented May 25, 2018 at 6:58
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    $\begingroup$ Consider it like this that water droplets are like a barrier to an originally blue sky caused by air scattering. As light travels through both the blue-ish and white cloud in the white cloud it scatters a lot and all wavelengths so it’s white. Now in the blue cloud there is much lower concentration of water droplets so the image appears more like its background which is the original blue scattered light of the sky. You can notice the color is almost like a certain patch of the sky in the photo. $\endgroup$
    – Tausif Hossain
    Commented May 25, 2018 at 7:05
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    $\begingroup$ I see, that makes sense. Thanks! Although, it is dense enough to be opaque to the cloud behind it. I guess this means it extends a good distance back, away from the camera, while still being low density? I know I've seen some thin clouds and mists which, while white-looking, were thin enough for things behind them to be discerned. $\endgroup$
    – gandalf3
    Commented May 25, 2018 at 7:35
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the cloud is "blue" because the microscopic droplets that make the cloud are bigger. Colors of clouds are determined by size of droplets, which governs the wavelengths of light they reflect. I'm sure others can post answers in more detail.

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  • $\begingroup$ Are you sure about this? To my eye, the difference between clouds (in general) seems to be one of darkness, not colour. $\endgroup$
    – David Richerby
    Commented May 25, 2018 at 13:15

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