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Title explains it all.

Assuming an earth-like atmosphere and cloud composition, how thick would the clouds have to be to completely blot out the sun, causing the surface to be pitch black?

How this cloud canopy arose, whether by magic or by some form of climate change, doesn't matter.

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  • $\begingroup$ Why the magic tag? $\endgroup$
    – L.Dutch
    May 12, 2018 at 15:01

2 Answers 2

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Very thick. The optical properties of clouds are discussed in depth in http://www.patarnott.com/satsens/pdf/opticalPropertiesCloudsReview.pdf

A key point of this is the fact that the ratio of absorbsion to extinction of light is very small (in the visible range) This means that even though a beam of light will be broken up by a cloud, it will won't get absorbed for much further. A fairly thin layer of cloud will obscure the disk of the sun long before it will cause darkness. Even under a massive cumulonimbus cloud, there is enough scattered light to see by.

Our eyes are good at adapting to a wide range of light levels so even if the light is attenuated by a factor of 100 it still remains doesn't get "pitch black" And under Venus's clouds, there was still enough light for cameras to capture images. It is dark on Venus, but not pitch black source.

Much depends on the details. Absorption in a cloud can be greatly changed by low levels of particles and aerosols. To get pitch black under clouds you would need very deep clouds (in the 10s of km) or change the composition so that the droplets that make the clouds absorb more light (in the terminology of the linked paper, you can vary the composition to add particulates that will increase the ratio $\sigma_{abs}/\sigma_{ext}$.)

While clouds are effective at scattering but not absorbing visible light, the same is not true of Infrared light. Water is a good absorber of Infrared, and this is why clouds can act like a "blanket", keeping temperatures high during cloudy nights.

Venus has a cloud layer of about 25km, and a further 25km of haze. We shall want to exceed that. For the sake of maintaining realism, a dense cloud of 50km thickness could plausibly render the surface pitch black.

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You can safely bet on something like the cumulonimbus

Towering cumulonimbus clouds are typically accompanied by smaller cumulus clouds. The cumulonimbus base may extend several miles across and occupy low to middle altitudes - formed at altitude from approximately 200 to 4,000 m (700 to 10,000 ft). Peaks typically reach to as much as 12,000 m (39,000 ft), with extreme instances as high as 21,000 m (69,000 ft) or more.

If you have been caught in a storm below them, you have surely experienced the darkness they bear.

cumulonimbus storm

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