system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do some clouds appear darker than others? Does it have to do with density and sunlight?
gpt
It's the different height of the cloud itself.  

The darkness is how much shadow the top of the cloud is casting on the bottom of the cloud.  

E.g. A dark storm cloud is a very tall anvil cloud casting a huge shadow on its own bottom layers.