Yes, and my approach does capture that to some extent. Or rather, Godot's ambient lighting approach does. The (ir?)radiance cube map(s?) cover all directions, including the ground. Right now I'm using a uniform gray from below — [albedo 0.39](https://en.wikipedia.org/wiki/Albedo_0.39), but I just learned that this value is outdated. For slightly more realism, I could colour this according to the terrain underneath. Future work!

Of course, there's a chicken-and-egg problem here that isn't captured: the very presence of the clouds also affects how much light is reflected back onto them. That would be a bit harder to achieve.