I'm saying that *in its current state* GI may be problematic for animations, particularly on clouds. That doesn't mean it will always be that way - there's lots of room for improvement.
I don't know for sure why it's an issue with clouds and not terrain but I would guess the light transmission and volumetric nature of the material has a lot to do with it. It's a much more complex interaction than light on an opaque surface. Perhaps it is subject to more variability, particularly where sampling is involved. It may be that upping your cloud samples or GI samples would help, but this would of course increase render time. We do plan to support GI baking though, so you may just have to wait for that.
The Enviro Light can be used without GI and it should still have an effect. You may then be able to still control the strength on surfaces and in the atmosphere separately. If so this might give you a more stable lighting solution. Fill lights would affect both terrain and sky, yes, but then so does GI. Do you have the Enviro Light affect turned down for terrain or something?
- Oshyan