Not sure how this is set up, but banding can be caused by sample count being too low.
Edit: Also as an aside, I’d look at Eric Bruneton’s paper for multi-scattering, as that’s what the Atmospheric Fog is based on, and it gives nicer results than O’Neil’s method (i.e., the NVIDIA GPU Gems link, which is single scattered).
Also, regarding my previous posts - I didn’t realize that UE4 already had atmospheric scattering from the ground perspective (i.e., Atmospheric Fog), so that’s why those posts make zero sense. Sorry about that.