The clouds are made by the same OpenVDB heterogeneous grid instanced multiple times at different scales and rotations. The air is an homogeneous medium with atmospheric scattering. The trees and grass are instances as well.
Hi Nicola. I’m playing with the demo right now and I was hoping you could tell me what attribute settings you used for your homogeneous medium. I’d like to be able to create god-rays 
Thanks!
Hi Kwayde,
The settings for the homogeneus medium are pretty much the default ones, I just adjusted the density scale. To get the god rays, make sure you have some clouds that cast shadows above the homogeneus medium, and scale up the intensity of the sun.