The 3D viewport now has display options for adding uniform fog effects. You can visualize this fog using OpenGL with controls for density, opacity, height and depth range. You can also add a distant light to act as the sun and control how the sunlight interacts with the fog. This fog can be rendered to disk with the OpenGL ROP.
This feature was under development for a future release but was backported into Houdini 18.0.499. Learn more – https://www.sidefx.com/community/houdini-18-backport/