Recently in a project I had a change the dwell into this subject .
Volumetric atmosphere is an easy thing to achieve ,but photoreal large scale skies its another.
Here's some of my findings
Terragen sky
Terragen sky is almost like a plugin for Terragen 4
it helps design a sky with ease, giving you realistic results out of the box.
The downside is that it still requires Terragen 4 and also its quite slow to render and not extremely stable for sequence rendering.
The second solution is to use Vray Sky and Sun with their cloud feature
It doesn't offer as much control as Terragen Sky, but its integrated inside any DCC that used V-ray
The other option is to use Unreal sky tool and the latest development with loading Vdb
Or like we did on every single show, all by hand and Houdini :)
e.g