Hi,
this is a very simple setup which - I think - came out quite nicely.
One localized Cloud v2, a SSS and the Density Shader.
Render Settings for this:
1920x1080 / MPD=0.5 / AA=6 / Soft Clip Effect=0.8 (Compensation=OFF) / Contrast=0.35 / Gamma Correction=2
Pixel Reconstruction Filter=Cubic B-Spline(soft)
I blurred the top end of the cloud just a little bit in PS because the grain did not go away within reasonable render time.
C&C welcome.
CHeers, Klaus