Website & download: Ifu - cloud simulator/renderer (Linux/Windows 64-bit)
Changelog/release notes: Changelog.txt
Video tutorials: Ifu Silent Basics - YouTube
A sample cloud for quick test rendering is included in the download.

The program is in beta, has been tested more thoroughly on Linux than on Windows, and may have a number of bugs etc. Always see the changelog, first. Rendering, clouds tend to be best viewed so that the sun is somewhat behind the camera.
The simulator runs a Navier-Stokes fluid solver and does a bunch of other stuff. Not 100% realistic, but decent. The renderer is a normal path tracer, the clouds being rendered with a rough approximation of Mie scattering (non-spectral). There's currently no in situ tonemapping; renderings can be exported into PFM.
Both the simulator and renderer are CPU-only, and heavy on the computations. They run fairly well on a Xeon E3-1230 v3, but not too quickly on an Athlon 64 X2 6000+. The simulator doesn't benefit much from multicore - memory access bottleneck. The renderer should scale up reasonably well with many cores, though odd things may happen if you have a few hundred.