Just quick composition of my cloud setup. Its a hybrid between Sidefx article about Rio and CMIvfx fractal video. I use simple basic sphere mesh to form basic shape. Then I convert to volume and apply two layers of metaballs with stamp and scatter. Then convert meta and again convert to volume. Then I use great setup from CMIvfx. Add double noise at rendertime. There is result. Background is wallpaper and lencflaire some picture (hope Sidefx add simple COP lencflaire). Comp in Houdini.