The heights of the clouds (from top to bottom) must be really high for these scales, and you may need more than one density fractal for diversity of clouds. Clouds are quite tricky to make well, so it's best if you use the search function and try to find 'global clouds', then read and perhaps study available tgd's and tgc's. The vortex shader needs to be put between density shader and cloud, and get a location, but you may know that.
I'm not much into clouds, but I hope this helps. If you're willing to spend some cash, try some NWDA cloud setups.
