I've been lucky enough to help out the Sundog Software guys beta test the next version of their SkyMAXX v2 for X-Plane. It uses X-plane's own live weather from the net and renders it beautifully - multiple cloud layers, overcast layers, high stratus, towering cumulus and storms, reflections, shadows, lens flare and god rays. Winds aloft data is usually set in X-Plane by the FMC plugins themselves for instance in the 757 I believe, the only thing that Active Sky NEXT does above and beyond this is historical weather I think.