I was thinking that the smoke Sim could be used at slow speeds to Sim clouds and cloud systems. video as soon as it finishes rendering.
["]](http://s405.photobucket.com/albums/pp131/Art_by_Amar/Blender%20Art/?action=view¤t=clouds2.jpg)
["]](http://s405.photobucket.com/albums/pp131/Art_by_Amar/Blender%20Art/?action=view¤t=clouds2.jpg)