Im not an expert by any means at this but I think one thing u could do to create the whisps of fog is perhaps to create a directional emitter. You could then assign a Newton field to the emitter and then parent the Newton field to a sphere. By animating the sphere, you can in effect direct the whisps of fog. and the Newton field it adds the whispy like effect. Obviously you can hide the sphere when u render. When I have some time i'll see if I can knock up an example for you