It's been ages since I programmed anything (so I'll eventually be starting python from square 1), but if I remember my quick pass through the back of the book, doesn't Vue Pro expose that functionality? I would think you could call the float values being used each frame, build a function table to match the implied velocity, and use that t alter the precipitation angle (are we talking particles of some sort, or an animated cylinder plane in front of the camera? IQMWTK!). I've been playing with forester's rain objects, and the most successful set up I've tested so far had the .obj doing a loop about the camera, with the upswing far enough back that there was no particle in the view fustrum. Haven't worked out the correct settings and linkages yet, and I don't think it would do it by itself, but as the close in part of a multistage animated effect, it could be rather impressive...