I know a trick (of my own) for those kind of FX. I use the same technic than photograph by adding filters in front of my camera.. To do that, I paint a rain effect (photoshop) with little white lines on a black layer. Once done I import the rain pic in Bryce and apply it to a 2D plane. In the material editor, I choose the pic as transparent chanel (A) and set the diff and Amb to white. Then I put the 2D plane in front of the camera. Don't forget to add some soft grey haze to your scene (because rain looks like fog in a distante vue).. hope this will help. Cheers. Pidjy