Creating Snow

I went to see Edward Scissorhands and they used a scrim and projected onto it. I'm not a lighting expert or anything so I can't give much detail. I think they had made a clip of falling snow or rain or something, then the scrim was right at the front of the stage and the actors behind it so they actually looked like they were in the snow, it wasn't just falling behind them on a backdrop. It was really effective. "I have two giraffes... the State requires me to learn the Harmonica..."