We propose a new GPU based technique to render realistic rain in real-time. It consists of two parts: off-line image analysis of rain videos, and real-time particle-based synthesis of rain. Videos of real rain are analyzed to extract the rain mattes; random samples of these rain stroke mattes are then used for online synthesis. We incorporate pre-computed radiance transfer (PRT) in our particle system to take into account the scene radiance. The transfer function of a raindrop can be highly efficiently evaluated with a closed-form solution. Our approach achieves high realism with low computation cost and a small memory footprint. Our technique applies to a variety of scenarios, from synthetic 3D scenes to real videos.