Currently, I have worked out the interface for this project. I used a dark background color and a light color for the rain. The rain is generated by a particle system in Unity. The physics system allows me to simulate more realistic rain by creating splash effects using the die out animation for each particle, as well as bypassing any object in the interface.
The other feature of the interaction is the ability to respond to the sound of the surrounding environment. The microphone could capture the sound level of the environment and pass on the parameter to the particle system. The rain would be heavier when the surrounding environment is noisy, and vise versa.
I still need the technical solution to connect Microsoft Kinect to my interaction in order to make it interactive when people standing in front of it.