Tutorial: Add Rain to ANY Scene in Blender
Learn how to use the compositor to simulate a rain effect.
Ethan writes:
Hey Everyone. In this tutorial, you guys will learn how to add fake rain in Blender’s compositor. This way, you can instantly add rain to ANY image that you have created, or any other image that you want to change. It’s very easy to do and provides quick results which can instantly change the mood of any scene.
I hope you enjoy.

Simple n effective … i tried using the particle system for this and it was a nightmare