Learn how to create a rainy scene in this videuotutorial by TheBlenderFanatic.
Get a "rainy" scene is not as complicated as it seems. Yout just have to pay attention to the details that usually indicate it is raining: wet surfaces, gray colors, a little blurry atmosphere and water, of course. However, using a fluid simulation takes too much time ...so here it is another way.
this method seems a little too high poly for my taste. usually I will use vertex colors to define the wetness, and define the transition using height biased blending between a height map and procedural noise texture. that way it is not dependent on high resolution geometry and able to be used on low polygon models in a believable way. you could also just use a texture mask for this purpose.
Cool tut. May come to be useful in the near future. Thanks for sharing!
My only suggestions with this tutorials and all other people when making tutorials; show final results at the very beginning (and the end) so people know what exactly they're trying to achieve, the end goal.
Blend-Dah! MAKE IT RAIN!!