I was doodling in After Effects and came up with a pritty good Rain system. 100% Particular. With a little Optical Flares to make it pretty. I posted this littel guide quite a while ago on Vimeo where it recived nice response so i tought i would put it here aswell.

The setup is realy quite simple:

The scene i made up from 2 solid layers called “Ground” and “Particular”. Make the “Ground” layer 3D and rotate it flat to represent the groundplane and add Particular to the other.

##Particular settings:

For the emitter type select “Box”. Scale it so it fits the area you want to “Rain” on, and position in out of frame where you won’t see the drops spawning. The drops motion is a mix between velocity and gravity (Play around to find the sweet spot). also i turn off direction spread, otherwise you rain will look more like snow.

Under Particles/Sec set the number to 100-1500 depending on the amount of rain you want. Under the “Physics” tab choose “Bounce” and under the “Bouce” tab specify the Floor layer (Your 3d solid). Also set Collision Event to “Kill” so your rain drops stop when hitting the ground.

##Under Aux System:

Set Emit to “At Bouce Event” with a probability of 100%. Particle/Collision: 10 (This can be varied to find a sweetspot between realism and rendertime) Set velocity to around 150 (May vary depending on your scene).You’r “Opacity over life” setting should slope down from “full” to “none” to make the splashes fade away naturally. Set “Color from Main” to 100%. Lastly turn on motion blur.

That is it, now it is just a matter of playing around with the numbers a bit to find an overall sweet spot. Side note: This is a pritty good solution but there may be a much simpler way to get a similar effect. Try looking at the filters called CC Rain and CC Drizzle. These are much more render time efficient and may do the trick for the majority of your shots.