This is a short beginner tutorial written by a short beginner!
The tutorial is in the downloadable example (RainEffect.capx) which is usually found on the upper left of this page. The example capx is fully documented. Simply download the capx and try it out and then examine (and modify) the settings.
There's only 1 small sprite needed for this effect to work: a raindrop sprite.
Things you should experiment with:
Select the splash particle object and adjust it's settings on the left side of the screen. Test and adjust some more.
In the actual code (in the event sheet tab) you'll find several settings that look like 'random(a,b)'. The first number is the lower range and the second is the higher range. Example : random(-3,3) would pick a random number between -3 and 3. I use this in my example to give a slight but very noticable variation in the angle that the raindrop falls - which also looks much more realistic than if all particles fell at the same angle!
Ok, thanks for reading and I hope I helped someone!
Here is a sciencey link for those that are interested :
Raindrops by the Weather Guys.