1. First create a polygon sphere
This will function as our rain drop and we will use it in the nparticle instancer. First thing we will do is setup an animationed "warble" on the droplet so that it isn't just a static sphere.
2. Now in the animation sub set menu goto create deformers>lattice
A lattice appears around our rain drop,
In your attributes on the right you can change it's resolution
3. Set it to 3 x 3 x 3
4. Right click the lattice and go to lattice point and taper the top a bit to make it a little more tear drop shaped.
5. Now we are going to turn the lattice deformer into a soft body. Select the lattice then goto dynamics menu set > soft/rigid bodies > create soft body settings box
6. Make sure "duplicate, make original soft" is selected and hide non-soft object is CHECKED and make non soft a goal is CHECKED. leave weight as it is.
7. Now make sure your lattice is selected and go to fields>turbulence
8. In the outliner select the turbulence node, then go to the channel editor on the far right set
Magnitude to 100
Frequency to 1000
Press play and you will see our water droplet wobble more like a blob of water should. you can play with these settings to get the desired effect.
OUR RAIN DROP IS READY
Now let's create our rain particle system.
9. Go to ndynamics menu set >nparticles>create nparticles> create emmiter.
10. Now go to the attributes for the emitter you created and select emitter1 node under basic emitter attributes choose Volume instead of Omni.
11. Now scale the volume out nice and wide and flat for your rain area. Set your play area to more like 150 and press play.
You should have something that looks more like
Now let's instance our shape out for all those particles.
12. Select our rain drop, hold shift and click on a particle. This should grab the particle NOT the emitter.
13. Then goto ndymamics menu set> nparticles>instancer replacement settings box.
You should see your nparticles and sphere listed in the next dialog box scroll down to a drop down menu that is called "particle object to instance"
14. It will probably have the lattice selected. CHANGE THIS to nparticleshape1 then click create
Now you should have a sea of rain drops
Now let's scale up our emitter volume to bring them to a better scale.
Should have something that looks a bit more like this.
Now at the moment every single drop is exactly the same let's add a little variation.
15. Click a particle to select them. Then in the attribute editor select the nparticleshape1 tab, expand the rotation drop down and choose compute rotation.
16. If you play nothing will have changed we have to make one more setting.
Expand the drop down Instancer (geometry replacement)
Then under rotation options choose Rotation >rotationPP
Aim Direction set to Acceleration
This will give the drops some variation in their rotations.
DROPS ARE DONE!
Now that we have some slowish falling rain drops. let's place a vray material on them
17. Select a particle to select nparticle one, then in attribute editor go to attributes menu>vray>per particle export.
18. At the bottom of atrtributes for nParticleShape1 should be a new extra Vray attributes drop down. expand that and turn on RGB.
19. Now select your original sphere shape that you are using to instance the rain drops and right click assign new material. Choose a vray Mtl
20. In your vray materials attributes editor. under Basic Parameters >Diffuse color to Black
under reflection >reflection color to white
Refraction > Refraction color to White
Refraction>Refraction IOR to 1.3
If you try to render everything may be black this is becuase it's water and there is nothing to
reflection. Add a vray dome light.
Now you should have a million tiny jewels rain drops each slightly different from the rest slowly falling if you need to get slower use the timewarp feature of maya.