Let's use what we've learnt so far and create a rain simulation. This isn't any more complex than the previous tutorial, but it is a more practical application, and it uses a couple of new techniques such as using the Hair module to render. You should definitely do the previous recipe before embarking on this one so you are familiar with the techniques involved.
Create a null object and rename it as Rain Emitter. Move the null up so the Y position value is 500cm.
Create three new null objects, and name them Gravity, Wind, and Up. Rotate the Gravity null by -90º on the p axis, and rotate the Up null by 90º on the p axis. We'll use these a little later to help control our simulation.
Create a Plane Object and make it nice and big, 3000cm on both Width and Height.
Create two particle groups by opening the Particle Settings dialog (Simulate | Thinking Particles | Thinking Particle Settings), and right-clicking...