You have control over how fast rain moves, how it looks, and how water ripples react to impacts. The process is simple: Rain is saved as an animated alpha channel billboard that you paint onto a plane. Make the plane face the viewer and the rain looks great. View the plane from edge on and it looks like hell. The most important thing is that rain can be easily made loopable and that water ripples can be rendered as seamless.

Here's some rain with splatters at the bottom. Notice that it's looping without hiccups. The rain falls a little slow in this animation but you get the idea. Now, if there weren't splatters, you could tile this as many times as you like and it'll fall smoothly. This is a "rain plane". It's the texture that gets painted onto a plane in your 3D application.

You can also do a sleet-like appearance by rendering the rain as snow but with splatters. The flakes are a little small for this animation but I think it illustrates the idea. You can select snow flake ranges but the effect isn't quite believeable yet...just so you know.

Here's what you get if you take the time to align the rain planes with the animated water ripple bump map.

Some people like to use the ripple bump maps as alpha channels for shockwaves, and the rain for gun fire. There's probably some other cool stuff you can do with it.

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