Using the Water preset brushes you can quickly animate rain and snow in your scenes. These brushes have flow animation settings.

To animate rain

1. Add a light to the scene.
2. In the Visor, select the rain preset brush (in the weather folder).
3. Paint strokes on the grid plane of the perspective view. Paint each stroke in the same direction so the rainfall comes from the same direction.
   
4. In the scene view click the Play Forward button to check that the animation is working the way you want it to.
5. Render the animation. See Render a scene with Paint Effects strokes.