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Close up, rain can be modeled as transparent balls of water with appropriate motion blur. This seems impractical for large volumes, which would be necessary for scenes of rain in the distance.

At distances for which the human eye cannot resolve individual raindrops, how can the effect of rain-filled atmosphere be modeled? This does not need to be efficient enough for real time use, but it does need to allow multiple frames to be produced offline for animation without jarring discontinuities or flickering.

What approaches are available for this?

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When you think about rain in the distance (generally), you will have rain at several different depths from the camera, some close up, some very far away, all of which will look slightly different as you won't be able to focus on all of them. But the effect of them all layering on top of each other as they go off into the distance is what helps give the look you're aiming for, you need to recreate that depth and layering to really give the look of rain further in the distance.

Particle emitters are the general method of creating rain, as usually they use very small meshes, or even just a texture sprite of a drop. This keeps them pretty efficient - to a degree as the more particles you use and have on-screen at a time will change how heavy or resource dependant the effect becomes.

Generally you would only use large detailed meshes for rain, when you have a close up of a puddle and wanted to animate the splash, or even a close up of the single drop for some reason.

This can be combined with distance fog to further increase the sense of depth in your scene by occluding the objects in the far distance.

Again you could use another particle emitter to create dark, heavy clouds that go off into the distance, as having clouds in the scene will help to sell the look.

A lot of this will depend on what software you're using, as well as its intended platform, as different software have different solutions but particles and fog are pretty commonplace.

tldr: Particle Effects & Distance Fog

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    $\begingroup$ Depth of field may help too. $\endgroup$ – John Calsbeek Aug 10 '15 at 15:31
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    $\begingroup$ As an addendum: Sebastien Lagarde did a series of blog posts that documented how they implemented dynamic rain in the game "Remember Me" $\endgroup$ – RichieSams Aug 10 '15 at 16:39

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