As good as it sounds to have precipitation that behaves naturally, this would be horribly laggy if done like this. First of all, normally, precipitation is calculated client-side. Your game only worries about the particles falling wherever your camera is, and it's not synchronized with the server. All the client gets are instructions for how to generate them. With projectiles, you'd first have to worry about how to apply them everywhere. You'd need some way of keeping them from spawning uselessly all over uninhabited parts of the map, however, by only spawning them around players, you get a heavy increase in density of rain or snow when several players are together in one place. Then there's the issue of sending hundreds of updates about the individual projectiles per second per player. It's not really worth the resources to process something like this server-side. We have to wait for some better precipitation in the engine.
