There are a few different ways to do rain, some more efficient than others. You could have individual droplets that spawn randomly, - which is extremely performance hungry - or animated sheets of rain - which are a pain in the *** to place. There's also the option of having very large groups of raindrops that fall, so there are less objects, but then there's the problem of having them go right through terrain.
Edit: and don't even try to use physics man, that's a nightmare.