I didn't want to hijack the snow topic, so here's a rain topic with a quick test. Based on our findings in the snow topic, but not entirely. Rain is more subtle, so (I think) I didn't even use the tiny cloud size here. I did use an image map shader to get the shower effect, but it can also be done by painted shader on the Y. The landscape would need refining, if I pursue this.
---Dune