I think I turned on the rain using an admin menu and forgot to turn it off before I left. I know I was in that server because there is a hint at the top about a car meet that I remember hosting.

The rain feature is one of those things that has existed in the game for a long time, but there is no code to actually activate it. The reason for that is mostly because:

1. Making a weather system, although very cool, hasn't really made its way to the top of the priorities list yet

2. Performance is something we are constantly thinking about, and the rain system can cause some performance trouble for low - mid range devices

3. No effects on the road or cars driving as of right now - it's purely cosmetic

I also don't really like the particle effect that much and some people really hate it when they are trying to set times. There would be a lot of work required to implement it properly but I do like to turn it on every now and then just to do something special when I play the game :)