Hrm, so it is. I always thought that, given it was set in the ARE, it was just a shader. But I can see the models for it, fx_Rain in the case of rain. Looks to be a simple fountain emitter covering a 6m x 6m area. Oddly, it's positioned at 4m under world zero, not sure what that is about.
I wonder how the placement/activation for it works? I can't see anything obvious. Hardcoded maybe? Scripted? You'd surely want to be able to toggle the effect off and on to account for indoor/under cover areas.