I think the sound of the wind when its raining is important.
Audio is probably more important for "English rain".
It's not uncommon to look out of the window and not be certain if it's raining or not unless you can see whether there are drops falling into puddles. That whole "rain streaking" effect just doesn't work for drizzle...
Which is probably why I was thinking of Morrowind. The rain effect is a bit naff, but the audio was great. Though I was playing with a mod. One of these, can't remember which:
With Autumn coming up, I'm sure there'll be lots of opportunites to capture some decent audio to work from...
For drizzle you tend to get a really slow dripping of water from places that it gathers, like the end or a leaf or window ledge. I think that sort of 'drip...... drip....' sound would go well with the atmosphere of the game.