The rain is coming down cats and dogs in DC today, so two pieces come to mind:
The “Raindrop” Prelude of Chopin (Op. 28, #15) played by Martha Argerich (as the comments note, a force of nature).
and
The third movement of Brahms’ First Violin Sonata, based on his rain motif, a gentle romantic patter, here played by Itzhak Perlman and Vladimir Ashkenazy (who is successful in keeping Itzahk from over egging the cream).
A Few Reasonable Words 