Both.
My answer goes directly to the question formulated on the title, and only to the question formulated on the title.
Sorry to put it this way, but this kind of questions is shameful and somewhat shows how little sim racing evolved as a game and genre in favor of put all the effort onto glofiried physics that, with some investigations and using wayback machine with PRC, you conclude that these fail as well.
Something like wet weather should be standard for every sim-racing product since Grand Prix 3 brung it in 1998.
This, and an option to search in multiplayer for races according to a predefined weather game setting would do the job.
A little example: [All, Changing, Dry, Overcast, Wet, Monsoon]