[Normally a little rain like this wouldn't be enough to make him sick so quickly, but thankfully(?) the world is giving him a hand with that.
So Arthur is stuck outside in the rain, looking utterly and completely miserable. Stuffy nose, bad cough, dizziness and lethargy... And, naturally, he's also a dumbass who's pretending this isn't the case.
If you don't find him outside, he'll eventually stumble his way down into the tunnels in search of something to drink because his throat is killing him. So... random sick idiot wandering around, too stubborn to realize he should be lying down.]