I'm going to say a bum knee. When the moisture builds in the air, my knee starts to hurt. So, when it rains, he doesn't want to walk up the stairs.
Or he has a girlfriend on the 7th floor, and his wife cuts grass for a living. When it rains, his wife can't cut grass so the man has to go right upstairs to his wife. But when it sunny, his wife works until the sun goes down to make up for the rain filled days. That's when he get to stop on the 7th floor to see his girlfriend.
I like the second answer better.
