We have a "wet season" (well we used to have). During the month or so before the atmosphere gets very humid and hot (~30C).
I also keep the butter in a porcelain dish in the cupboard and it gets pretty soft but I put up with it.
BUT it grows a little black "mold" during this month. It generally grows on the contact between the dish & butter. Its really bad if I use unsalted butter.
I put in in the fridge overnight and its enough to keep it at bay.