It's probably always there because it's always raining - in about 90 minutes of play I had maybe 5 minutes of no rain. But even if it is intentional for it to be foggy when it rains I still think it doesn't look good and would really love the option to disable it :)
My problem is that it doesn't feel like normal fog, it feels like my view distance has been reduced to the minimum which makes the game look washed out and bland - I want the old happy colors back! Also, it also affects the inside of the house which doesn't feel right.