Car Climatology

So this is my question of life for today: why do you need an advanced degree in climatology to clear a windshield? SC has this annoying quality where it makes windshields foggy no matter what time of the year you happen to be in. I have tried cold air, hot air, warm air, no air, air from outside, air conditioned air, and just when I think I've got it right the next day I do the same thing and it doesn't clear it! This morning at one point I actually had to hit the brakes because I literally could see nothing because the windshield was getting worse rather than better and the sun was making it so I could seen NOTHING!

So this is my request - someone who knows how to make windshields clear every time, please, please tell me how - oh and I'm going to need to know why that method works because I have issues remembering things if I don't know why they work (I know, I know, it's annoying, but it's just how my brain works - I've accepted it and moved on).