I'm always amazed when I see someone driving with fogged-up windows. Driving is dangerous enough when you can see, don't you think? I wonder if their heater works? Do they have several large dogs in the car? I'll never know.
This happens much more often when it's raining. When it rains, people roll up their windows (fair enough). The air inside the car becomes warmer than outside because there are warm bodies inside, and the already humid air gets even wetter. Foggy windows are the result.
How can we avoid this? Turn on your fan. It may seem like common sense, but the fact that I see people driving around in a fog seems to argue otherwise. Possibly these people simply aren't familiar with their vehicle's heating system. Perhaps they don't realize that you can turn on the fan without turning up the heat. Can it be that they don't even notice the problem?
I never have this problem. That's because I leave my fan running on low all the time. When it's raining, I make sure to shoot the air onto the windshield by turning on "defrost" mode. Keeping the air moving like this eliminates foggy windows. Simple, huh?
Oh, and you know that "recirculate" button/knob? Make sure that stays off. Recirculating the air will fog up your windows quicker than you can say, "Was that a red light?" You may not be familiar with this control as it's often labeled obscurely. Look for a two position switch: one position shows an arrow coming into the car and the other shows air moving around in circles inside the car. You want the position where air comes into the car. The other position is only useful if it's really cold outside and you want to warm up the air inside the car quickly. Just remember to switch it back when you start driving.
Okay, so I hear complaints from people who wear contacts: "Leaving the fan on all the time dries out my eyes!" Sorry, I haven't got an answer for that.