Your car is a box of homemade weather, produced by your air intakes, your heater, your air conditioner and your passengers.
But sometimes, because of your passengers’ irrepressible habit of breathing, the car fills up with a lot of water vapor and some of it condenses onto the cold windshield, fogging it up. What do you do?
When it’s very humid in your car and the wind-shield fogs up with condensed moisture on the inside, turn on the air conditioner, no matter how cold it may be outside. (You can always turn on the heater, even while the AC is on.) Direct the conditioned air onto the windshield and it will clear up in a jiffy.
What happened was that the air conditioner took in the water vapor (along with the air that it’s mixed with) and cooled it down to a lower temperature at which the water would much rather be liquid. It condensed into liquid at the AC, where it was thrown away outside the car. There was then not enough water vapor in the car for the temperature, and the liquid on the windshield restored the balance by turning itself into vapor. Voilà! A dry windshield.
But what about the rear window? When it fogs up, there’s no way to blow air-conditioned air onto it; the cool air all comes out of duct work up front, where the driver needs it, and there is no blower for the rear window.
So what did those clever car designers do? They embedded heater wires in the rear glass. Instead of blowing dried-out, cool air on it, you just heat the glass. That raises the glass’s temperature above the point at which water prefers to be liquid, so it turns to vapor and the fog disappears.
Odd, isn’t it? To defog the windshield you cool the air, but to defog the rear window you heat the glass, and the end result is the same. Why don’t the car manuals ever explain this to you? How many people are driving around with fogged-up glass, not having the foggiest notion of what to do about it?
Now how about your bathroom mirror back home? After your shower, it’s fogged up worse than any windshield ever got in the steamiest jungle, and just when you want to shave or put on your makeup. I’ll bet you have neither an air conditioner in your bathroom nor heating wires embedded in the mirror, so you can’t use either of the car window tricks on it. But you probably have a hair dryer handy. Just sweep across the mirror with it, as if you were painting the glass with hot air.
The dryer’s air will heat the condensed water on the glass enough that it will prefer to be vapor rather than liquid and it will evaporate, just as it does on your electrically heated rear car window.