As I was driving down the road one day in my little car, I had an instance of my windshield being suddenly very foggy. Feeling a little panicked, I tried every setting on my heater/defroster controls. In desperation, I even tried turning on the air conditioner. And you know what? It worked! Within just a few seconds, the air conditioner began clearing the fog from my windshield.

Sometime later, I happened to mention my discovery to a friend. She said that tip would really help her, and probably lots of other drivers. Recently she saw some information that backed up my theory. The source said the air conditioner has a dehumidifier. I thought it removed the fog because it was equalizing the inside and outside temperature of the windshield. However it did it, I was grateful.
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Remember to set your knob toward defrost and put the blower on the highest setting when you defrost with the air conditioner. Another source I found online said that if you have no air conditioner you should set your air to “outside air” rather than “re-circulate”, set the blower on the highest setting, and open the windows slightly. If you should be stuck with a foggy windshield any time soon, I hope this helps.