Why Does the Inside of My Car Fog Up?
Fogged car windows are not only an inconvenience, they can be dangerous. When the fog is on the outside it is easy to deal with, but on the inside it can be difficult to clear the view.
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Water in the Air
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Air almost always has some moisture in it. How much water the air can hold is largely determined by the temperature of the air. Warm air can hold more moisture than cold air. The air inside a car, especially a closed car, is almost always warmer than the air outside the car. However, the air right next to the inside surface of the window is colder than the rest of the interior air, so some water must come out of the air and onto the window. The "fog" on the window is the water that condensed out of the warm air onto the cold glass.
People are the Problem
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The humidity, and heat, from the passengers also contributes to window fogging. The solution to the fogging problem is to equalize the temperature, and humidity, on the inside and outside of the window. That is why lowering the car door windows an inch can get rid of the fog--it equalizes the temperature and humidity inside and outside of the car.
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Clean Windows
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A clean windshield is less likely to fog. When the moisture starts to come out of the air--because of the temperature difference--it condenses on a dirty windshield more easily than it would on a clean windshield. Keeping your windshield clean reduces fogging.
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