How to Defog a Windshieldby Chris Moore
What is windshield fog, exactly? Water on Earth is in constant flux between states, and it takes only a tiny change in temperature or air pressure to affect that balance. Fog is what happens when warm water vapor in the car's interior air comes into contact with a cold windshield. The temperature change causes the vapor's molecules to slow down and collect together into microscopic water droplets called "fog." There are two basic ways to defog a window: get rid of the temperature difference between the air and windshield, get rid of the water vapor in the air, or both.
Allow fresh air into the car by cracking the windows open. The circulation of outer air alone will often be enough to defog the windshield without the heater-air conditioner. This might not be possible sometimes due to cold or rain, but installing vent visors on the windows can reduce splash from rain.
Turn the heater-air conditioner on to the maximum fan setting, set it to the "defroster/defogger" setting at a warm temperature. The warm air from the vents will locally heat the windshield and vaporize the water collected on it. However, the effect is temporary since all you've done is send the water vapor back into the air. As long as the windshield is cold and the air next to it is warm and humid, the fog will come back.
Reduce the temperature of the vents' air gradually. Get the temperature of the vents' air very close to that of the air outside the car so the air on either side of the windshield is the same temperature.
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