What to Use to Defog Car Window Glass?

[FinnMarie]

To defog car windows, use the cold air setting, press the defog button, turn on the air conditioning (A/C), set the temperature to the lowest, and activate the external air circulation. Both cold and warm air can defog car windows, but warm air is slower than cold air. The most effective method is cold air, which blows dry air directly onto the glass, preventing water vapor from condensing and achieving defogging. The warm air method is similar to cold air, but avoid using warm air on rainy days as it may initially worsen fogging. Always use external air circulation when defogging car windows. The reason for car window fogging: There is a temperature difference between the inside and outside of the car. If the temperatures differ, the side with the lower temperature has a lower saturated vapor pressure, causing water vapor to accumulate on the glass surface, forming tiny droplets and creating fog. Methods to defog car windows: Air Conditioning Cooling: Use the air conditioning's dehumidification function to reduce air humidity and remove fog. In summer, especially when multiple people enter the car without turning on the AC, the exhaled humid air quickly causes the windshield to fog up. Turn on the AC to blow cold air onto the windshield, using the dehumidification function to remove fog quickly. If humidity is too high and the AC is ineffective, slightly open a window to rapidly reduce cabin humidity. Combining both methods yields faster results. Air Conditioning Heating: Reduce fogging by minimizing the temperature difference. In winter, blow warm air onto the windshield to quickly raise its temperature and reduce the temperature difference between the inner and outer surfaces, preventing heavy fogging. Note that the rear and side windows heat up more slowly, requiring more time to fully defog. Whether using automatic or manual AC, turning it on aids dehumidification. Avoid setting the AC to the coldest temperature, as prolonged use may cause fogging on the outer windshield. Applying Anti-Fog Spray: Spray a small amount of defogger onto the car windows and wipe it clean to remove dirt and stains. This leaves a transparent protective film that prevents water vapor from condensing into fog, ideal for cold winters. Alternatives like dish soap, soapy water, glycerin, alcohol, or saline can be used—let them dry before wiping with a chamois or soft cloth to remove excess residue, keeping windows fog-free for days. Opening Windows for Airflow: If the fog is light, open both side windows to create airflow between the inside and outside of the car, reducing the temperature difference and gradually eliminating fog. This method works similarly to using cold air from the AC. However, it is unsuitable during rain, snow, high-speed driving, or heavy fog. Rear Window Heating: Most cars have a rear windshield heating function, with varying button locations but the same purpose. Turn on the heating switch, and the rear window fog will disappear in about a minute.