Press the defog button, turn on the air conditioning cooling (A/C) switch, set the temperature to the lowest, and use the external circulation to defog. Both cold and warm air can defog car windows, but warm air is slower than cold air. The most effective method is using cold air, which blows dry cold air directly onto the glass, preventing water vapor from condensing on the glass and achieving defogging. The method for defogging with warm air is the same as with cold air, but note that warm air should not be used for defogging on rainy days as it may initially increase fogging. Always use external circulation when car windows fog up. The reason for car window fogging: There is a certain temperature difference between the inside and outside of the car. If the temperatures inside and outside are different, the surface with the lower temperature has a lower saturated vapor pressure than the surrounding environment, causing water vapor to gather on the glass surface in tiny droplets, forming fog. Methods to defog car windows: Air Conditioning Cooling: Use the air conditioning's cooling and dehumidifying function to reduce air humidity and remove fog. In summer, especially when multiple people enter the car and the air conditioning is not turned on promptly, the exhaled air has high humidity, quickly causing the windshield to fog up. At this point, turn on the air conditioning to blow cold air onto the windshield, using the dehumidifying function to remove fog quickly. If the humidity is too high and the air conditioning is not effective enough, slightly open a window to rapidly reduce the humidity inside the car. Combining this with the air conditioning can speed up the process. Air Conditioning Warm Air: Use the method of reducing the temperature difference to remove fog. In winter, blowing warm air onto the glass quickly raises the temperature of the windshield, reducing the temperature difference between the inside and outside surfaces of the glass, thus preventing excessive fogging. However, note that the rear and side windows heat up more slowly, so it takes longer to remove all the fog. Whether the air conditioning is automatic or manual, turning it on helps with dehumidification. Do not set the air conditioning temperature to the coldest position, as this can cause the outer surface of the windshield to fog up over time. Applying Anti-Fog Agents for Prevention: Spray a small amount of defogging agent on the car windows and wipe it clean to remove dirt and stains. While polishing the glass, a thin transparent protective film forms, effectively preventing water vapor from condensing into fog, especially suitable for cold winters. Alternatives to anti-fog agents include dish soap, soapy water, glycerin, alcohol, or saline solution. After applying, let it dry and then wipe off excess residue with a chamois or soft dry cloth, ensuring the windows remain fog-free for several days. Opening Windows for Airflow: If the fog inside the car is not heavy, open both side windows to create airflow between the inside and outside, reducing the temperature difference and gradually dissipating the fog. The principle of this method is the same as using cold air from the air conditioning. The downside of opening windows is obvious—it cannot be used in rain, snow, on highways, or when the fog is heavy. Rear Windshield Heating: Most cars have a rear windshield electric heating function, with the button location varying but the function being the same. Turn on the windshield heating switch, and the fog on the rear windshield will disappear in about a minute.
