What to Do When the Car Windows Fog Up After Turning on the Heater in Winter?
2 Answers
Press the defogging button, turn on the air conditioning (A/C) switch, set the temperature to the lowest, and switch to external circulation to defog. Both cold and warm air can be used to 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 window and achieving defogging. The method for using warm air is the same as cold air, but note that warm air should not be used on rainy days as it can initially worsen the fog. Always use external circulation when defogging car windows. The reason car windows fog up: There is a temperature difference between the inside and outside of the car. If the temperatures differ, the side with the lower temperature will have a lower saturated vapor pressure on its surface compared to the surrounding environment, causing water vapor to gather on the glass surface and form tiny droplets, resulting in 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 without turning on the air conditioning promptly, the exhaled humid air can quickly cause the windshield to fog up. Turning on the air conditioning to blow cold air onto the windshield and using the dehumidifying function can quickly remove the fog. If humidity is too high and the air conditioning is ineffective, slightly opening the windows can rapidly reduce cabin humidity. Combining this with the air conditioning speeds up the process. Air Conditioning Heating: Use warm air to reduce the temperature difference and remove fog. In winter, blowing warm air onto the glass quickly raises its temperature, reducing the temperature difference between the inner and outer surfaces and preventing heavy fogging. However, the rear and side windows heat up slower, so it takes longer to fully defog them. Whether using automatic or manual air conditioning, turning it on aids dehumidification. Avoid setting the temperature to the coldest to prevent fogging on the outer windshield over time. Applying Anti-Fog Spray or Other Preventative Measures: Spray a small amount of defogging agent on the car windows and wipe it clean to remove dirt and stains. This leaves a thin, transparent protective film that effectively prevents water vapor from condensing into fog, especially useful in cold winters. Alternatives to anti-fog spray include dish soap, soapy water, glycerin, alcohol, or saline solution. After applying and letting it dry, wipe off excess residue with a chamois or soft cloth to keep the windows fog-free for days. Opening Windows for Airflow: If the fog is light, opening the side windows allows air to flow between the inside and outside, reducing the temperature difference and gradually clearing the fog. This method works similarly to using cold air from the air conditioning. However, it is unsuitable during rain, snow, on highways, or when fog is heavy. Rear Windshield Heating: Most cars have a rear windshield heating function, with varying button placements but the same purpose. Turning on the heating switch clears the rear windshield fog in about a minute.
I've been driving for thirty years, and fogging up the windshield after turning on the heater in winter is quite common, mainly due to the temperature difference between the inside and outside of the car, causing moisture to condense on the glass. To deal with it, I first press the A/C button to activate the dehumidification mode, as it helps remove moisture from the car; if that doesn’t work, I slightly lower the window to let fresh air circulate. I also make it a habit to keep an anti-fog cloth in the car—when fog appears, I wipe it a few times, but remember to pull over safely before doing so. Safety always comes first. Regularly cleaning the glass and keeping the interior dry, like not leaving wet umbrellas inside, can significantly reduce fogging. For long-term prevention, buying an anti-fog spray and applying it to the glass works even better. In short, don’t panic—try a few methods, and you’ll find the one that works best for your car.
