What to Do When the Car Windows Fog Up While Driving?

[DeSpencer]

Press the defog button, turn on the air conditioning (A/C) switch, set the temperature to the lowest, and activate the external circulation to defog. You can use either cold or warm air to remove fog from the car windows, but warm air is slower than cold air. The most effective method is to use cold air, which blows dry cold air directly onto the glass, preventing water vapor from condensing on the surface 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 may 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 accumulate on the glass in the form of tiny droplets, forming fog. Methods to defog car windows: Air Conditioning (Cold Air): Use the air conditioning's dehumidification function to reduce air humidity and remove fog. In summer, especially when multiple people enter the car, if the air conditioning is not turned on promptly, the exhaled humid air can quickly cause the windshield to fog up. In this case, turn on the air conditioning and blow cold air onto the windshield. The dehumidification function will quickly remove the fog. If the humidity is too high and the air conditioning is ineffective, slightly open the windows to rapidly reduce the humidity inside the cabin. Combining this with the air conditioning can speed up the process. Air Conditioning (Warm Air): Use warm air to reduce the temperature difference and remove fog. In winter, blowing warm air onto the glass quickly raises the windshield's temperature, reducing the temperature difference between the inner and outer surfaces and preventing heavy fogging. However, note that the rear and side windows heat up more slowly, so it takes longer to fully remove the fog. Whether using automatic or manual air conditioning, turning it on helps with dehumidification. Avoid setting the temperature to the coldest level, as prolonged use may cause frost to form on the outer surface of the windshield. Applying Anti-Fog Spray or Other Preventive Measures: Spray a small amount of defogging agent onto the car windows and wipe it clean to remove dirt and stains. This leaves a thin transparent protective film on the glass, effectively preventing water vapor from condensing and forming fog. This is especially useful in cold winters. Alternatives to anti-fog spray include dish soap, soapy water, glycerin, alcohol, or saline solution. After applying, let it dry and then wipe off any excess with a chamois or soft dry cloth. This can keep the windows fog-free for several days. Opening Windows for Airflow: If the fog inside the car is not too heavy, opening the side windows can create airflow between the inside and outside, reducing the temperature difference and gradually eliminating the fog. This method works on the same principle as using cold air from the air conditioning. However, opening windows has obvious drawbacks—it cannot be used during rain, snow, on highways, or when the fog is too heavy. Rear Window Heating: Most cars are equipped with a rear window heating function, though the button location may vary. Turning on the rear defroster will clear the fog from the rear window in about a minute.

[DiNoah]

When the windshield fogs up while driving, my top concern is safety. Fogging occurs due to condensation when moisture inside the car meets the cold glass. I immediately slow down or pull over, then turn on the defogging mode of the AC: set the fan speed to maximum, adjust the temperature to cold air, and press the windshield vent button. This quickly dries the glass with cold air. If the fogging is mild, cracking the windows slightly to let moisture escape can also help. However, wiping the glass with a towel while driving is unsafe as it can distract. On humid summer days, I pre-treat the glass with anti-fog spray for better results. In winter when heating causes fogging, I first use cold air to defog before switching to warm air for safer driving. Keeping the car interior clean and avoiding damp items reduces the chance of fogging. Safe driving always comes first.