Should You Use Recirculation or Fresh Air Mode to Defog Car Windows?

[DeclanMarie]

You should use the fresh air mode and press the defog button, turn on the air conditioning (A/C) switch, set the temperature to the lowest, and activate fresh air mode to remove fog. Both cold and warm air can defog car windows, but warm air works 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 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 for defogging on rainy days as it may initially worsen the fogging. The reason for car window fogging: There is a temperature difference between the inside and outside of the car. If the temperatures inside and outside the car differ, the surface with the lower temperature has a lower saturation vapor pressure than the surrounding environment, causing water vapor to accumulate on the glass surface and form tiny water 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, if the air conditioning is not turned on promptly, the high humidity from exhaled breath can quickly cause the windshield to fog up. In this case, turn on the air conditioning to blow cold air onto the windshield, using its dehumidifying function to quickly remove fog. If the humidity is too high and the air conditioning is not effective, slightly open a window to rapidly reduce the humidity inside the cabin. Combining this with the air conditioning can speed up the process. Air Conditioning Heating: 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 inner and outer surfaces, thus preventing heavy fogging. However, note that the rear and side windows heat up more slowly, so it takes longer to completely remove fog. Whether using automatic or manual air conditioning, turning it on helps with dehumidification. Avoid setting the temperature to the coldest position, as prolonged use can cause the outer surface of the windshield to fog up. Applying Anti-Fog Spray: Spray a small amount of defogging agent on the car windows and wipe it clean to remove dirt and stains. This also forms a thin, transparent protective film on the glass, effectively preventing water vapor from condensing and forming fog, especially useful in cold winters. Alternatives to defogging 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. This can keep the windows fog-free for several days. Opening Windows for Airflow: If the fogging is not severe, opening the side windows can create airflow between the inside and outside of the car, 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, it has obvious drawbacks: it cannot be used during rain or snow, on highways, or when the fogging is heavy. Rear Window Heating: Most cars have a rear window heating function, though the button location may vary. Turning on the rear window heating switch will typically clear the fog in about a minute.

[LeJulian]

Foggy car windows are mainly caused by high humidity inside the vehicle, where moisture condenses on the cold glass. In this case, decisively turning on the external air circulation is more effective—it draws in dry air from outside to reduce humidity. When I first started driving, I also tried using the internal circulation, but the fog actually worsened, especially on rainy days or in winter. The exhaled moisture got trapped inside the car and couldn’t dissipate, quickly fogging up the windshield. If it’s so cold you get goosebumps, first turn on the air conditioning in cooling mode + external circulation to quickly dehumidify. Once the fog clears, switch to warm air to maintain temperature. By the way, the defog button actually automatically switches to external circulation + turns on the compressor, making it more convenient than manual operation. If there’s a big temperature difference between inside and outside the car, cracking the windows a bit can also be quite effective.