Should You Use Internal or External Air Circulation When Defogging Windows?
2 Answers
To defog the windows, you should turn on the external air circulation, press the defog button, activate the air conditioning (A/C), set the temperature to the lowest, and use external air circulation. 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 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 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 for defogging 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. In this case, turn on the air conditioning to blow cold air onto the windshield. The dehumidifying function will quickly remove the fog. If the humidity is too high and the air conditioning is ineffective, slightly open a window to rapidly reduce the cabin's humidity. Combining this with the air conditioning will yield faster results. Air Conditioning Heating: Use warm air to reduce the temperature difference and remove fog. In winter, blowing warm air onto the windshield quickly raises its temperature, reducing the temperature difference between the inside and outside surfaces and preventing heavy fogging. However, the rear and side windows heat up more slowly, so it takes longer to defog them completely. Whether using automatic or manual air conditioning, turning it on helps with dehumidification. Avoid setting the temperature to the coldest, as prolonged use may cause frost to form on the outside of the windshield. Applying Anti-Fog Spray or Other Preventive 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 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 minor, opening the side windows can create airflow between the inside and outside of the car, reducing the temperature difference and gradually dissipating 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, snow, on highways, or when the fog is heavy. Rear Windshield Heating: Most cars have a rear windshield heating function, though the button location may vary. Turning on the heating switch will clear the fog from the rear windshield in about a minute.
Every time I encounter foggy windshields while driving, my favorite solution is to use the external air circulation. It draws in drier air from outside the car, which helps remove moisture, and the fog naturally dissipates faster. Pairing it with the air conditioner's defogging mode, increasing the fan speed, directing the airflow toward the windshield, and setting a cooler temperature works even better. In contrast, internal circulation tends to trap moisture inside the car, causing fog to build up. I remember last time it was foggy while driving in the rain—using internal circulation made the fog worse, but switching to external circulation cleared it up in minutes. It's also important to regularly check that the car's ventilation vents aren't blocked. New drivers, remember this trick—long drives won't be a problem with fog obstructing your view, and you'll feel much safer!
