Should You Use Internal or External Air Circulation When the Windshield Fogs Up?
2 Answers
You should turn on the external air circulation, press the defogging button, and activate the air conditioning (A/C) with the temperature set to the lowest. This will help remove the fog. Both cold and warm air can defog the windshield, but warm air is slower than cold air. The most effective method is using cold air, as it blows dry, cold air directly onto the glass, preventing water vapor from condensing and achieving defogging. The method for warm air defogging is the same as cold air, but note that using warm air on rainy days may initially worsen the fog. 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, creating fog. Methods to defog car windows: Air Conditioning Cooling: Use the air conditioning's dehumidification function to reduce air humidity and remove fog. In summer, especially when multiple people enter the car without turning on the AC, the exhaled humid air can quickly cause the windshield to fog up. Turning on the AC and blowing cold air onto the windshield can dehumidify and quickly remove fog. If humidity is too high and the AC alone is ineffective, slightly opening the windows can rapidly reduce cabin humidity. Combining both methods works even faster. 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 inside and outside surfaces and preventing heavy fogging. However, the side and rear windows heat up more slowly, requiring more time to fully defog. Whether using automatic or manual AC, turning it on aids dehumidification. Avoid setting the temperature to the coldest, as prolonged use can cause frost to form on the outer surface of the windshield. Anti-Fog Sprays and 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 solutions. After applying and letting them dry, wipe off excess residue with a chamois or soft cloth to keep windows fog-free for days. Opening Windows for Airflow: If the fog is light, 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 similarly to using cold air from the AC. However, it has clear drawbacks: it’s unusable during rain, snow, on highways, or when fog is heavy. Rear Window Heating: Most cars have a rear window heating function, though the button location may vary. Turning on the rear defroster will clear fog from the rear window in about a minute.
When I first started driving, the windshield would easily fog up on cold or rainy days, which was quite frustrating. After consulting a mechanic, I learned that turning on the external air circulation is the most effective solution because it draws in dry air from outside to expel the moisture inside the car. Fogging is mostly caused by moisture condensing on the cold glass, and using the external circulation to reduce humidity, combined with the air conditioner blowing air, quickly clears the fog. I often turn on the external circulation before starting the car and adjust the airflow to blow onto the windshield for faster defogging. Sometimes, if the outside air is too humid or it's raining, I briefly switch to internal circulation as a temporary measure, but I rely on external circulation in the long run. Developing this habit helps reduce safety risks, especially when driving in rainy or foggy conditions, as visibility improves significantly.
