How to Deal with Foggy Windows in a BMW?

[MacEzra]

Press the defogging button, turn on the air conditioning (A/C) switch, set the temperature to the lowest, and switch to external circulation to remove fog. 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 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 for foggy car windows: 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 and form tiny droplets, resulting in 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 high humidity from exhaled breath can quickly cause the windshield to fog up. In this case, turn on the air conditioning and direct cold air toward 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 car. Combining both methods will yield faster results. Air Conditioning (Warm Air): Use warm air to reduce the temperature difference and eliminate fog. In winter, blowing warm air onto the glass quickly raises the temperature of the windshield, reducing the temperature difference between the inside and outside surfaces and preventing excessive 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 may cause frost to form on the outside of the windshield. Applying Anti-Fog Agents for Prevention: Spray a small amount of defogging agent onto the car windows and wipe it clean to remove dirt and stains. While polishing the glass, a thin transparent protective film forms, 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 chamois or a soft dry cloth. This will keep the windows fog-free for several days. Opening Windows for Airflow: If the fog inside the car is not severe, open both side windows to create airflow between the inside and outside of the car. This reduces the temperature difference and gradually eliminates the fog. The principle is the same as using cold air from the air conditioning. However, this method has obvious drawbacks and cannot be used during rain, snow, on highways, or when the fog is heavy. Rear Window Heating: Most cars come with a rear window heating function, though the button location may vary. Turn on the rear defroster, and the fog on the rear window will disappear in about a minute.