How to Deal with Foggy Windows in a BMW?

[LeHenry]

To defog the windows in a BMW, press the defogging button, turn on the air conditioning (A/C) switch, set the temperature to the lowest, and switch to external circulation. Both cold and warm air can be used to defog the windows, but warm air is slower than cold air. Using cold air is more effective as it blows dry cold air directly onto the glass, preventing water vapor from condensing on the glass and achieving the purpose of defogging. Similar to using cold air, avoid using warm air for defogging on rainy days as it may initially worsen the fog. The reason for foggy windows is that the temperature of the window glass in contact with the outside is lower, causing water vapor inside the car to condense on the glass and form fog. The cause of foggy car windows: There is a certain temperature difference between the inside and outside of the car. If the temperatures inside and outside are different, the surface of the cooler side has a lower saturated vapor pressure than the surrounding environment, causing water vapor to gather 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 dehumidification function to reduce air humidity and remove fog. In summer, especially when multiple people enter the car and the air conditioning is not turned on promptly, the high humidity from exhaled breath can quickly cause the windshield to fog up. Turning on the air conditioning to blow cold air onto the windshield and using the dehumidification function can quickly remove the fog. If the humidity is too high and the air conditioning is not effective, slightly opening the windows can rapidly reduce the humidity inside the cabin. Combining this with the air conditioning can speed up the process. Air Conditioning Heating: Use warm air to reduce the temperature difference and remove fog. In winter, blowing warm air onto the glass quickly raises the temperature of the front glass, reducing the temperature difference between the inside and outside surfaces and preventing excessive fogging. However, the rear and side windows heat up more slowly, so it takes longer to completely remove the fog. Whether the air conditioning is automatic or manual, turning it on helps with dehumidification. Avoid setting the temperature to the coldest position to prevent fogging on the outside of the windshield over time. Applying Anti-Fog Agents: 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 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 dissipating the fog. This method works on the same principle as using cold air from the air conditioning. However, this method has obvious drawbacks and is not suitable during rain, snow, on highways, or when the fog 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 usually clear the fog in about a minute.

[VanHarrison]

I've encountered the issue of foggy car windows multiple times with my BMW, especially when it rains. Initially, I didn't know better and just wiped the windows with a towel, only to make them even blurrier and more distracting. Later, I figured out a better method: BMWs come with a dedicated defogging button, usually located on the air conditioning control panel. Just press the button with the wavy line icon. The fan will automatically switch to high speed to blow air onto the windshield. Depending on the weather, it's advisable to choose either cold or warm air and set it to external circulation mode to let dry air in, which works better. Avoid using internal circulation to prevent moisture buildup. I also regularly replace the cabin air filter to prevent blockages that could affect defogging. Occasionally, stopping by the roadside to wipe the windows with a clean cloth is just an emergency measure. Remember, don't panic when the windows fog up—first, slow down to ensure safety before dealing with it.