How to Deal with Foggy Car Windows on Rainy Days?

[Tommy]

Press the defogging button, turn on the air conditioning (A/C) switch, set the temperature to the lowest, and switch to external circulation to defog. 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 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 gather on the glass and form tiny 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 without turning on the air conditioning promptly, the exhaled humid air 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 ineffective, slightly opening the windows can rapidly reduce the cabin's humidity. 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 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 remove all the fog. Whether the air conditioning is automatic or manual, turning it on helps with dehumidification. Avoid setting the temperature to the coldest position, as prolonged use may cause the outer windshield to fog up. Applying Anti-Fog Spray or 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 saltwater. After applying and letting it dry, wipe off excess residue with a chamois or soft dry cloth to 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 approach has obvious drawbacks and cannot be used during rain, snow, on highways, or when the fog is too 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 within about a minute.

[LeHannah]

I've been driving for over a decade and have found three most effective tricks for dealing with foggy windows on rainy days. The first trick is to turn on the AC cold air at maximum blower speed directed at the windshield, while activating the external air circulation button to let dry air in. This works because fogging is essentially caused by temperature difference between inside and outside the car plus humidity - the AC cold air quickly lowers the glass temperature and dehumidifies. The second trick works better in winter: switch to warm air to gradually dry the moisture, which takes about five minutes for mild fogging. The third is an emergency solution: crack open a side window to balance the temperature difference, and keep a clean cloth handy for quick wipes. I also always keep anti-fog spray in my car - applying it during car washes provides about two weeks of protection, worth trying. Remember never to wipe the windshield with your hand while driving - it's too dangerous!