
Press the defog button, turn on the air conditioning (A/C) switch, set the temperature to the lowest, and activate the external circulation to defog. 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, which blows dry, cold air directly onto the glass, preventing water vapor from condensing on the surface and achieving defogging. The method for defogging with warm air is the same as with cold air, but note that using warm air on rainy days may initially worsen the fog. Always use external circulation when defogging the windshield. The reason for windshield 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 and form tiny droplets, creating fog. Methods to defog the windshield: 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 promptly, the exhaled humid air can quickly fog up the windshield. Turning on the AC to blow cold air onto the windshield and using the dehumidification function can quickly clear the fog. If humidity is too high and the AC alone is ineffective, slightly opening the windows can rapidly reduce cabin humidity. Combining both methods yields faster results. Air Conditioning (Heating): Use the heating function to reduce the temperature difference and eliminate fog. In winter, blowing warm air onto the windshield raises its temperature, reducing the temperature difference between the inside and outside surfaces and preventing heavy fogging. However, the rear and side windows warm 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 may cause frost or fog on the outer windshield. Applying Anti-Fog Spray: Spray a small amount of defogging agent onto the windshield and wipe it clean to remove dirt and stains. This leaves a transparent protective film that effectively prevents water vapor from condensing into fog, especially useful in cold winters. Alternatives like dish soap, soapy water, glycerin, alcohol, or saline can also be applied, left to dry, and then wiped with a chamois or soft cloth to keep the windshield fog-free for days. Opening Windows for Airflow: If the fog is light, opening both side windows creates airflow between the inside and outside of the car, reducing the temperature difference and gradually clearing the fog. This method works similarly to using cold air from the AC. However, it has clear drawbacks—it's unsuitable during rain, snow, high-speed driving, or heavy fog. Rear Windshield Heating: Most cars have a rear windshield heating function, with varying button locations but the same purpose. Turning on the heating switch clears rear windshield fog in about a minute.

I've encountered the issue of windshield fogging up when using the heater on rainy days many times over my 20+ years of driving. While the heater keeps you warm, raising the cabin temperature causes moisture to condense on the cold glass, creating fog. In this situation, immediately turning on the AC is crucial because the cold air can quickly dehumidify. Set the fan to maximum speed, select warm temperature, and direct the vents toward the windshield. Also, switch to recirculation mode to let dry, cool air in and dilute the moisture. If the fog is too heavy, first wipe it briefly with a clean cloth or glove, but don't get distracted while driving—wait until you're safely parked. Regularly cleaning both the inside and outside of your windows to prevent oil residue buildup can help avoid this. Remember, safety comes first: foggy conditions reduce visibility and increase accident risks, so slow down or pull over until the fog clears, especially during heavy rain when extra caution is needed.

As a young outdoor enthusiast who enjoys driving, I often encounter the annoyance of fogged-up windows when using the heater on rainy days. The key issue is that the warm air isn't effective at dehumidifying when humidity is high, and can actually worsen the fogging. My habit is to turn on the AC button as soon as I get in the car, regardless of the weather, as it helps absorb moisture. I set the fan speed higher and keep the temperature moderate—not too high—while directing the airflow toward the windshield. I also switch to external circulation for ventilation, avoiding prolonged use of internal recirculation. I usually keep an anti-fog cloth in the car and wipe the windows beforehand when rain is expected; applying a defogging spray from the supermarket can provide longer-lasting results. If fogging occurs while driving, don't panic—first, turn on the hazard lights to alert others, then gently brake and find a safe spot to pull over. Safety always comes first—never push through. Developing these small habits can make driving more comfortable and reduce unexpected issues.

From a physical perspective, fogging occurs when the interior temperature rises, causing moisture to condense into tiny water droplets upon contact with the cold glass. Rainy weather with high external humidity exacerbates this phenomenon. My advice: Immediately activate the AC mode in your car's climate control system, as it cools and dehumidifies. Adjust the fan speed and temperature appropriately—for instance, moderate warm air with increased airflow directed toward the windshield. External air circulation is essential to introduce drier outside air and dilute interior humidity. In cases of severe fogging, park and use warm air to thoroughly dry the glass before resuming your journey. Avoid relying solely on warm air without AC, as its effectiveness is significantly lower. Fogging isn't just a comfort issue—it directly impacts driving safety, so address any anomalies promptly without hesitation. Regularly cleaning your windows also helps minimize residue that can contribute to fog buildup.

Fogging on the windshield when using the heater is especially dangerous in rainy weather, and I must emphasize that safety should never be overlooked. When the windshield fogs up, visibility becomes blurred, making it difficult to see the road clearly, which can easily lead to collisions or loss of control. The priority is to pull over safely and turn on the hazard warning lights. Then, inside the car: turn on the AC button to force dehumidification, adjust all air vents to point toward the windshield, and increase the fan speed to quickly clear the fog. Switch to external air circulation to draw in fresh, dry air. It’s a good habit to preheat the car and wipe the inner surface of the windshield before rain to avoid oil buildup that attracts fog. For family cars, parents are advised to keep anti-fog cloths or sprays on hand for emergencies—children’s safety in the car comes first. Don’t worry about using more fuel by running the AC; driving comfortably and safely ensures a better mood on the road.


