What to Do When the Windshield Fogs Up with the Heater on a Rainy Day?

[MacAmara]

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.