Should You Use Hot or Cold Air to Defog Windshield on Rainy Days?
2 Answers
On rainy days when the car windows fog up, turn on the warm air and increase the fan speed. The dry, warm air will evaporate the water molecules in the car's interior air, preventing fog from forming on the windows. Here are some solutions for defogging windows: Spray anti-fog agent: Apply a small amount of defogging agent to the car windows and wipe it clean. This will remove dirt and stains from the glass while forming a thin transparent protective film that effectively prevents moisture condensation and fog formation. Use the air conditioning's dehumidification function: Reduce air humidity by turning on the AC to blow cold air. The dehumidification feature will quickly remove fog from the windshield. Utilize temperature difference reduction: In winter, direct warm air at the windshield to rapidly increase its temperature, reducing the temperature difference between the inner and outer surfaces of the glass. This helps prevent excessive fogging on the windshield.
When I used to drive long distances in rainy weather, the windows would often fog up. Now I simply use the hot air setting because it quickly heats the glass, allowing the moisture to evaporate. Fogging mainly occurs due to condensation caused by the temperature difference between the warm interior and the cold glass surface, and hot air directly disrupts this balance by raising the temperature. While turning on the cold air might temporarily clear the fog, the cool air makes the glass colder, causing the fog to return faster—its effectiveness is short-lived. After decades of driving, I've found that using hot air combined with the A/C dehumidification function works best. Set the temperature to a moderate level—not too high—for both safety and convenience. Blurred vision on rainy roads is a serious issue, so it's wiser to turn on the hot air in advance as a preventive measure. Never wait until the fog becomes severe before taking action.
