Should You Use Cold or Hot Air to Defog Car Windows on Rainy Days?
2 Answers
The most effective way to defog windows on rainy days is to use cold air, which blows dry cold air directly onto the glass, preventing water vapor from condensing on the surface and achieving the purpose of defogging. The method of using warm air to defog is similar to using cold air, but it's important to note that warm air should not be used for defogging on rainy days, as it can initially increase fogging. When car windows fog up, the external air circulation should be turned on. The reason for car window fogging: There is a certain temperature difference between the inside and outside of the car. If the temperatures inside and outside the car are different, the surface with the lower temperature will have a lower saturated vapor pressure than the surrounding environment, causing water vapor to gather on the glass surface 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, if the air conditioning is not turned on promptly, the high humidity from exhaled breath can quickly cause the windshield to fog up. In this case, turn on the air conditioning to blow cold air onto the windshield, utilizing its dehumidification function to quickly remove fog. If the humidity is too high and the air conditioning is not effective, slightly opening a window can rapidly reduce the humidity inside the car. Combining this with the air conditioning can yield faster results. Air Conditioning Heating: Use the method of reducing the temperature difference to remove fog. In winter, blowing warm air onto the glass quickly raises the temperature of the windshield, reducing the temperature difference between the inner and outer surfaces of the glass, thereby preventing excessive fogging. However, it's worth noting that the rear and side windows heat up more slowly, so it takes longer to completely remove fog from these areas. Whether the air conditioning is automatic or manual, turning it on helps with dehumidification. The temperature control should not be set to the coldest position, as this can cause frost to form on the outer surface of the windshield over time. Applying Anti-Fog Agents for Prevention: Spray a small amount of anti-fog agent onto the car windows and wipe it clean to remove dirt and stains. While polishing the glass, a thin, transparent protective film forms, effectively preventing water vapor from condensing into fog. This is especially useful in cold winters. Alternatives to anti-fog agents include dish soap, soapy water, glycerin, alcohol, or saline solution. After applying and letting it dry, use a chamois or soft dry cloth to remove excess residue, ensuring the windows remain 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 of the car, reducing the temperature difference and gradually eliminating the fog. The principle behind this method is the same as using cold air from the air conditioning. However, the downside of opening windows for airflow is obvious—it cannot be used during rain, snow, on highways, or when the fog is heavy. Rear Window Heating: Most cars are equipped with a rear window heating function, though the button location may vary. Turning on the rear window heating switch will typically clear the fog within about a minute.
I remember once when I was driving in a hurry, it suddenly started pouring rain, and the car windows fogged up instantly, making it impossible to see anything. At that moment, I quickly turned on the cold air and directed it toward the windows. The cold air worked remarkably fast—within seconds, the fog was gone. However, keeping the cold air on for too long can make you feel chilly, and if you only use the recirculation mode, the fog tends to come back quickly. So, my experience is to start with cold air to clear the fog, then switch to warm air to maintain dryness once the fog is gone. Also, turn on the external air circulation to keep the air flowing. If there are elderly passengers in the car, remember to turn off the cold air in advance to avoid making them feel cold. And don’t forget to press the front defroster button—the small fan-shaped symbol—which automatically adjusts the air direction for you, making it very convenient. If the rear windshield fogs up, press the rear window heating button, which is the square icon with wavy lines.
