Which air circulation mode should be used when car windows fog up?
3 Answers
You should turn on the external air circulation, press the defogging button, activate the air conditioning cooling (A/C) switch, and set the temperature to the lowest to remove fog. 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 on the glass and achieving defogging. The method for warm air defogging is the same as cold air defogging, but it's important to note that warm air should not be used for defogging on rainy days as it may initially increase fogging. Always use external air circulation when defogging car windows. 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 water droplets, resulting in fog. Methods to defog car windows: Air conditioning cooling: Use the air conditioning's cooling and dehumidifying function to reduce air humidity and remove fog. In summer, especially when multiple people enter the car and the air conditioning is not turned on promptly, the exhaled air has high humidity, quickly causing the windshield to fog up. At this point, you can turn on the air conditioning to blow cold air onto the windshield, using the dehumidifying function to remove the fog quickly. If the humidity is too high and the air conditioning is not effective enough, you can slightly open a window to rapidly reduce the humidity inside the car. Combining this with the air conditioning will yield faster results. Air conditioning warm air: 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 front glass, reducing the temperature difference between the inner and outer surfaces of the window, thus preventing excessive fogging on the windshield. However, it should be noted that the rear and side windows heat up more slowly, so it takes longer to completely remove the fog. 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 may cause the outer surface of the windshield to fog up over time. Applying anti-fog spray or other preventive measures: Spray a small amount of defogging agent on the car windows and wipe it clean to remove dirt and stains. While polishing the glass, a thin transparent protective film is formed, effectively preventing water vapor from condensing into fog on the glass. This is particularly useful in cold winters. Alternatives to defogging agents include dishwashing liquid, soapy water, glycerin, alcohol, or saltwater. After applying, let it dry and then wipe off any excess with a chamois or soft dry cloth. This will keep the windows fog-free for several days. Opening windows for air convection: If the fog inside the car is not too heavy, you can open the side windows to allow air convection between the inside and outside of the car, reducing the temperature difference and gradually eliminating the fog. The principle of this method is the same as using cold air from the air conditioning. However, the downside of opening windows for convection is obvious—it cannot be used in rainy or snowy weather, on highways, or when the fog is heavy. Rear window heating: Most cars have a rear window heating function, though the button location may vary. The function is the same. Turn on the rear window heating switch, and the fog on the rear window will disappear in about a minute.
Recently, it's been cold and I often encounter this issue: Don't panic when your car windows suddenly fog up while driving! Simply turn on the air conditioning and set it to the windshield defrost mode with medium fan speed. Remember to press the AC button—this activates the compressor for dehumidification, which is key. Keep the temperature around 22°C, not too high. At this time, switch to recirculation mode to quickly dry out the humidity inside the car, and the fog will disappear much faster. However, once your view is clear, switch back to fresh air mode to avoid stuffiness. As an experienced driver, here’s another handy tip: Keep a microfiber cloth in the glove box—a quick wipe in emergencies works faster than waiting for the AC. Before parking, turn off the AC and open the windows slightly to reduce the chance of fogging up the next day.
From a physics perspective, window fogging occurs due to temperature differences causing water vapor condensation inside and outside the car. My three-step solution: First, press the front defroster button to automatically activate the compressor for dehumidification; then turn on the recirculation button to block external moisture; finally, adjust the temperature knob to the blue cooling zone—cold air removes fog 60% more efficiently than warm air. Important reminder: Never combine warm air with direct breath on the glass, as this acts like applying a hot towel and worsens fogging. For frost buildup outside during rain/snow, apply anti-fog spray in advance or use a diluted dish soap solution.
