What to Do When the Car Windshield Fogs Up on Rainy Days?
4 Answers
Press the defog button, turn on the air conditioning (A/C) switch, set the temperature to the lowest, and switch to external circulation to defog. Both cold and warm air can be used to defog the 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 the purpose of defogging. The method for using warm air to defog is the same as using cold air, but it should be noted that warm air should not be used for defogging on rainy days, as it will initially worsen the fog. When the car windows fog up, external circulation should be used. 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 many people enter the car and the air conditioning is not turned on in time, the high humidity of exhaled air can quickly cause the windshield to fog up. In this case, turn on the air conditioning to blow cold air onto the windshield. Using the air conditioning's dehumidifying function, the fog on the windshield can be removed quickly. However, if the humidity is too high and the air conditioning is not effective, slightly open the window to quickly reduce the humidity inside the car. Of course, combining this with the air conditioning will yield faster results. Air Conditioning Heating: Use the method of reducing the temperature difference to remove fog. In winter, use warm air to blow hot air onto the glass, quickly raising the temperature of the front glass and reducing the temperature difference between the inside and outside surfaces of the window. This can prevent excessive fogging of the windshield in time. However, it should be noted that the rear and side windows heat up more slowly, so it takes longer to remove all 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 can 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 on the glass, effectively preventing water vapor from condensing and forming fog. This is especially useful in cold winters. Alternatives to anti-fog spray include dishwashing liquid, soapy water, glycerin, alcohol, or salt water. After drying, wipe off any excess residue with a chamois or soft dry cloth. This can keep the car windows fog-free for several days. Opening Windows for Airflow: If the fog inside the car is not too heavy, you can open the windows on both sides to allow air to flow between the inside and outside of the car, reducing the temperature difference and causing the fog to disappear slowly. The principle of this method is the same as using cold air from the air conditioning. The downside of opening windows for airflow 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. The button position may vary, but 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.
As someone who works with cars every day, I'm all too familiar with windshield fogging on rainy days. The most effective method is to immediately press the defogger button—the one with the fan-shaped arrow symbol—while adjusting the air vents to direct airflow toward the windshield. Set the temperature just right, not too high or too low, around 23°C is perfect. Remember to turn off recirculation mode! Using fresh air mode allows dry outside air to quickly enter. If the fogging is severe, turning on the AC cooling can help, as the cold air has an instant dehumidifying effect. Cracking the window slightly also works, but use this trick cautiously on rainy days. Keeping an anti-fog cloth in the car is practical, but never wipe the windshield while driving—safety first!
Last year during the rainy season, I had a rear-end collision due to fogged-up windows, but now I've finally figured out the tricks. After starting the car, immediately roll down the windows for a few seconds to balance the temperature difference, then turn the AC to the defogging setting. The key is to use the external air circulation! Internal circulation just traps moisture inside the car. Keep the temperature between 22-25°C, alternating between cool and warm air for better results. Prepare a small spray bottle with soapy water and wipe the inside of the windows with a towel while parked—it's cost-effective and eco-friendly. If the rear window fogs up, remember to press the heating button; the electric heating strips will sizzle and clear the fog away. Develop the habit of turning off the AC before parking to let the fan dry out the ducts, making it less likely for fog to form the next time you start the car.
The worst thing about driving in the rain is when the windshield turns foggy. Don't panic when you notice fogging—just follow these three simple steps: press the windshield defogger button, switch the AC to external circulation, and crank the fan speed to the maximum. The fog will clear in 30 seconds. If you're in a real hurry, blast cold air directly at the windshield—it might freeze your arms, but it works fast. Always keep a rain repellent spray in the car; applying it monthly forms a hydrophobic coating. Hanging an anti-fog strip on the rearview mirror is also handy. Once, during a heavy rainstorm while picking up my kid from school, the windshield was completely fogged up. My child taught me to wipe the glass with newspaper dipped in dish soap—once dried, moisture couldn’t gather anymore. This old-school trick worked surprisingly well!
