What to Do When the Front Windshield Fogs Up?
2 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 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 glass and achieving defogging. The method for using warm air is the same as cold air, but note that warm air should not be used for defogging on rainy days as it 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 inside and outside are different, 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 accumulate on the glass surface in tiny droplets, forming 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 air conditioning promptly, the exhaled air has high humidity, quickly causing the windshield to fog up. At this point, turn on the air conditioning to blow cold air onto the windshield, using the dehumidification function to remove the fog quickly. If the humidity is too high and the air conditioning is not effective, slightly open a window to quickly reduce the humidity inside the car. Combining this with the air conditioning can speed up the process. Air Conditioning Warm Air: Use warm air to reduce the temperature difference and remove fog. In winter, blowing warm air onto the windshield quickly raises its temperature, reducing the temperature difference between the inside and outside surfaces of the glass and preventing excessive fogging. However, the rear and side windows warm 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. Avoid setting the temperature to the coldest position, as prolonged use can cause the outer surface of the windshield to fog up. Applying Anti-Fog Agents for Prevention: 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 forms, effectively preventing water vapor from condensing into fog, especially useful in cold winters. Alternatives to defogging agents include dish soap, soapy water, glycerin, alcohol, or saltwater. After applying, let it dry and then wipe off excess residue with a chamois or soft dry cloth, ensuring the glass remains fog-free for several days. Opening Windows for Airflow: If the fog inside the car is not heavy, opening the windows on both sides allows air to circulate between the inside and outside, reducing the temperature difference and gradually dissipating the fog. This method works on the same principle as using cold air from the air conditioning. However, the downside is obvious: it cannot be used during rain, snow, on highways, or when the fog is heavy.
I've been driving for over a decade, and foggy windshields are a common occurrence, usually caused by significant temperature differences that lead to condensation forming on the glass. In such cases, my first move is to press the defogger button—the one with the fan-shaped symbol and arrows—then turn the fan to its highest setting, directing the airflow at the windshield. If the weather is warm, using the air conditioner in cold mode for dehumidification works well; in winter, heating the glass with warm air also does the trick. Remember to crack open a window slightly to balance interior and exterior humidity, preventing the fog from recurring. Regular glass cleaning is also important, as oil and dust buildup can make fogging more likely—I use a damp cloth for periodic cleaning. If heavy fog severely obstructs visibility, avoid slamming on the brakes. First, activate the defogging system and wait a few minutes to see if it clears. If not, pull over safely and wipe the glass. For prevention, on rainy or highly humid days, switching to recirculation mode in advance can reduce the entry of outside moisture.
