Why Does the Car Window Fog Up When the Air Conditioning Is On?
2 Answers
Fogging up inside the car when the air conditioning is on is caused by the significant temperature difference between the inner surface of the glass and the air inside the car. Here is some related information about defogging: 1. Causes of fogging inside the car: The main reason for fogging inside the car is the temperature issue. When the temperature inside the car is higher than outside, the windows are more prone to fogging. Another reason is the high humidity inside the car, combined with the warm air exhaled by passengers. When this warm air meets the cold glass, it condenses, leading to fogging on the windows. 2. Best solution: The quickest and most effective method is to turn on the air conditioning's cold air. By activating the air conditioning's cooling function along with the external circulation, the fog on the windows should disappear in less than a minute. During autumn or winter, using the air conditioning's warm air to blow on the windshield is the preferred option.
This is something I often encounter during daily driving, especially on rainy days or winter mornings when I leave home. As soon as I turn on the AC, the front windshield gets foggy, which affects visibility and is quite dangerous. The main reason is that the temperature and humidity inside the car are relatively high, and the cold air from the AC chills the glass surface. When warm air meets the cold surface, moisture condenses into tiny water droplets, similar to how a chilled drink glass fogs up. I immediately switch the AC to external circulation mode to let dry outside air dilute the humidity, or simply press the defog button—within a minute or two, the fog clears. Remember not to use internal circulation for too long, as it traps moisture. Every time this happens, I remind myself to check the window seals to ensure there are no leaks that could worsen the problem. Safety first—distractions during fogging can easily lead to accidents.
