Why Does the Windshield Fog Up When the Air Conditioner is On?
2 Answers
The reason for the windshield fogging up is the significant temperature difference between the inner surface of the glass and the air inside the car. Generally, the windshield defrosting function is used to address windshield fogging based on the driving environment. If the vehicle is equipped with automatic air conditioning and allows selection of air vent positions, it is recommended to direct airflow to both the windshield and the central vents while activating the rear windshield heating function. This keeps the interior glass temperature regulated and prevents fogging. If it's not possible to direct airflow to both positions simultaneously, when the side windows fog up, slightly opening the windows can quickly remove the fog. After closing the windows, once the interior temperature rises, fogging will no longer occur. It's important to manage the duration of air conditioning usage. Prolonged use increases the burden on the condenser, which not only affects the cooling efficiency but may also cause some damage to the engine. Therefore, intermittent use of the air conditioner is advisable.
I've been driving for decades and often encounter the issue of foggy windows when turning on the air conditioning. Especially during the rainy season when the weather is humid, as soon as I turn on the cold AC, the windshield becomes misty, which feels particularly dangerous. This happens because the cold air from the AC rapidly lowers the temperature of the glass surface, causing the moisture in the car's air to condense into tiny water droplets upon contact with the cold glass. My usual approach is to immediately press the defog button or turn on the fan to blow air directly at the windshield, speeding up air circulation. At the same time, I switch to external circulation mode to let fresh air in and reduce humidity. To prevent this, don't turn off the AC too abruptly before parking; gradually increase the temperature instead. Additionally, checking if the AC filter is clean during car maintenance can also help. In short, paying attention to weather changes and maintaining a balanced temperature inside the car can minimize such issues.
