What Causes Windshield Fogging When Using Air Conditioning?
3 Answers
When the temperature inside the car is low and the outside temperature is high, the moisture in the outside air condenses into tiny water droplets upon contact with the cold glass surface, forming what we commonly refer to as fog. Fog essentially consists of these minute water droplets. The detailed explanation is as follows: 1. Principle: Under conditions of abundant moisture, light winds, and stable atmospheric layers, when relative humidity reaches 100%, the water vapor in the air condenses into fine water droplets suspended in the atmosphere, reducing horizontal visibility at ground level. This weather phenomenon is called fog. Fog occurs more frequently between February and April during spring. 2. Formation conditions: In meteorology, when suspended water vapor condensation in the atmosphere reduces visibility below 1 kilometer, this weather phenomenon is termed as fog. The conditions for fog formation are: first, cooling; second, humidification to increase water vapor content. Types of fog include radiation fog, advection fog, mixing fog, evaporation fog, and smog.
Fogging on the windshield after turning on the AC is mainly caused by the significant temperature difference between the inside and outside of the car. The outside temperature is low, while the interior is warmed by the AC, making the windshield the focal point of this temperature contrast. Moisture in the air condenses on the cold glass, similar to water droplets forming on a chilled drink bottle. This effect is especially pronounced on rainy days or in winter when external humidity is high, compounded by moisture from passengers' breathing, leading to more frequent fogging. Instead of just wiping the glass, use the AC's defogging feature: turn on the AC to cool and dehumidify, adjust the airflow to direct it toward the windshield, and slightly open the external air circulation for ventilation. Avoid blasting hot air onto cold glass, as this increases the temperature difference and worsens fogging.
I've researched this issue when encountering fogging while driving. Fogging on the glass is essentially condensation of moisture. In summer, when the cold air from the AC meets the sun-heated glass, fog forms. In winter, warm air hitting the cold glass makes it worse. The solution is simple: turn on the AC's dehumidification mode, which dries out the moisture inside the car, and increase the airflow to blow directly at the windshield. Additionally, a dirty cabin air filter reduces ventilation efficiency and can also cause fogging, so regular replacement is important. On rainy days, switch to external air circulation to prevent moisture buildup inside the car, and don't forget to turn on the front and rear window defogging functions.
