Why does the outside of the car glass fog up when the AC is on?
3 Answers
The reason for fog forming on the outside of the car glass when the AC is on is that the temperature inside the car is much lower than the outside temperature. When moisture in the outside air comes into contact with the very cold windshield, it condenses into fog. Here is relevant information: Solution: First, you can use the windshield wipers since the fog is on the outside of the glass, where the wipers can be effective. If the fog is heavy and the wipers don't help much, you can also use warm air to blow on the windshield. Adjust the AC temperature to produce warm air, increase the temperature for automatic AC, rotate the direction switch to direct air toward the glass, and the fog will disappear shortly. This happens when the temperature is set too low or the outside weather is humid. Generally, it's best to use the recirculation mode with the AC vents directed toward the feet and head.
I think this is quite common, especially when driving with the air conditioning on in summer, and the windshield fogs up on the outside. The reason is that the weather is hot and the air humidity is high. When the cold air from the AC blows, the surface temperature of the glass drops too low, and the hot air outside condenses into water droplets when it hits the cold glass, forming fog. From my experience, for example, after rain or just after washing the car, when the glass is wet, it's more prone to fogging up. When encountering this issue, I usually just turn on the wipers for a quick swipe or adjust the AC direction to avoid blowing directly on the glass—aiming it at the body or feet instead. For regular car maintenance, I also recommend periodically cleaning the windshield and using an anti-fog spray to reduce moisture buildup. In short, this is a physical phenomenon, not a car malfunction, but it's important to address it promptly for safety—just make sure it doesn't obstruct your view.
When I turn on the cold air conditioning while driving, the outside glass fogs up. The reason is that the cold air from the AC chills the glass, causing condensation when it meets the hot and humid air outside. As someone who frequently drives long distances, this happens more often during the rainy season in the south. To prevent it, I first check if the outside temperature is too high. If the humidity is high, I reduce how much I open the windows. For the AC settings, I avoid pointing the vents directly at the glass at high speed and instead set them to medium or low airflow directed inside the car. Additionally, keeping the glass clean is crucial. I regularly wipe it down with a damp cloth and sometimes use anti-fog coating sprays, which work well for prevention. If fog does appear while driving, a quick swipe with the wipers clears it up without affecting driving safety.
