Why Does the Car Window Glass Fog Up on the Outside in Summer?

[SanAaron]

In summer, car windows fog up on the outside due to the low temperature setting of the car's air conditioning, which creates a significant temperature difference. Additionally, improper air conditioning vent direction can contribute to this issue, such as when the air is directed toward the front windshield or the side windows. Here are some additional details: Detergent-based solutions: Dish soap or soap can be used to prevent fogging on car windows. The principle involves disrupting the attraction between water molecules, reducing surface tension, and evenly distributing water molecules across the treated glass surface. This forms a water film that adheres to the glass—the more rainwater, the larger the water film—keeping the car glass consistently clear. Anti-fog sprays: Anti-fog sprays can be used to remove fog from car windows. The principle involves absorbing moisture on the glass surface to form a water film, which prevents fogging by stopping moisture from adhering to the glass. Wiping the window glass dry with a cloth ensures clarity, with effective and long-lasting results.

[DeEmery]

Fogging on the outside of car windows is common in summer, primarily because the glass surface temperature is lower than the surrounding air temperature, causing highly humid water vapor to condense into tiny droplets. In hot weather, if we turn on the air conditioning with cold air blowing directly at the windshield, the glass cools rapidly, and the warm, humid outside air quickly forms fog upon contact. This situation is especially prone to occur after rain or in humid areas, as humidity levels above 90% accelerate condensation. When dealing with it, don't panic—adjust the air vents to avoid the glass, switch to external circulation mode, or slightly open a window to balance the temperature difference inside and outside, which can reduce fogging. A gentle swipe with the wipers can clear it, but if it occurs frequently, check the window seals and air conditioning system to ensure proper airflow. This helps avoid obstructing the driver's view and ensures safe driving. It's advisable to carry simple defogging tools during peak summer travel periods as a backup.

[McPatrick]

As a seasoned driver, I encounter the issue of external window fogging every summer, especially during long-distance driving. This is caused by hot weather combined with excessive air conditioning use, where the cold air cools the glass, causing moisture from the external hot air to condense into fog. Common scenarios include sunny weather after rain or being stuck in traffic on the highway, where the AC cooling effect is strong but the glass surface temperature drops too much. My experience suggests avoiding direct airflow from the AC vents onto the windshield and instead directing it toward the legs or seat area. Alternatively, cracking a window slightly to let in fresh air can help balance humidity. Using the windshield wipers promptly can clear the fog, making it both convenient and safe. It's also a good habit to check the weather forecast before setting off—on high-humidity days, reduce AC usage and rely more on natural airflow. Regularly cleaning external dirt from the glass can also prevent fogging from obstructing visibility, ensuring a more relaxed drive.

[CharlieLee]

External fogging on car windows is mostly caused by the air conditioning's cold air. When strong cold air hits the glass and cools it down, the external high-temperature moisture condenses instantly. I often pay attention to this during car maintenance and recommend adjusting the direction of the air vents away from the windshield—either upward toward the interior space or downward toward the footwell area. Check if the cabin air filter is clean, as a clogged filter increases the risk of condensation. Using the wipers can quickly clear the fog, ensuring unobstructed visibility. Simple measures like temporarily switching to recirculation mode or reducing the temperature difference can also help prevent fogging.