What Causes Windshield Fogging in Summer?
3 Answers
Fogging on the inside of the front windshield in summer is caused by the temperature difference between the interior and exterior. When the window temperature is lower than the dew point, the air's moisture content is insufficient to form water droplets, resulting in tiny water particles that condense on the window surface, creating the fogging effect. Solutions for window fogging: Using detergent-based methods: Dish soap or soap can be used to prevent fogging on car windows. Soften the soap by soaking it in water, then apply it evenly on the glass surface. Once the soap dries, remove it with a tissue or newspaper. For dish soap, mix water and soap in a 9:1 ratio and spray it on fog-prone windows during rainy days. After drying, gently wipe with a soft tissue to prevent fogging. External circulation defogging: If rain is frequent and opening windows is not feasible, turn on external circulation for defogging. Proper vent settings should direct airflow to both the front windshield and footwell or just the windshield. For automatic AC models, the compressor may not activate at lower temperatures, requiring air exchange between the interior and exterior to disperse moisture and achieve defogging. Anti-fogging agents: Commercial anti-fogging sprays designed for car windows absorb moisture from the glass surface, forming a water-repellent film that prevents fogging. After application, wiping the window with a cloth ensures clarity and provides long-lasting defogging. Window adjustment: After driving outdoors, park and open the windows to let warm air in. The temperature difference will decrease within a few minutes, causing the fog to dissipate. If parking isn't possible, use windshield wipers for the front and rear windows, and adjust the side windows to scrape off fog with the rubber seals.
When driving long distances in summer, I often encounter windshield fogging, mainly caused by the large temperature difference between inside and outside the car plus high humidity. For example, on a hot day with the AC on, the car interior is cool, but outside it's stuffy and humid, so moisture tends to condense on the cold glass, forming fog; and on rainy days, with windows closed, passengers' breathing or damp clothing increases interior humidity, causing fog to form quickly on the cold glass. I remember once when I was in a hurry, the fog blocked my view and I almost hit the curb. It's a reminder to everyone to check if the AC defog function works properly before heading out, ensuring clear visibility for safe driving.
Windshield fogging in summer is essentially caused by condensation: water vapor in warm air turns into mist droplets when it hits a cold surface. On hot and stuffy days, excessive air conditioning cooling makes the inner side of the glass much colder than the outside temperature, causing fog to form when it encounters high-humidity air. Alternatively, improperly adjusted AC vents blowing directly onto the glass can accelerate cooling and become a root cause of fogging. For instance, after a rainy day, I once tried reducing the fan speed and temperature, which noticeably decreased the fog. In daily use, adjusting the AC to a moderate temperature and frequently using the external circulation mode to expel moisture is more hassle-free than dealing with fog after it forms—prevention is better than cure.
