What Causes Car Windows to Fog Up in Summer?
3 Answers
Introduction to the causes of car window fogging: 1. Temperature difference between inside and outside the car: Due to the temperature difference between the inside and outside of the car, windows are prone to fogging. This happens because the window glass, in contact with the outside, is cooler, causing water vapor inside the car to condense on the glass, forming fog. 2. Window temperature below the dew point: When the air humidity is constant, if the window temperature is below the dew point, condensation easily forms on the surface of the window, leading to the commonly seen fogging on the windshield. 3. Temperature difference between the two sides of the window: If the temperatures on both sides of the window are different, the surface water vapor pressure on the cooler side is lower than the surrounding environment's vapor pressure. This causes water vapor to gather on the glass surface, forming tiny water droplets that appear as common fog.
I often encounter foggy car windows in summer, especially when driving after rain in hot and humid weather. The windshield becomes as blurry as if covered by a thin veil. The reason is that the outside air is hot and humid, while the car's air conditioning makes the interior cool. The large temperature difference causes water vapor to condense into tiny droplets on the outer surface of the glass, forming fog. In severe cases, it can completely obscure the road conditions, which is very dangerous. Once, during a trip to the countryside, I nearly hit a tree because of this situation, which was quite frightening. Now I've learned to be smarter: I turn on the defogging function of the air conditioning before driving, increase the fan speed to blow directly on the glass, and avoid setting the air conditioning temperature too low to reduce the temperature difference. I also avoid placing wet towels or rain gear inside the car to prevent increasing humidity. Regularly cleaning the windshield to remove oil films and residue helps prevent attracting moisture and affecting visibility. In short, safety comes first—address fogging immediately and don't stubbornly continue driving.
Having driven for decades, I've seen plenty of summer window fogging, mainly due to temperature differences and humidity. When it's scorching hot outside and the AC is blasting cold air inside, the glass surface cools down, causing external moisture to condense into fog on the windows—especially common after rain. When this happens, I immediately turn the fan to maximum speed, directing airflow onto the glass to evaporate the droplets, or crack the windows slightly to improve air circulation and disperse moisture. For prevention, I recommend regularly checking the AC system and cleaning filters to control humidity, ensuring door seals are intact to prevent drafts and additional moisture ingress, placing a dehumidifier box to absorb excess moisture, and cultivating a habit of ventilating while driving to create airflow and avoid fog buildup. The key is timely action to prevent obstructed visibility—don’t delay.
