Why Does Car Window Glass Fog Up?
2 Answers
Car window glass fogs up due to the following reasons: 1. The temperature difference between the inside and outside of the car causes the windows to fog easily. The window glass, being in contact with the outside, is cooler, causing the water vapor inside the car to condense on the glass, forming fog. 2. The window temperature is below the dew point. At a certain air humidity, if the window temperature is below the dew point, condensation easily forms on the surface of the window, leading to fogging on the windshield. 3. There is a temperature difference between the two sides of the window. If the temperatures on either side of the window are different, the side with the lower temperature has a surface water saturation vapor pressure lower than the surrounding environment's vapor pressure, causing water vapor to gather on the glass surface and form tiny droplets, resulting in fog.
I've seen quite a few cases of window fogging, mainly caused by the large temperature difference between the inside and outside of the car, coupled with high humidity inside the vehicle. Take winter for example: when you first get in the car, the warm air you exhale or the warm air blown by the AC meets the still-cold glass, causing moisture to condense on it instantly, appearing like fog. The water vapor from breathing and moisture from clothes accumulate, making it easier for condensation to form on the windows when humidity is high. This is especially common on rainy days or during humid seasons. It's quite a nuisance—fog obstructing your view while driving can easily lead to accidents, so it's best to develop good habits to prevent it. I usually turn on the AC's dehumidification mode or use the fan to blow air onto the windshield before driving, and the fog clears up quickly. If fog has already formed, simply opening the windows to improve air circulation helps disperse the moisture, making the glass much clearer. Also, make sure there are no leaks inside the car, such as poorly sealed doors or windows, which can increase interior moisture. Safety first—if the windows fog up, don't force yourself to keep driving; pull over and deal with it properly.
