Why Does Car Glass Fog Up?
3 Answers
Car glass fogs up due to high humidity and low temperature, both conditions being necessary. Below are the relevant explanations: 1. Humidity Reason: When the air humidity is constant and the window temperature falls below the dew point, condensation forms on the window surface, which is commonly seen as fogging on the windshield. 2. Temperature Reason: When there is a certain temperature difference on both sides of the window, and the saturated vapor pressure of water on the colder surface is lower than the surrounding environment's vapor pressure, moisture gathers on the glass surface, appearing as tiny water droplets, thus forming fog.
The core reason for car window fogging lies in temperature differences and humidity changes. When the outside temperature drops suddenly or during rainy weather, the relatively warm and humid air inside the car comes into contact with the colder glass surface, causing moisture to condense into tiny water droplets. This is similar to how glasses fog up when moving from a warm room to the cold outdoors in winter. The humidity inside the car mainly comes from passengers' breathing, moisture brought in on rainy days, or evaporation from drinks in the vehicle. Fogging typically starts on the windshield first, as it's the area closest to the cold outside air. To quickly defog, turn on the air conditioning at its coldest setting while activating the external air circulation. The cold air can rapidly lower the glass surface temperature and absorb moisture, showing results in about three minutes.
As a long-distance driver, I'm all too familiar with this situation. When there are many people in the car or it has just rained, the moisture from breathing and the rainwater brought in by shoes evaporates into the air, condensing into fog when it hits the cold glass. This is especially noticeable on rainy days because the outside air is humid, and there's a significant temperature difference between the inside and outside of the car. Experienced drivers use the air conditioning to defog: first turn on the AC switch, then switch to defog mode, directing cold air to the windshield. In winter, warm air is used for defogging, though it works more slowly. Normally, you can also use a mixture of dish soap and water to wipe the glass, which can prevent fogging for several hours. Remember not to wipe with your hands, as it will only make the glass blurrier, and driving with the windows open actually lets in more moisture.
