What Causes Car Windows to Fog Up in Summer?

[LaLevi]

Reasons for 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. The reason is that the window glass, being in contact with the outside, has a lower temperature, causing water vapor inside the car to condense on the glass and form fog. 2. Window temperature below the dew point: When the air humidity is constant, if the window temperature is lower than the dew point, condensation is likely to form on the surface of the window, resulting in the common fogging phenomenon 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 will be lower than the surrounding environment's vapor pressure, causing water vapor to gather on the glass surface and form tiny water droplets, resulting in the usual fogging.

[VonAlexa]

Window fogging in summer is a common yet annoying issue while driving. The main reason is the high temperature causing people to sweat, and the air conditioning cools the glass. When warm, humid air inside the car meets the cold glass, moisture condenses into fog. After years of driving, I've observed that high humidity in summer, with external temperatures soaring above 30°C, causes the glass temperature to drop sharply when the AC is turned on, increasing the temperature difference and making fogging more likely. Additionally, having more passengers increases humidity through breathing and drinking. To prevent this, I usually turn on the external circulation for ventilation or pre-cool the car with the AC to dehumidify before starting, ensuring clear visibility for safety. If fogging occurs while driving, adjusting the air vents to blow on the windshield clears it up in minutes, so it doesn’t delay your journey.

[OJosie]

As a young driver who just bought a car, I initially thought my car was broken when the windows fogged up for the first time. After researching, I learned the simple principle behind summer window fogging: high external temperatures cause the glass to cool rapidly when the AC is on, while warm, moisture-laden air inside the car (like from breathing) condenses into fog upon contact with the cold glass. During humid summers, large temperature differences between morning and evening make this more likely. I've experienced blurred vision while driving, which scared me into slowing down. Now I know the solution: use recirculation mode to reduce humidity or crack a window for ventilation. Though a minor issue, it can have major consequences, so I remind myself to check my car's condition.

[DiMicah]

From a physics perspective, car window fogging is quite fascinating. In summer when temperatures are high, the air has higher humidity and can hold more water vapor. When the air conditioning is turned on, the window temperature drops below the dew point, causing warm, moist air to release moisture upon cooling, which condenses on the glass to form fog. A simple analogy is like water droplets forming on the outside of a cold drink placed on a table. The temperature difference inside the car is greater in summer, and the air conditioning intensifies the cooling effect, leading to frequent fogging. In practice, I've found that lowering the air conditioning temperature or changing the airflow direction can help reduce it. This principle applies year-round but is most prominent in summer.