What Causes Persistent Fogging on the Windshield?
3 Answers
Window fogging is primarily caused by temperature differences. Temperature Difference: Due to the temperature disparity between the interior and exterior of the vehicle, windows are prone to fogging. This occurs because the window glass, being in contact with the outside environment, remains cooler, causing the water vapor inside the car to condense on the glass and form fog. When the window temperature is below the dew point at a certain air humidity level, condensation readily forms on the window surface, leading to the common fogging phenomenon on the windshield. Principle: If the temperatures on either side of the window differ, the surface with the lower temperature will have a lower saturation vapor pressure than the surrounding environment. This causes water vapor to accumulate on the glass surface, emerging as tiny water droplets that create fog.
When it comes to the windshield constantly fogging up, I’ve really experienced it firsthand while driving in damp and cold weather. This usually happens because the humidity inside the car is too high—our breathing, sweat, or even moisture brought in on rainy days can accumulate and condense into fog when it hits the cold glass. Another reason is not using the air conditioning’s external circulation mode, which keeps the interior air recirculating and traps moisture inside. I remember it being especially noticeable in winter: the cabin gets warm from the heater, but the outside of the glass stays icy cold, creating a big temperature difference that causes fog to form. To fix this, avoid leaving wet clothes in the car, turn on the defroster button as soon as you start the car, or manually switch to external circulation to let fresh, dry air in. Regularly checking the window seals is also crucial—if they’re leaking, outside moisture will seep in, making the fog even more stubborn and seriously compromising driving safety.
I take safe driving very seriously, and fogging up on the inside of the windshield is no small matter—it can obscure your view at any moment and lead to an accident. The reason is simple: excessive humidity inside the car, like when there are many people exhaling heavily, combined with poor ventilation, causes moisture to accumulate. Additionally, if the air conditioning system isn’t functioning properly—such as a clogged filter or a faulty fan—it can make the humidity even worse. Temperature differences also play a role; condensation is more likely when the outside of the glass is cold while the inside is warm. I remember one time during a heavy rainstorm last year when the window seals were worn out and let water in, soaking the entire interior. The fog wouldn’t clear for ages, and it was truly dangerous. So, I recommend developing good habits: turn on the auto-defog function of the AC when you start the car, switch to external air circulation promptly, and avoid eating in the car to reduce moisture. Regularly maintain the vents and defogging functions—safety comes first.
