What Causes Fogging on Car Windows?
3 Answers
Fogging on car windows occurs due to the temperature difference between the inside and outside of the vehicle. The window glass, being in contact with the colder external environment, causes the moisture inside the car to condense on the glass, forming fog. For minor fogging, opening the windows to ventilate and remove the humid air inside can help. For heavier fogging, using absorbent paper or a towel to wipe the windows is effective, or you can apply a professional anti-fog spray. Methods to defog car windows: The car's air conditioning system has a defogging function. Turn on the automatic air conditioning and set the mode switch to the defogging position. The air conditioning will automatically switch to external circulation, allowing fresh air from outside to enter the cabin and begin the defogging process. For manual air conditioning, first adjust the air direction switch to the defogging position and activate external circulation to let fresh air in. Increasing the compressor and fan speed can speed up the defogging process.
Having driven for over a decade, I find foggy windows on winter rainy days most annoying, mainly caused by the temperature difference between inside and outside the car. For instance, in winter when the heater keeps the interior at around 20°C while it's just a few degrees outside, the moisture from your breath instantly condenses into tiny droplets upon hitting the cold windshield. Rainy days are especially prone to fogging because evaporating rainwater increases interior humidity, and opening doors brings in more damp air. I remember one family trip where four adults and kids were packed in the car—within minutes the windows turned completely white, all from exhaled moisture. If window seals age or doors don't close tightly, outside humid air sneaks in, worsening the fogging. The solution is simple: the AC defogger works fastest, or crack windows slightly for airflow to save fuel.
Last week, while driving my mom to the hospital, I realized how dangerous fogging can be, especially when the windshield fogs up and makes it nearly impossible to see the road. After consulting a mechanic, I learned the principle is quite simple: just like how a cold soda can 'sweats' when taken out of the fridge, fog forms on car windows when their temperature drops below the dew point of the interior air. There are two key factors: temperature difference and humidity. Fog can also appear after a summer rainstorm because humidity skyrockets, and blasting the AC too hard can cause the glass temperature to drop suddenly. Spilled drinks or wet umbrellas in the car add to the problem by evaporating moisture. Turning on the AC is the fastest way to defog, but avoid using recirculation mode, as it just cycles the humid air inside. Rubbing the glass with soapy water can actually keep fog away for a couple of days and costs just pennies.
