Why Does Fog Form on Car Windows?
3 Answers
Explanation for fog formation on car windows: 1. Causes: Fog tends to form on car windows due to the temperature difference between the interior and exterior of the vehicle. When the window glass, which is in contact with the colder outside temperature, causes water vapor inside the car to condense on the glass, forming fog. When the window temperature falls below the dew point at a given air humidity level, condensation readily forms on the window surface, resulting in the common fogging phenomenon observed on the windshield. 2. Solutions: Using anti-fog spray: Apply a small amount of defogger to the car windows and wipe it clean. This not only polishes the glass but also forms a thin transparent protective film that effectively prevents moisture from condensing into fog on the glass. Utilizing air convection: Seasonal temperature variations can create significant differences between the inside and outside of the car. When warm, moist air inside the car meets the cold windshield, it releases heat and liquefies, easily forming fog. To prevent this, slightly opening the window allows air to circulate between the inside and outside of the car, balancing the temperature difference and reducing fogging. Using the car's heating system: Turn on the warm air conditioning and direct the warm air towards the windows. This quickly raises the temperature of the front glass, eliminating the conditions that cause warm, moist air inside the car to release heat and liquefy on the windows, thereby preventing fog formation.
I have so much experience with foggy windows while driving, especially when taking kids out in winter. When there are more people in the car, the warm breath hits the cold glass and instantly turns into a layer of white fog. The windshield is the most obvious, and the rearview mirror gets completely blocked. I usually turn on the air conditioning defroster right away, letting the cold air blow onto the glass. If the rear windows fog up, cracking the opposite window slightly works wonders by allowing air circulation inside the car. Another little trick is to wipe the glass with a mixture of dish soap and water—it keeps the fog away for days. I’ve heard some people buy anti-fog sprays, but I find using the AC the easiest solution.
A seasoned driver tells you that fogging is all about the temperature difference inside and outside. When it's cold outside and warm inside, the glass in between becomes a condensation panel. I used to encounter this issue often when hauling goods, with the truck filled with fresh produce and the fog so thick I couldn't see the rearview mirror. Turning on the AC cools things down quickly but makes it too cold, while using the heater is slower but more comfortable. Blow hot air at the windshield for two or three minutes, and the fog will turn into water droplets. Remember not to turn off the AC too soon, or the droplets will turn back into fog. Using the wipers won't clear the fog inside—you need to crank the fan up to the highest setting.
