What Causes Fogging on Car Windows in Rainy Weather?

[LeCatherine]

Fogging on side windows during rainy weather can be addressed using toothpaste, water repellent, or air conditioning defogging. Below are specific solutions for fogging: On rainy days, evenly apply toothpaste or soap to the car's side windows and let it sit for a while until the applied soap hardens. Then, wipe it clean with a towel. This will form a wax layer on the glass surface, which has excellent water-repellent properties and can effectively prevent fogging or water droplets on the rearview mirror, ensuring clear visibility. Purchase a specialized water repellent and evenly spray or apply it to the rearview mirror. The rearview mirror surface will form a protective film, which also has excellent hydrophobic properties, effectively preventing fogging or water droplets and ensuring a clear rearview mirror. Turn on the car's air conditioning to cold air and adjust the vents to blow toward the front windshield to quickly clear fogging. For a more thorough solution, turn on the air conditioning to hot air before starting the car and blow it directly at the front windshield for 3 minutes. This can fundamentally prevent glass fogging. Air conditioning cooling can also dehumidify and achieve the fastest defogging effect. The driver can turn on the air conditioning's cooling mode and adjust the air flow to the windshield setting. The fog should disappear in about ten seconds.

[MacMaddox]

I've been driving big trucks for long hauls for over 30 years, and foggy windows on rainy days are all too familiar—it's all caused by temperature differences. On rainy days, the cold wind and rain outside chill the windows, while we sit warm inside exhaling moist air that condenses into fog when it hits the cold glass. Turning on the heater or AC makes it worse, as warm air carries more moisture, thickening the fog. I still remember nearly hitting a tree in the mountains last year because the fog blocked my vision—it was terrifying. Now, as soon as I get in the truck, I switch the AC to external circulation, directing the airflow at the windows to dehumidify and turning up the temperature to speed up water droplet evaporation. I also keep an anti-fog spray handy and apply it regularly. Don’t get lazy when driving in the rain—safety comes first. Everyone should practice defogging techniques beforehand, not just when trouble strikes.

[DelFernando]

As a new driver, I was completely at a loss when the windows fogged up on rainy days. After researching, I understood: the high humidity in rainy weather cools the exterior glass with rainwater, while our breathing or damp clothes inside release moisture, creating a temperature difference that causes water droplets to condense into fog. Initially, I foolishly tried wiping the fog away by hand, only for it to reappear. Later, I learned to use the AC for cooling and dehumidifying or the warm air on external circulation to quickly defog. Setting up the air conditioning mode on my phone helps reduce mistakes. During summer rains, avoid keeping the internal circulation on for too long; opening windows for ventilation also helps. Poor visibility on the road is extremely dangerous, so new drivers should drive slower and check the car's condition more frequently, ensuring the wipers and defogging system are in good working order.

[Abigail]

I usually drive a sedan to pick up and drop off my kids at school, but the foggy windows on rainy days are so annoying. High humidity outside combined with the cold rain on the windows causes moisture inside the car—like from kids breathing or wet umbrellas—to condense on the glass, creating fog. This seriously affects driving visibility and safety. I always turn on the air conditioning in dehumidifier mode or adjust the vents to blow air toward the windows, which clears the fog in just a few minutes. For prevention, I spray anti-fog solution weekly to keep the windows clean, avoid leaving wet items in the car, and open the doors to ventilate before heading out on rainy days. Safety first—don’t overlook these small things. Stay-at-home moms all know these little tricks save both money and effort.