Why is there frost on the inside of car windows?

[Charlie]

Frost forms on the inside of car windows because: after parking, the humidity inside the car is relatively high. As the car's temperature drops, the moisture in the air inside the car condenses on the windows when it encounters the cold. When the temperature drops to a certain level, the water droplets adhering to the windows freeze into ice. Car windows are an important part of the vehicle body, designed to meet the needs of interior lighting, ventilation, and the visibility of the driver and passengers. Methods to remove frost from the inside of car windows include: 1. Using a hard plastic scraper to chip away the ice; 2. Spraying a de-icing agent onto the windows; 3. Covering the windows with a cloth after parking; 4. Turning on the vehicle's warm air to blow on the windows once the engine's coolant temperature rises; 5. Opening the doors to ventilate after parking to equalize the interior and exterior temperatures.

[VonRobert]

Frost forming on car windows in winter is extremely common, especially for vehicles parked outdoors overnight. I've figured it mainly comes down to these reasons: First, the moisture from your breath and dampness on your body accumulates inside the car after you get in, then temperatures plummet after parking at night. Second, aging door seals or clogged sunroof drainage holes allow rainwater to seep in, increasing humidity. Third, leaving wet items like umbrellas or snow boots in the car is like having a built-in humidifier. Once I forgot to remove a damp down jacket, and the next morning the entire windshield looked like frosted glass. Now I've learned my lesson - I run the external air circulation for a few minutes before parking to expel moisture, and keep a box of silica gel on the dashboard.

[OHeidi]

Window frost formation is a matter of physics: when it's below freezing outside, residual warm air inside the car condenses upon contact with the cold. The most common cause is failing to fully vent interior humidity when parking—like bringing in moisture by driving with windows open on rainy days, or heading home immediately after washing floor mats. My routine is parking before sunset, preferably in a garage or sunny spot. I remotely start the car five minutes before departure to let the AC warm air defrost the windshield first, and keep an old bank card as a scraper. For emergencies, an anti-fog spray from Taobao applied with a cloth lasts 2-3 days. Never pour hot water on glass—the thermal shock can crack it, as my friend learned the hard way when his windshield shattered.

[BarrettDella]

My old Focus does this every winter, and I've figured out the pattern: the inside of the car is warmer than the outside at night, causing moisture to condense and frost on the cold glass. Sometimes it's because I forgot to switch from recirculation mode when sleeping, and other times it's due to cracked door seals letting in snowmelt. Now I do three things: turn off the heater and switch to outside air circulation for five minutes before parking at home; hang moisture-absorbing bags bought from Taobao on the rearview mirror; and empty any wet items from the trunk. If frost does form, I turn the heater to maximum and aim it at the windshield, then scrape the edges with a credit card from my wallet—usually, the road becomes clear in three to five minutes.