Should You Use Cold or Hot Air to Defog Car Windows on Rainy Days?

[MaximusAnn]

Using cold air is more effective because when the outside temperature is higher or lower than the inside temperature, moisture will form on the windows. Alternatively, you can also open the windows to make the fog disappear. Here are the correct methods to remove fog from the windows: 1. Use the air conditioning cold air to defog: The most effective way to deal with foggy windows is to turn on the air conditioning (A/C) cooling, activate the external circulation, set the air direction to blow towards the windshield, and adjust to the maximum wind speed. 2. The dry cold air will blow directly onto the windshield, creating a 'air film' on the surface that prevents water vapor from condensing on the glass.

[Bo]

Last time during a heavy rainstorm while driving my child to school, my windshield suddenly became completely fogged up. I immediately pressed the defogger button with the fan and arrow symbol (the front windshield defogging indicator), and the automatic air conditioning defaulted to cold air at maximum fan speed. The fog cleared in about ten seconds—the principle is that cold air removes moisture quickly. But blowing cold air in the middle of winter is too uncomfortable—what should I do? Once the fog clears, you can switch to warm air for dehumidification. Just remember to turn on the external air circulation to ventilate; turning it off may cause the fog to return. A reminder: Never use warm air directly for defogging! Moisture meeting warm glass will make it even blurrier, which is extremely dangerous!

[LeMariana]

Fogging is essentially the condensation of moisture inside the car into tiny water droplets on cold glass. Having personally tested several methods: cold air works the fastest but is only for emergencies, prolonged use can cause joint pain; warm air provides stable dehumidification and keeps you warm, but requires a few minutes of preheating to take effect. Experienced drivers first use cold air for quick defogging, then switch to warm air once the glass is clear, keeping the AC button on to continue dehumidification. Cars with automatic climate control are more convenient—just press the defrost button, and the system automatically adjusts temperature and fan speed. Keeping a dry towel handy to wipe side windows in rainy weather is practical, and remember to identify the rear defroster button (the one with a square and wavy lines) for electric heating wire defogging.

[EvanFitz]

This is a common question from customers during car repairs. Simply put: Fogging occurs on rainy days or in winter due to large temperature differences. Using cold air to defog is the fastest because the compressor simultaneously dehumidifies. However, running the cold air continuously increases fuel consumption and feels uncomfortable. It's recommended to first use cold air to clear the view, then switch to warm air around 24°C + AC mode to maintain dehumidification. For owners with automatic air conditioning, pressing the MAX defog button is the most scientific approach. If fogging repeatedly occurs after turning on the warm air, it may be due to a clogged cabin air filter or a moldy evaporator causing dehumidification failure, requiring timely filter replacement. Some older cars lack automatic defogging functions, but manually turning on AC + external circulation + windshield mode can also solve the issue.