Should You Use Internal or External Air Circulation for Defogging on Rainy Days?
4 Answers
If you want to quickly defog, it is recommended to turn on the external air circulation. If it's not raining heavily, you can also open the windows slightly for faster defogging. Below is an introduction to homemade and professional defogging sprays: Defogging with Homemade Spray: Mix dish soap and water in a ratio of 1:10 (or dishwashing liquid and water in a ratio of 1:6), then apply it to the inside of the front and rear windshields with a soft, clean cloth. After it dries, wipe off any remaining watermarks on the inside of the glass. This method can generally keep the glass fog-free for up to 20 hours, is simple, practical, and cost-effective. The effect is comparable to commercial defogging products—just spray and wipe to solve the problem. Defogging with Professional Defogging Spray: Before using a defogging spray, first clean the inside of the front windshield thoroughly. Then, spray an appropriate amount of the defogging agent and wait for it to solidify before wiping it off with a clean towel. The defogging agent forms a hydrophobic layer between water molecules and the inner surface of the glass, keeping the glass surface clean and preventing fogging.
When driving long distances on rainy nights, the windshield often fogs up, which can be nerve-wracking. At first, I tried using the recirculation mode, thinking it would help retain warmth, but the fog accumulated even faster because the humidity inside the car was being recirculated. Later, I learned my lesson and always switched to fresh air mode—letting outside air in. Even though it's humid outside on rainy days, using the air conditioning (AC) in combination helps absorb moisture, clearing the glass in just a few minutes. Don’t worry about fuel consumption; just set the AC to defog mode. Once I got into the habit, I would also turn up the fan speed first and direct it at the windshield. Safety comes first when driving in the rain—fog severely impairs visibility, so quickly switching to fresh air mode is a simple and more reliable solution than recirculation.
I've done quite a few experiments with defogging because I'm a car enthusiast. From a technical perspective, you should use the external air circulation system on rainy days to clear fog. The reason is simple: internal circulation only recirculates cabin air, allowing humidity to accumulate and actually increasing fogging. External circulation introduces fresh, relatively dry outside air, and when combined with air conditioning for dehumidification, it can quickly reduce fog. Remember to set the fan to the windshield position and increase the fan speed to accelerate airflow. Using internal circulation for extended periods can also degrade cabin air quality and cause fatigue. I recommend checking your air conditioning settings before each trip, prioritizing external circulation in rainy conditions for safety.
As a parent who often drives kids to school, I know how dangerous fogging can be in rainy weather – poor visibility makes everything blurry. I tried using the recirculation mode to defog, but the mist lingered for ages. Later, I learned that the fresh air mode is actually the right choice. It brings in dry outside air, and when paired with the AC blowing for a while, the windshield clears up. When operating, don’t just turn on the fan—activate both the AC and heater simultaneously. Make it a habit to adjust the settings a few minutes before driving in the rain. Safety outweighs comfort; pulling over to clear fog is a lifesaving move. A simple mode adjustment can significantly reduce accident risks.
