Why Should You Use the External Air Circulation in the Car on Rainy Days?

[JayceDella]

On cold rainy days, the windows can fog up, affecting visibility. Therefore, it is best to first use the external air circulation and adjust the airflow direction toward the windshield. Turning on the external circulation helps expel the moisture exhaled by passengers from the car, while the outside cold air is less likely to condense. Important Notes: 1. When the outside air quality is poor or heavily polluted—such as on dusty roads, in congested traffic, or near chemical plants with foul odors—use the internal air circulation to prevent contaminated air from entering the car. 2. In summer, to quickly cool down the car with air conditioning, you can use the internal circulation. 3. In winter, using the internal circulation can help keep the car warmer.

[LaStella]

Driving with the external air circulation on during rainy days is mainly to prevent window fogging that can obstruct vision. Having driven for so many years, I understand that during rain, the air humidity is high. If the internal circulation is turned on, the air inside the car gets recirculated repeatedly, causing moisture to accumulate and more easily condense into fog on the windows. Once fogging occurs, visibility of the road is reduced, increasing the risk of accidents. Turning on external circulation introduces outside air; even though it's raining outside, the fresh air flow helps carry away moisture, keeping the car interior dry and ventilated. I remember one rainy day when I mistakenly used the internal circulation, and the windows quickly fogged up. I was frantically wiping them for a long time and almost got into danger. Since then, I always set the air conditioning to external circulation—safe and hassle-free. I also recommend using the windshield wipers and the air conditioning’s dehumidification function together for double the effect. Remembering these small details can greatly enhance driving safety in the rain and avoid unnecessary trouble. Regular vehicle maintenance is also crucial.

[Athena]

Every time it rains while driving, I don't hesitate to turn on the external air circulation. The reason is simple: it keeps the air inside the car comfortable and prevents fogging. When using internal circulation, the air only recirculates inside, trapping moisture, and the windows immediately become blurry, making it stuffy and uncomfortable. External circulation brings in fresh air, promotes ventilation, and reduces humidity. I usually focus on comfort while driving, and on rainy days, the air outside is often stale—using external circulation makes breathing easier and reduces fatigue. With long-term use, the windows stay clear and bright, minimizing bacterial growth. My personal tip is to switch to external circulation right after starting the car, rather than waiting for fog to form. Pair it with the fan blowing on the windshield, and the fog disappears quickly. Rainy days already make roads slippery, so clear visibility makes driving more relaxed and enjoyable. After making this a habit, my driving experience has improved significantly. This approach can also be useful in other conditions, such as foggy weather.

[BradyAnn]

I remember when I was first learning to drive, I didn't know to use the external air circulation on rainy days. I turned on the internal circulation instead, and the windows fogged up badly. I couldn't see anything and was really scared. Later, I learned that using external circulation on rainy days is because rainwater increases air humidity. Internal circulation traps moisture inside the car, while external circulation brings in fresh air to dehumidify and prevent fogging. Now I always switch to external circulation when it rains, and it really works. Much safer. New drivers should pay attention to this point and avoid making the same mistake I did. Simple and practical.