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

[Kylie]

It is recommended to use the external air circulation mode in the car on rainy days. In cold rainy weather, both side windows tend to fog up, which can obstruct the driver's vision. Therefore, first adjust the external circulation airflow to direct it toward the windshield, then activate external circulation to expel the moisture exhaled by passengers out of the vehicle. Below are situations suitable for using external circulation: 1. When internal circulation has been used for an extended period and opening windows is inconvenient, periodically switch to external circulation to allow fresh air into the cabin. 2. When there are multiple passengers in the car, using external circulation helps maintain air freshness while preventing excessive humidity that could cause window fogging.

[MacAllie]

On rainy days, I generally recommend using the external air circulation mode as the primary setting, as it effectively prevents window fogging. Fogging occurs due to the significant temperature difference between the inside and outside of the vehicle. Activating external circulation draws in outside air, helping to balance humidity and dissipate fog—especially effective during light rain or right after it stops. However, during heavy downpours when external air is excessively humid, prolonged use of external circulation can make the cabin stuffy and damp. In such cases, I briefly switch to internal air circulation for a few minutes to prevent moisture buildup, while simultaneously engaging the A/C mode for dehumidification. Safe driving is crucial, as poor visibility increases accident risks. Hence, I constantly monitor rainfall and fog conditions to decide: when fog appears, I promptly activate external circulation with airflow; when rain intensifies, I switch to internal circulation. Additionally, regular replacement of the cabin air filter is essential—otherwise, poor airflow can cause dizziness. I learned this the hard way last rainy season when my windows fogged up badly in traffic, and timely external circulation saved the day. In short, flexible adjustment is key—avoid rigidly sticking to one mode, prioritizing safety and comfort above all.

[MacEmiliano]

I pay attention to the air quality inside the car while driving. On rainy days, I mostly use the recirculation mode to prevent external humid air and rain odors from seeping in, protecting my family's health and avoiding allergies. However, if the windows fog up, I switch to the fresh air mode to quickly defog them. I also use the A/C button to speed up dehumidification—without A/C, relying solely on the fan makes defogging slower. Prolonged use of recirculation mode can make the air stuffy and cause dizziness, so I open the windows for a few seconds to ventilate during breaks in the rain, though this isn’t practical during heavy rain. Comfort is crucial, especially when traveling with children, and controlling humidity inside the car helps prevent mold. My personal habit is to keep some defogging spray on hand to reduce frequent mode switching. Maintaining the air conditioning system is also important—a dirty filter affects efficiency, so I recommend checking it monthly. Safety comes first when driving in the rain—act immediately when fog appears and don’t hesitate.

[McHayden]

I usually turn on the exterior air circulation to prevent fogging when driving in the rain, as fogging is a common issue. However, when the rain is heavy, the external humidity is high, and using exterior air circulation can make the car interior damp. In such cases, it's better to switch to interior air circulation to avoid this. Using the air conditioning with the A/C button is effective for dehumidification. When the rain is light, exterior air circulation can be used for ventilation. New drivers should prioritize safety. Keeping a defogging cloth in the car for backup is convenient.

[DeDante]

The principle of fogging on rainy days is caused by the temperature difference between the inside and outside of the car. Turning on the external circulation introduces outside air to balance humidity and eliminate fog, and it's best to turn on the A/C to enhance dehumidification. However, if the outside air is too humid, prolonged use can make the car damp. Using internal circulation prevents moisture but has poor air circulation, which can easily cause dizziness. My suggestion: When fogging is severe, use external circulation with A/C. After the fog dissipates, use external circulation for ventilation when the rain is light, and switch to internal circulation when the rain is heavy. The automatic air conditioning mode is convenient, and turning on the A/C activates the compressor for quick cooling. Regular system maintenance prevents malfunctions.

[OPaul]

Many people habitually use only the recirculation mode on rainy days to prevent water ingress, but this mistake can lead to stale air and carbon dioxide buildup. Alternatively, excessive use of fresh air mode introduces moisture. Personally, I handle it with balance: when fogging starts, I use fresh air mode to defog, turning on the A/C for enhanced effect; when the rain intensifies, I switch to recirculation to avoid prolonged moisture retention. Setting the air conditioning to the windshield defrost mode helps quickly clear fog. A common mistake is ignoring external rainfall—I adjust based on real-time road conditions, prioritizing anti-fog for visibility at high speeds. Keeping defogging tools like a damp cloth handy for emergencies is wise. Always focus on safe driving without distractions.