Using Internal or External Air Circulation on Rainy Days

On rainy days, using internal air circulation (recirculation mode) is generally more effective. This prevents humid external air from entering the cabin, reducing windscreen fogging and maintaining a comfortable temperature. However, prolonged use can lead to stale air and drowsiness.

1. Fog Prevention: Internal circulation stops moist air from outside, reducing condensation on the windscreen. Pairing this with the air conditioning (AC) helps dehumidify the cabin.
2. Air Quality: After 10-15 minutes, switch to external circulation briefly to refresh oxygen levels, especially with passengers.
3. Defogging: If the windscreen fogs, briefly switch to external air and increase fan speed to clear it faster.

Modern cars like the BMW iX or Tesla Model 3 often have automatic climate systems that adjust circulation based on humidity. For older models, manual control is necessary.

Avoid using internal circulation exclusively for hours, as CO₂ buildup can cause fatigue. If driving in heavy traffic with exhaust fumes (e.g., ULEZ zones), temporary internal circulation is safer.

A mechanic’s view: Internal circulation is better initially to stop damp air fogging up your windscreen. But don’t leave it on too long – stale air isn’t great for alertness. If your car’s AC has a dehumidify function, use it alongside recirc to keep things clear. Older cars might need more manual toggling between modes to balance airflow and visibility.

On rainy days, it's generally advisable to activate the external air circulation to allow thorough air exchange between the interior and exterior of the vehicle. Besides preventing fog formation, rainwater significantly reduces airborne impurities and dust, making external circulation beneficial for passengers' respiratory health by providing fresher air.

However, using external circulation during rainfall may allow water ingress. This system draws air through the window vents, creating continuous air circulation within the cabin. During prolonged rainy drives, precipitation may gradually enter with the airflow, causing interior humidity to increase over time. In such cases, switching to external circulation becomes necessary to replenish fresh air.

Motorists may initially engage internal circulation when activating climate control, as this rapidly adjusts cabin temperature to desired levels. Once optimal temperature is achieved, external circulation can be engaged - though this should only be used when external air quality is good. During heavy pollution episodes, such as in traffic congestion, external circulation should be disabled in favour of internal.

Internal circulation effectively blocks pollutant entry, while external circulation introduces fresh outside air. For short journeys under an hour, internal circulation usually suffices. On longer drives, switching to external circulation for approximately hourly intervals provides adequate ventilation.

To summarise, engaging external circulation during rainfall facilitates proper air exchange between vehicle interior and exterior.