It’s a familiar scene that many people have to contend with during the summer: returning to your car after several hours away, opening the driver’s door, and feeling the blast of hot, sticky air inside.
Of course, it’s tempting to immediately turn on the air conditioning, but that method is likely to take a while to make a noticeable difference to your car’s temperature. Instead, consider a different approach, inspired by the principles of fluid dynamics (the study of how liquids and gases behave in motion).
This simple technique involves lowering a window on the passenger’s side of the car and then quickly opening and closing the driver’s door. Repeating this “fanning” process several times creates an area of low pressure outside of the driver’s door, resulting in the hot air quickly exiting via the open door, while fresher and cooler air comes in through the window. This trick for enhancing airflow has recently gone viral on social media, thanks to an Instagram video posted by University of Cambridge math professor and BBC presenter Dr. Hannah Fry.
However, although this method is effective at removing much of the hot air trapped in the car, its cooling potential is limited by the outside air temperature. Ideally, after fanning the door several times, you’ll want to get in your car, close any open windows, turn the air conditioning on, and start driving. The motion of the vehicle will allow the A/C system to start doing its job by transferring heat as wind passes over the condenser.
More about cooling off your car:
- Thrillist also covered this topic in a 2016 article. Texas-based writer Aaron Miller tried four different methods to cool down his car in the summer heat. He found that the least effective method was to get into his car, turn on the A/C at full blast, and wait. According to his calculations, turning on "MAX A/C" in a stationary vehicle cooled the inside temperature by 1.8°F per minute during the first 10 minutes.
- On another sweltering day, Miller tried turning the air conditioning to maximum and immediately starting to drive. He found that it was better to leave the windows up rather than rolling them down, which lets too much cold air out and delays the cooling process. Driving with the windows down and air conditioning on at full blast resulted in a temperature decrease of 2.5°F per minute during the first 10 minutes. Notably, when he repeated this the next day but kept the windows closed, the temperature cooled by 3.1°F degrees per minute during the first 10 minutes.
- Just as discussed in Dr. Fry's viral video, Miller found that the most effective method was repeatedly opening and closing the driver’s side door (he actually “fanned” the door around six times) with a passenger-side window open. After this, Miller started driving with the windows up and the air conditioning on maximum. The temperature changed by 3.6 degrees per minute in the first 10 minutes, making this method the clear winner.
