Jenny Brown

Sun, August 10, 2025

Yahoo is using AI to generate takeaways from this article. This means the info may not always match what's in the article. Reporting mistakes helps us improve the experience.Generate Key Takeaways

Getting into a car that's been baking in the heat is among the most unpleasant parts of summer. The air inside a car can get much hotter than the air outside—according to one study, a car's interior can reach 99°F in just 10 minutes on an 80°F day—and it also grows stale and stuffy from being trapped inside.

Well, a little thing called fluid dynamics can help sweep out the stifling air and usher in fresh air. Hannah Fry, a mathematics professor at the University of Cambridge, recently dazzled the internet with a social media video explaining the phenomenon. "Don't bother putting on the aircon," she says in the video (that's British speak for "air conditioner"). "That would take five minutes. It's not worth it."

Fry then demonstrates the trick, which she says she uses regularly, though she warns it will make you look a bit goofy. Check out the video below, and read on for a how-to.

How to Do It

As Fry shows in the video, with the car parked, roll the passenger side rear window all the way down. Then, standing outside the car, rapidly open and close the driver's side door a few times. Simple!

What's going on? Fry, who has a PhD in fluid dynamics—the study of how liquids and gases, including air, move—gives a brief explainer in the video. "When you open and close the door, especially if you do it quickly," Fry says, "the door, as it's moving outwards, sweeps out all of the air that's in its way."

With the "hot, sweaty, horrible air inside the car" drawn outward, she adds, there's now an area of low air pressure by the driver's seat. Low-pressure areas (where air is moving up and away) create a kind of vacuum. "And the only way to refill it is with nice, fresh, easy air" that comes in through the rear window on the opposite side. The phenomenon of air swooping into a low-pressure zone—which is also how wind works—is called bulk flow.

"All it takes is about two or three goes" of opening and closing the door, "and suddenly it's absolutely beautifully temperate," Fry says in the video.

Does It Really Work?

According to many comments on the video (which is on Instagram, TikTok, and YouTube), yes. "Glad there's actual science to back me up when I do this because I feel so silly doing it but it 100% works!!" @Kate on TikTok wrote.

Many other commenters say they didn't feel an effect, though. I've tried it a few times myself this summer and haven't noticed a difference in the temperature. Since Fry wasn't available for comment, I asked car experts to weigh in on why I wasn't having success.

To begin with, if the outside air is also stiflingly hot—as it has been during my state's extended heat wave—you're unlikely to feel the nice, fresh, easy air Fry mentions, points out Robert Dillman, the founder of NEVO Driving Academy in Florida. "If the air inside and outside the vehicle is similar, the effect of cooler air exchange will be felt significantly less," he says. Commenters on the video noted this as well: "In Arizona, you're exchanging 140°F air for 110°F air," wrote @thelougalan on Instagram.

Secondly, "interior materials make a huge difference in how long cars stay hot," notes Marc Skirvin, president of Cash Auto Salvage, who says cars that have been sitting outside in his lots can reach 150°F in the summer. While cloth seats cool down faster than dark leather seats, "here's what really gets people: the dashboard and steering wheel," he says. "That plastic dashboard acts like a heat radiator, pumping hot air into the cabin long after you've started cooling it down."

(By the way, that greenhouse effect—i.e., the car's interior surfaces converting the sun's light into heat, which gets trapped inside—is why cars get so hot in the first place.)

Skirvin says that the door-pumping method can work for him, but it takes 30 or 40 pumps to make a difference, "and you look ridiculous doing it." Plus, over time it will wear out the door's hinges, he notes.

Both Dillman and Skirvin recommend using the good-old AC to cool down your car. But Fry's method is still worth a shot—it might work for you and, at the very least, you can impress your friends and confound your enemies with your knowledge of fluid dynamics.