2026年10月13日 / Last updated : 2026年10月12日 桑子 研 Science

Experiment! What Happens When You Take a Bag of Potato Chips Up a Mountain? Almost Bursting at 2,237 m?! (The Mystery of Air Pressure)

Hi, I’m Ken Kuwako, your Science Trainer. Every day is an experiment!

I pulled a bag of potato chips out of my backpack on a mountaintop at 2,237 meters, and it was puffed up like a balloon. It looked perfectly normal when I bought it at the supermarket down below. So what happened? And the setting was just as strange: Tsuboniwa in the northern Yatsugatake range, a garden strewn with lumps of lava.

Taking a Bag of Chips Up to Tsuboniwa, 2,237 m Above Sea Level

I recently visited Tsuboniwa in the northern Yatsugatake mountains, which sits at an elevation of 2,237 meters. To get there, I rode the Kitayatsugatake Ropeway, which whisks you from the base station at 1,771 meters up to the summit station in about 7 minutes.

For the experiment, I brought along a bag of potato chips I’d bought at a supermarket in Tokyo (only a few meters above sea level). The question: would it puff up on the mountain?

The Results! The Bag Was Puffed Up Almost to the Bursting Point

And the verdict is… it swelled up so much that it looked ready to burst! I’m thrilled!

I also took a bag of chips along when I visited the Nobeyama site of the National Astronomical Observatory of Japan. Here’s how the bags looked in Tokyo compared with Nobeyama…

Why the Bag Puffs Up: There’s Less “Weight of Air” Pressing on It

The air around us has weight, and the force that weight exerts on everything is called air pressure (atmospheric pressure). Near sea level, the whole column of air above you is pressing down, so the pressure is high. As you climb a mountain, there’s less air above you, so the pressure drops. In a standard atmosphere, the pressure at 2,237 meters is roughly 70 to 80 percent of what it is near sea level, or about 770 hPa.

A bag of chips is sealed, so the air inside can’t escape. Down at the base, the air inside the bag and the air outside pushed against each other with equal force, so everything was balanced. Up on the mountain, though, the push from the outside gets weaker. The air inside hasn’t changed, so now it’s pushing outward harder than the air outside is pushing in, and the bag swells up.

This can be explained by Boyle’s law: at a constant temperature, the volume of a gas is inversely proportional to its pressure. If the pressure drops to about three quarters, the volume grows to about 1.3 times its original size. In reality, other things come into play, like the colder temperatures up the mountain and how stiff the bag material is. Even so, seeing the very same bag visibly swell up is pretty impressive, isn’t it?

By the way, many snack bags are filled with a gas like nitrogen to keep the snack from oxidizing and to protect it from getting crushed. The bags are still sealed, so they puff up in basically the same way.

Tsuboniwa: A Giant Garden Made of Lava

Tsuboniwa, the star of this experiment, is actually a strange landscape created by a volcano. Mt. Kita-Yokodake in the northern Yatsugatake range is an active volcano, and it is said to have spewed out about 3 million cubic meters of lava in the past, roughly the volume of two Tokyo Domes. That lava cooled and hardened into a lava plateau covered with huge, rugged black rocks, spreading across about 330,000 square meters.

In the gaps between the lava rocks, Siberian dwarf pines grow thick, and alpine plants such as lingonberry and crowberry (ganko-ran) carpet the ground. It looks like a giant garden created by nature itself.

Interestingly, Tsuboniwa isn’t high enough to be in the alpine zone, where trees can’t grow at all. Even so, there are few tall trees and you see dwarf pines and alpine plants instead, which is explained by the fact that the lava flowed out relatively recently. That said, it seems nobody knows exactly when the lava erupted. As you look at the rocks beneath your feet, it’s fun to imagine what this volcano looked like long, long ago.

References

・Kitayatsugatake Ropeway official website (https://www.kitayatu.jp/)
・Introduction to Tsuboniwa on the travel info site “Tabimagajin” (https://tabi-mag.jp/nn715/)

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