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Q & A: mountain top air pressure

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Most recent answer: 02/09/2008
Q:
i took a plastic bottle up to the high mountains and emptied my drink up there. i closed the cap and when i brought it down the mountain, the bottle "concaved" in. i know it"s something about pressure but i do not really understand.
- crystal (age 13)
singapore
A:
You're right that the pressure (and density) are higher at the bottom of the mountain than at the top. So when you sealed the bottle at the top and brought it down, there was more pressure outside than in. That means more force pushing in on the sides than pushing out, so the sides caved in.

 So why is the pressure lower higher up? Actually, you can think of the question a little differently. Think of water. It flows down as low as it can go. Air tends to concentrate lower-down, but not to the same degree as water, not to the point where the molecules are squeezed into contact.

Each individual air molecule is more likely to be found at the foot of the mountain than at the top, as it wanders around. The reason is that at the bottom it has lower gravitational energy (the same reason things fall), and the extra energy can get out to increase the number of states available to the rest of the world. So the same thing that makes the air spread out (getting to as many states as possible) also tends to make it settle lower down. We can calculate just how the trae-off goes and how the concentration should (and does) decrease with height.

Mike W.

(published on 02/09/2008)

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