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Mountain air pressure

By David, 12 November 2020 Activity

Holding up an empty drink bottle with mountain in the background.

What happens when you take air up a mountain?

Here’s a great excuse for a trip up a mountain! Grab some bottles and learn why it’s hard to breathe at the top of tall mountains.

You will need

  • 2 empty and dry soft drink bottles of the same size
  • A bag to carry the bottles in
  • A mountain, at least 100 metres taller than the surrounding area
  • A way to get to the top of the mountain (e.g. you could climb, or ask an adult to drive you)
  • Marker (optional)

outdoor hazard iconSafety: Ask an adult to go with you, and remember to be sun smart! If you’re going to climb the mountain take plenty of water to drink.

What to do

  1. This activity is sensitive to temperature. Start with the bottles at room temperature. For example, if you just got out of a hot car, wait five minutes for the bottles to cool down.
  2. Holding up an empty drink bottle.At the bottom of the mountain, put a lid on one of the bottles and keep the other one open.
  3. Go to the top of the mountain.
  4. Put a lid on the other bottle. (You may wish to label the bottles with a marker, so you know which is which.)
  5. Two empty drink bottles.Feel both bottles. Do they feel like they’re at the same pressure? (Don’t worry if the difference is not very noticeable – we found it was a lot better on the way back down.)
  6. Two empty drink bottles, one slightly crushed.Go back to the bottom of the mountain. Do the two bottles feel the same as each other, or different? Do they feel the same as they did at the top of the mountain? How do they look?

What’s happening?

Earth’s atmosphere is heavy. It might not seem like it, but air weighs something, and Earth has quite a lot of air. If you drew a one-metre square on the ground, there would be about 10 000 kilograms of air directly above that square. That’s about the weight of two elephants!

If you go upwards, by climbing a mountain or hopping in a plane, there will be less air above you pushing down, and that means less air pressure. For every 100 metres you climb, the air pressure drops about 1%.

When air pressure drops, the volume of that air increases by the same percentage. That means 100 metres up, what was a litre of air is now about one litre and 10 mL. That should be enough of a difference for you to feel it when you squeeze the bottle.

On the way down, the process reverses. For every 100 metres you go down, a litre of air becomes about 990 mL. Depending on how tall your mountain is, you might be able to see the bottle being crushed a bit by the increased pressure.

These percentages keep adding up the further you go. The top of Mount Everest is above more than two-thirds of Earth’s air, and the air there is not being squeezed very much at all so it’s thin and hard to breathe. One litre of air captured at Everest’s summit would shrink to about 330 mL if you took it back to sea level!

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  1. How would you convince people that air has volume and that volume can change without mass changing? Because the measurement of volume is taught in the maths curriculum, there are people who think that air doesn’t have volume because it doesn’t have a boundary (no values to substitute into a formula), or that its volume is conserved because its mass is conserved (and also because in 2D, when we cut up a shape, ‘area’ is conserved).

    1. There are some profound questions in there!

      I think volume changing without mass changing is the easier one. You could demonstrate that with a sponge. If the class doesn’t like that because ‘It’s only shrinking because you’re squeezing out all the air’, you could try putting a piece of sponge inside a syringe and blocking up the end of the syringe with hot glue or something.

      For the ‘air has volume’ question, a soft drink bottle full of air is not bad. If you have a second, crumpled bottle, half-full of air,you might be able to make some comparisons. I guess you need to make sure your examples involve air that is in a completely sealed container.

      I guess you could have a demo with a fishtank full of oil, and two beakers on the bottom. Fill one of them with water – how much water is in that beaker? How much oil is in the other?

      I guess the fundamental piece to the puzzle is the gas law PV=nRT. Maybe showing them a gas formula with volume in it will help, even if they don’t need to learn it just yet.

      I suspect that every student you have has a different problem, so you’ll probably need several different strategies, but hopefully one or more of these is helpful in some way.

      Good luck!

  2. Simple demonstration of different volumes at different pressures:
    Try using a hand pump (you know, the old kind you hold still with your foot while pushing a handle down with both hands) to pump up a car tyre. How much volume is in the cylinder of the pump? What is the change in volume of the car tyre as it is pumped up? (not much!)
    The pump cylinder (around one litre?) fills with air at normal atmospheric pressure, which is then squeezed into the tyre whose volume hardly changes.


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