Field of Science

Weird Words of Science: Hypsometer

Every time I write an exam, I think about this story, where a physics professor asks on an exam how to measure the height of a building using a barometer. A student answered that he would tie a string to the barometer, lower it down, then measure the length of the string. Given no credit, he protests, and the professor offers him a second chance to provide an answer that is both correct and demonstrates some knowledge of physics taught in the course. The student goes on to give several answers (in some versions the student is averred to be Niels Bohr - though the origin of the story is apparently in a textbook on the teaching of math and science by Alexander Calandra, and unrelated to Bohr) all demonstrating a knowledge of physics, and none the one he seems to know the professor is fishing for (which has to do with the - probably unmeasurably small - pressure differential between the ground and the top of the building).

Here is a chemistry exam question I sometimes ask - how would you measure the height of a mountain with a thermometer? This is a well-known technique,not a trick question, the apparatus is called a hypsometer, from the Greek for "height-measure". The underlying science is that the boiling point of a liquid changes in a known way with altitude. Hypsometers were used before portable aneroid barometers became widely available, and were used in high altitude balloon measurements of pressure as late as the 1960s.

Bonus question: Is it easier to drink a liquid using a straw at the top of Mt. Everest or on the beach in Florida? (Disregard temperature differences and explain your answer for full credit!)


  1. In the spirit of the story: On the beach in Florida.

    Why? Because the oxygen apparatus would get in the way at the top of Everest...

  2. The atmosphere is more rarefied at the top of Mt. Everest (high altitude, low pressure).. Hence, it would be easier to drink using a straw..

  3. Well the density of the liquid doesn't change but the pressure of the air does. At altitude, air pressure is lower than at sea level. Therefore, the pressure in the mouth needs to be reduced by a greater amount than at sea level because there is a lower atmospheric pressure to push the liquid up the straw. It is harder to drink a liquid through a straw at the top of Everst than on a beach in Florida.

  4. Other than the breathing equipment, it doesn't matter. The change in pressure needed to change the liquid level a fixed amount (the amount the straw is above the surface) is the same whether on the beach or on a mountain.

  5. I reckon pretty much everything is harder on top of Everest, due to being exhausted and short of oxygen.

  6. Are you going to tell us the answer?

  7. Neil - full points for style!

    Matt has nailed it. A straw in a glass with one end sealed (in this case by your mouth) is essentially a barometer. The pressure at the top of Everest is low, so you get less "help" pushing the liquid up the tube. (Kathryn, the liquid in the straw would rise less above the surface of the liquid under lower external pressure.)

    Overall, I'd rather be in Florida than at the top of Everest for a plethora of reasons!! Even given the humidity....

  8. Ah got it - smaller change in pressure due to lower external-to-the-straw pressure. Cool!

    I love a puzzle although I'm bummed I got it wrong; engineers tend to think only in inches of head pressure without reference to external pressure. My scientist brain pleads contamination.

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  10. No but yes. At sea level you get more help from the ambient pressure to push the liquid into the straw; but this cancels out with the fact that your jaw, chins and tongue have to do work against the same higher air pressure in order to create suction. It's not as if "create an absolute pressure of 28 kPa inside your mouth" is as easy in Florida as it is on Everest.

    Assuming that the density and viscosity of the liquid do not change significantly with pressure, the same pressure difference moves liquid through the straw at the same rate, and the same tension in jaw/chin/tongue will create the same pressure difference.

    Since we're ignoring temperature differences, the only remaining effect seems to be that the liquid has a larger buoyancy in the dense Florida air, so it will take (very slightly) less work to lift it from cup height to mouth height.


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