Science · 6 pages · 12 min
Why water boils at a lower temperature on a mountain
On a mountain the air presses less, so water boils below 100 degrees Celsius.
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Water boils when its vapor can push back as hard as the air pressing on the surface. On a mountain the air column is shorter, the pressure is lower, and that match happens below the sea-level boiling temperature.
The mountain does not supply a weaker flame, and the water is not a different substance. A thermometer in a rolling boil up high reads lower than a thermometer in a rolling boil at the shore, even if both pots are raging.
The mechanism is more particular than the slogan. Molecules leave the liquid all the time, but a bubble inside the pot survives only when the vapor pressure in it equals the surrounding pressure. Lower the pressure on the water and a cooler bubble is already strong enough, so the boil arrives earlier on the way up the thermometer.
Follow it one step further. At ordinary sea-level pressure that balance sits at 100 degrees Celsius. Lower the pressure and you slide down the same vapor-pressure curve, with no need for the stove itself to have changed.
The case worth keeping in mind is specific. A pot at a high camp can sit at a full boil while the food cooks slowly, because that boil is cooler than an open pot at the shore. The bubbles are real, and the bath around the food is simply not as hot.
That case has a boundary. The account is for an open pot, where the surface feels the local atmosphere. A closed pressure vessel does not care what the mountain air is doing, because the pressure that matters is the pressure trapped inside the lid.
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Inside
- Boiling as a pressure match
- Why the sea-level mark is 100
- What an open pot feels
- Why a hard boil can be cooler
- Where the idea stops applying
- A distinction worth keeping
Sources
- The vapor-pressure curve of water
- Atmospheric pressure falling with altitude