Why does water boil at a lower temperature at a higher altitude?
In short, a liquid’s boiling temperature is dependent on its composition and the atmospheric pressure on the boundary between the liquid and the air above it. For water, the boiling point at sea level is 100 degrees Celsius (212 degrees Fahrenheit). The atmospheric pressure is roughly 29 mmHg at sea level, but this number is dependent on altitude and is lower the higher you get from sea level.
Boiling is the process in which the molecules in a liquid have enough energy to overcome the opposing pressure of the atmosphere. When these liquid molecules start turning into gas molecules, we say the liquid is boiling. If you lower the opposing pressure, there will be less resistance to the water molecules turning into gas molecules and entering the air, and the liquid will boil at a lower temperature.
It follows that water would boil quicker on a day with lower atmospheric pressure than on a day with a higher pressure. This is true although the difference in barometric pressure on any given day at the same altitude isn’t as great as a change from differing altitudes.
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