How do boiling points change at high altitudes?

At higher altitudes, boiling points are lower than at sea level becasue of the lower atmospheric pressure.

A substance boils when its vapor pressure equals the pressure of its surroundings. At sea level, atmospheric pressure equals 1 atm, which means that when a substance gets hot enough that its vapor pressure equals 1 atm, it starts to boil.

At higher altitudes, atmospheric pressure drops, as you can see here

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This means that the substance will not be as hot when boiling starts, since a lower atmospheric pressure implies a lower vapor pressure for boling, which in turn can be reached faster.

For example, let's take a look at water. At sea level, water boils at `100^@"C"`. However, at 2400 m, water boils at `92^@"C"`, at 4400 m, it boils at `85^@"C"`, and at 8848 m, which is the hight on top of Mount Everest, water boils at `71^@"C"`.

By comparison, atmospheric pressure at sea level is 1 atm; at the top of Mount Everest, atmospheric pressure is 0.36 atm.


Here is a video demonstrating the fact that water will boil at lower temperatures when the atmospheric pressure is reduced.

video from: Noel Pauller