# Explanation for different boiling points of water on different altitudes

I understand water boils at different temperatures depending on altitude.

I am seeking to get an illustrative explanation for this, including a diagram if possible.

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Boiling point of water changes with altitude because atmospheric pressure changes with altitude.

So, how/why boiling point changes with pressure.There is good explanation of this at hyperphysics (with diagrams).

Now, why does pressure change with altitude? Imagine you are swimming in water. Deeper you go more pressure you feel, because there is more water above you.

$P = P_0 + dh$

$P$ - pressure, $P_0$- pressure at the surface, $d$ - density of fluid, $h$- height/depth to free surface

There is huge column of air above our heads.

$\rightarrow h$ at sea level > $h$ at hill station.

$\rightarrow P$ at sea level > $P$ at hill station.

$\rightarrow$ boiling point at seal level > boiling point at hill station.

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The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid.
Atmospheric pressure is due to air above any given point. At greater heights, there is less air above you and hence pressure decreases.
Lesser pressure hence lower boiling points as you go higher.

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+ It works the other way too. Underwater, at a depth of 1km, the water can get up to around 300C without boiling, as shown in this article. –  Mike Dunlavey Mar 18 '13 at 19:12