Why Does Boiling Temperature Decrease at Higher Altitudes?

As I pointed out in my article about adding salt to water to make pasta cook faster, the boiling temperature of water decreases 1° F for every 500 feet climb in altitude. This means, incredibly, that if you boil water on top of Mt. Everest, which is around 29,000 feet, water would boil at around 154° F. Why does this happen? Why Does Boiling Temperature Decrease at Higher Altitudes?

As water or any liquid heats, there is a point at which it begins to change to a gas. This point is the boiling point. When water reaches 212°F or 100°C, it turns to steam or vapor. Once this point is reached, the water will not get any hotter.

This process of the water being changed to steam is different than evaporation, because it does not just take place at the surface of the water, but takes place within the volume of water. The vapor being produced in the water causes pressure called vapor pressure. This pressure has the pressure of the atmosphere pushing on it, so the water cannot “boil” and begin to release the vapor until the vapor pressure is strong enough to overcome atmospheric pressure. This is the boiling point.

As altitude increases, the atmospheric pressure decreases. Thus, the vapor pressure has less and less pressure acting against it, causing the boiling point to decrease as the vapor can be released more easily.

When you first begin to heat water, some air bubbles begin to rise to the top, long before it boils, but water vapor bubbles may actually form on the bottom and the sides of the pan, where the heating action is quicker. Once the vapor begins being released as steam, however, it is invisible and only becomes visible as ‘steam’ as it cools down. This is the visible cloud that we associate with water vapor. To see this for yourself, bring your eyes level with the top of a pan of boiling water. You’ll notice that you can’t see the steam until a certain point above the pan (after it cools a bit).

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Boiling Point of Water at Various Altitudes

For reference, below is listed some of the boiling points of water at various altitudes. However, you can figure out the boiling point at any altitude by simply subtracting 1° F for every 500 additional feet above sea level, or 2° F per 1000 feet climb, or 1° C for every 960 feet of elevation.

See also High-Altitude Baking Adjustments

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