Why water boils at a lower temperature at altitude

Water boils when its vapour pressure matches the pressure of the air above it. Air pressure falls as you climb, so in the mountains water reaches that point at a lower temperature: roughly 1 °C lower for every 300 m of altitude. In Denver it boils at about 94.7 °C, and on the summit of Everest at about 70 °C. The boiling point at altitude calculator works it out for any elevation or barometer reading.

Why pressure sets the boiling point

Molecules are always escaping from the surface of liquid water as vapour. The hotter the water, the more escape, and the higher its vapour pressure. Boiling is the point where that vapour pressure equals the surrounding pressure, so bubbles of vapour can form inside the liquid rather than only at the surface. At standard sea-level pressure of 101.325 kPa, that happens at 100 °C. Lower the outside pressure and the water needs less heat to match it, so it boils cooler. Adding heat after that does not make boiling water hotter; it just makes it boil faster. The pressure converter translates between the kPa, hPa and inches of mercury that barometers use.

How much it drops

The calculator does two steps. First it estimates the air pressure at your altitude with the standard-atmosphere barometric formula. Then it finds the temperature at which water's vapour pressure equals that pressure, using the Antoine equation. Run across a range of heights, the result is close to linear: about 0.99 to 1.0 °C per 300 m up to around 5,000 m, which is where the familiar rule of thumb comes from. In Fahrenheit, the usual cooking rule is about 1 °F per 500 ft. The real pressure on a given day also shifts with the weather by a few percent, enough to move the boiling point by up to about 1 °C, so if you have a barometer reading you can enter it directly.

Boiling points around the world

• Sea level: 100 °C (212 °F).
• Denver, 1,609 m: about 94.7 °C, with air pressure around 83% of sea level.
• Mexico City, 2,240 m: about 92.6 °C.
• La Paz, 3,640 m: about 87.9 °C.
• Everest Base Camp, 5,364 m: about 82 °C.
• Everest summit, 8,849 m: about 70 °C, with less than a third of sea-level pressure.

The same falling pressure means less oxygen in each breath; the altitude oxygen calculator shows that side of it, and Altitude sickness: why thin air affects you and how to prevent it explains what it does to the body.

What it means for cooking

Cooking in water depends on temperature, not on bubbles. At 94 °C pasta, rice, potatoes, eggs and dried beans all cook more slowly than at 100 °C, and the effect grows with height. At a few hundred metres the difference is too small to notice. In a city like Denver, boiled and simmered foods simply need extra time; check doneness rather than trusting the packet. Above about 3,000 m, beans and tough cuts can take far longer, and some foods are hard to cook properly in an open pot at all. Water also evaporates faster at altitude, so long simmers may need extra liquid.

Baking is a separate problem. Lower pressure makes gases in dough and batter expand more, so high-altitude baking adjusts leavening, liquid and oven temperature rather than just time. And adding salt to the pot raises the boiling point only by a fraction of a degree, so it is not a fix.

Pressure cookers and other fixes

The direct fix is to raise the pressure. A pressure cooker running at about one atmosphere above outside pressure lifts the boiling point to around 120 °C at sea level, and still well above 100 °C at altitude, which is why pressure cookers are a staple in high cities and mountain huts. Otherwise, use a lid to save energy and keep the temperature steady, allow longer cooking times, and use a thermometer for anything where the exact temperature matters, such as sugar work. The cooking converter helps when you are adapting a recipe written in other units.