Why water boils at 72°C on top of Everest

On the summit of Everest, water boils at about 72°C (161°F)161°F. At Everest Base Camp it boils at 83°C181°F, at Barafu Camp on Kilimanjaro at 85°C185°F and at High Camp on Denali at 83°C181°F. Ice melts at 0°C32°F at every one of them, exactly as it does at home. Those two facts explain most of what changes about drinking, cooking and fuel on a high mountain, so this post works them out camp by camp.

Air pressure sets the boiling point

Water boils when the pressure of its own vapour matches the pressure of the air pressing down on it. At sea level the air presses with about 101 kPa, and water has to reach 100°C (212°F)212°F before its vapour can push back that hard. Climb, and there is less air above you. Somewhere between 5,500 and 6,000 m18,045 and 19,685 ft the pressure has fallen to half its sea-level value, and water boils at around 81 to 82°C178 to 180°F.

The rule of thumb is one degree Celsius lower for every 300 m (about 1,000 ft)about 1,000 ft of height, which is close to two degrees Fahrenheit. It is good enough for planning. The tables below are worked out from the pressure itself, which matters at the top end where the rule starts to drift.

Water's boiling point at every camp

The figures come from two published pieces of science. The pressure at each height is taken from the model atmosphere John West published in 1996, built from balloon soundings and from measurements made on Everest itself, which fits the air at Everest's latitude in the climbing seasons. The boiling point at that pressure comes from the Antoine equation for water, the standard formula chemists use. Denali gets its own treatment, because its air is different (more on that below). The camp heights are the ones used in our guides.

Kilimanjaro, Machame route

Aconcagua, Normal Route

Denali, West Buttress

Everest, South Col route (Nepal)

Everest, North Col route (Tibet)

Weather moves these numbers a little. A deep low-pressure system lowers the pressure by a kilopascal or two, and each kilopascal takes about half a degree off the boiling point at base camp. Read the tables as accurate to about a degree.

Why Everest's summit is 72°C162°F, not 70°C158°F

Many tables online put the summit figure at 70°C158°F. That number comes from the International Standard Atmosphere, the average air that aviation uses, which describes the middle latitudes in an average year. The air over the Himalaya in the climbing season is warmer and holds a higher pressure at the same height. Chris Pizzo, a doctor on the American Medical Research Expedition, measured the pressure on the summit in October 1981 and found 253 mmHg (33.7 kPa). At that pressure water boils at about 72°C162°F. West's model, used for the tables above, was built partly on that measurement.

Two degrees sounds like a footnote. At the South Col it is the difference between water that softens a freeze-dried meal and water that leaves it chewy, and the warmer figure is the one climbers actually get.

Latitude: Denali has thinner air than its height suggests

The column of air over the tropics is warmer and deeper than the column over the poles, so pressure falls off more slowly with height near the equator. Denali stands at 63°N, the furthest north of the 7 summits, and its air is correspondingly thin. Terris Moore, writing in the American Alpine Journal in 1968, gave the mean pressure on the summit as 459 millibars in July and 426 in January.

At 459 millibars water boils at 79°C174°F. At the same height near Everest, by West's model, the pressure would be about 481 millibars and water would boil at 80°C176°F. Using the same model the other way round, Denali's summer summit pressure is what you meet at about 6,550 m21,490 ft in the Himalaya. In January, at 426 millibars, the boiling point drops to about 77.5°C (171°F)171°F.

Kilimanjaro sits almost on the equator, 3° south. Its air is, if anything, a touch denser than West's model gives, so the water on Uhuru may boil a fraction of a degree above the 81°C178°F in the table. Aconcagua, at 33°S in the southern summer, is close to Everest's 28°N and the model fits it well.

Temperature works through the pressure

A cold morning at Barafu does not lower the boiling point by itself. The boiling point follows the pressure, and the air temperature only matters because cold air is denser, so a cold column of air loses its pressure faster with height. That is the same effect that makes Denali's summit feel higher in winter, and it is why the January figure is lower than the July one.

What cold changes directly is the time it takes to get there. A pot loses heat to freezing air and to the wind, and snow at −20°C−4°F has to be warmed to 0°C32°F before it can even start to melt. The fuel post in this series goes into what that costs.

Why the freezing point stays at 0°C32°F

When water boils it turns into vapour that takes up about 1,600 times the space the liquid did, and the air above has to be pushed aside to make room. That is why the air pressure has such a large say in when boiling can happen. Freezing changes the volume by only about 9 percent, so the pressure has almost nothing to push against. Halving the air pressure raises the melting point of ice by less than one hundredth of a degree, about four thousandths.

So a water bottle at Camp 2 freezes at 0°C32°F, and the snow in your pot melts at 0°C32°F, at any height. What shrinks is the distance between the two. At home there are 100 degrees between ice and boiling water. At Everest Base Camp there are 83, and on the summit 72.

What a lower boiling point changes

Drinking water

Heat kills germs over time, and the hotter the water the faster it works. The US Centers for Disease Control and Prevention asks for a rolling boil of one minute, and three minutes above 2,000 m (6,500 ft)6,500 ft, which makes up for the lower temperature. On Kilimanjaro the crew does this for you, and the FAQ on whether the water is safe to drink on Kilimanjaro covers the rest.

Tea and coffee

Black tea is normally brewed with water close to 100°C212°F, and at base camp it comes out paler and thinner. Give it longer or use two bags. Green tea, herbal tea, instant coffee and hot chocolate are all happy at 80°C176°F.

Cooking

Anything that has to cook, rather than simply soak up water, takes longer. Rice and pasta still cook, slowly; dried beans and lentils hardly do. The third post in this series is about what to cook instead, and what tastes good once it is cooked.

What the big kitchen tents use

On a guided climb somebody else does most of this, and each mountain has its own way of doing it.

Kilimanjaro

The national park banned firewood after fuel collection stripped the areas around the huts, and the crews moved to gas stoves in the 2000s. Porters carry the gas cylinders up, and the cook works from a kitchen tent at every camp, boiling water for drinks and washing and cooking three meals a day. Our page on guides, porters and your crew explains who does what.

Aconcagua

At Plaza de Mulas and Plaza Argentina the base-camp service companies run kitchen domes and sell cooked meals for most of the season. Above base camp nearly every group cooks for itself on its own stoves, and white gas is the usual fuel. The practical issues chapter of the Aconcagua guide has the quantities.

Everest

Most expeditions have their own cooks in the main camps, sometimes as high as the North Col, working on gas from big cylinders. Above Advanced Base Camp the climbers use small canister stoves. Base-camp kitchens in Nepal and Tibet also use pressure cookers, which put the boiling point back above 100°C212°F. A cooker running at about one bar above the outside air reaches about 112°C234°F at Everest Base Camp, which is how dal bhat and rice come out properly cooked at 5,360 m17,585 ft.

Denali

There are no camp staff on Denali. Guided and independent teams alike cook on white-gas stoves, with the fuel bought from the air-taxi company and flown in with them. The National Park Service suggests planning for 0.15 to 0.30 litres of white gas per person per day. Most of that goes into melting snow, because above Kahiltna Base Camp every drop of water starts as snow.