Why Mountain Water Isn’t Hot Enough

What the Bubbles Are

As water heats up, its molecules move faster and faster, and some break free as water vapour. The hotter the water, the harder that vapour pushes: its vapour pressure.

The big bubbles of a boil are pockets of water vapour forming inside the water. But for a bubble to open up, its push has to hold back the air pressing down on the water’s surface, plus the little bit of water above it.

A pot of water · bubbles, schematic

The Air Above Us

A thick blanket of air presses down on us. At the seaside its pressure is about 1,013 hectopascals, and water must reach 100°C before its vapour can push back that hard.

The higher you go, the less air is above you and the lower the pressure. At the top of Everest, only about a third of it is left.

Air pressure with height: International Standard Atmosphere

One Mountain, Four Pots

The same pot of water boils at 100°C by the sea; at about 95°C in Denver, about 1,600 metres up; at about 88°C in La Paz, Bolivia, at 3,640 metres; and at about 70°C on the summit of Everest, at 8,849 metres.

Near the ground, the boiling point drops about 1°C for every 300 metres you climb.

The Real Summit

This is the real Everest massif, 24 kilometres across, from satellite elevation data, with its heights unstretched. The summit stands 8,849 metres high, by the survey China and Nepal announced in 2020. A pot of water there boils at about 70°C.

Base camp on the Nepal side, beside the Khumbu Glacier, is about 5,300 metres up. The air there presses with about half its weight at the seaside, and water boils at about 82°C. In the last three and a half kilometres of the climb, the boiling point falls by about 12°C more.

Nepal–China border · 28.0°N 86.9°E · heights not stretched · pots not to scale

One City, Three Boiling Points

La Paz lies on the edge of the Altiplano, the high plain of the Andes, filling a canyon cut several hundred metres into it. El Alto spreads across the plain above the rim.

From El Alto down to the southern districts, the ground falls about 750 metres. The same pot boils at about 86°C at the top of the city, about 88°C in the centre and about 89°C at the bottom.

Bolivia · 16.5°S 68.1°W · heights × 2.5 · pots not to scale

Why Mountain Kitchens Use Pressure Cookers

Once water boils, it stops getting hotter: any extra heat goes into turning it to vapour. So high up, however hard the pot boils, the water stays at 88°C or 70°C, and pasta and eggs take longer to cook.

A pressure cooker seals its lid so the pressure inside climbs about one atmosphere above the air outside, and the boiling point climbs with it: in La Paz, to about 114°C, hotter than an open pot by the sea. That is why pressure cookers are common in high places.