From the Dead Sea shore to Everest

The temperature at which water boils, by altitude

In London, at 11 metres, water boils at 100.0 °C.

100.0°C

the same as at sea level London · 11 metres Standard pressure 101.2 kPa

Altitude 11 metres

The calculation uses standard pressure, 101.325 kPa at sea level. A day's weather moves the ceiling by a few tenths of a degree. The margin of the calculation is ±0.5 °C.

A bigger flame makes more steam. The ceiling stays.

Once water boils, extra heat goes into steam. The boiling temperature depends on the pressure of the place, and the pressure depends on altitude.

100.0 °C

London

Steam against a medium flame: 1.0×

The boil holds at 100.0 °C. A bigger flame makes more steam and leaves the temperature where it is.

Flame power 60%

The sealed lid adds 100 kPa on top of the pressure of the place. Pressure inside the pot takes the place of the air above it, and the ceiling rises.

The climb, from the shore of the Dead Sea to Everest

Every point on the curve comes out of the same formula, at another altitude. A point, or a row in the table, puts that place on the screen above.

Place	Altitude	Standard pressure	Boiling point
Dead Sea shore	−430 metres	106.6 kPa	101.4 °C
Constanța	25 metres	101.0 kPa	99.9 °C
London	11 metres	101.2 kPa	100.0 °C
Bucharest	85 metres	100.3 kPa	99.7 °C
Cluj-Napoca	360 metres	97.1 kPa	98.8 °C
Brașov	538 metres	95.0 kPa	98.2 °C
Predeal	1,050 metres	89.3 kPa	96.5 °C
Ben Nevis	1,345 metres	86.2 kPa	95.5 °C
Omu Peak	2,505 metres	74.6 kPa	91.7 °C
Moldoveanu Peak	2,544 metres	74.3 kPa	91.6 °C
Everest	8,848 metres	31.4 kPa	70.3 °C

The weight of air holds the bubbles down

A steam bubble forming inside water has to push outward against the weight of the water and the air above it. While the vapour pressure inside the bubble stays below the pressure of the place, the bubble collapses. At the boiling point the two pressures meet, and the bubbles grow, break free and burst at the surface in a rolling boil.

High on a mountain, the column of air above the pot is shorter and lighter. The meeting point arrives sooner, at a lower temperature. Bucharest, the Romanian capital in the plain north of the Danube, stops 0.3 °C short of the sea-level hundred. Brașov, a Transylvanian city under Mount Tâmpa, stops 1.8 °C short. At the weather station on Omu Peak, the highest summit of the Bucegi range, 8.3 °C are missing, and at standard pressure tea at the refuge starts from water of at most 91.7 °C, however long the kettle stays on the flame.

The link between boiling and the barometer is as old as the scale itself. In 1742 Anders Celsius measured how far the boiling point moves with air pressure, and found a dependence modern measurements confirm. His scale put zero at boiling and a hundred at freezing; the turn to today's direction came after his death. The 100.0 °C mark stayed tied to standard pressure at sea level: 101.325 kPa.

Evaporation at the surface is a different matter: molecules escape one by one, at any temperature. A rolling boil starts only when the whole body of water can boil at once.

The calculation, with its formulas

1. The pressure of the place, standard atmosphere:
P(h) = 101.325 × (1 − 2.25577×10⁻⁵ × h)^5.25588 kPa, with h in metres.

2. The boiling point, the Antoine equation for water, NIST constants:
T = 1730.63 / (8.07131 − log₁₀P_mmHg) − 233.426 °C.

At zero altitude the calculation returns 100.0 °C. With the second published set of constants, Omu Peak comes out at 91.4 °C instead of 91.7 °C: the honest margin of this page is ±0.5 °C, and headlines round accordingly.

Reference altitudes: Bucharest 85 metres and Cluj-Napoca 360 metres (Romania's National Institute of Statistics), Brașov 538 metres, Omu Peak 2,505 metres, Moldoveanu Peak 2,544 metres, Everest 8,848 metres. A city's altitude is a reference point for the locality; its streets can sit higher or lower, and the instrument accepts any value between −430 and 8,848 metres.

Public sources, in the project record: ISO 2533 (standard atmosphere), the NIST Chemistry WebBook (Antoine constants for water), Romania's National Institute of Statistics (altitudes of county seats), Uppsala Astronomical Observatory (the history of the Celsius scale).