Boiling Point at Altitude & Vapour Pressure Calculator

About the Boiling Point at Altitude & Vapour Pressure Calculator

Water boils when its vapour pressure equals the pressure of the air on it, so the higher you go, the cooler it boils: about 99.97 °C at sea level, 93.3 °C at 2,000 m and only 70.2 °C on the summit of Everest. This calculator works it out three ways:

• Water at altitude: the air pressure of the U.S. Standard Atmosphere at any height from −500 m to 32 km, or a barometer reading you enter, and the boiling point from the international steam equations (IAPWS-IF97), with what a pressure cooker reaches at that altitude;
• Vapour pressure: for water and 44 other liquids, from methanol and acetone to mercury and liquid nitrogen, at any temperature within their measured data (the Antoine equation with the NIST Chemistry WebBook’s parameters), and the temperature at which each boils under a given pressure;
• Clausius–Clapeyron: from one known point and the enthalpy of vaporization, the boiling point at a new pressure, the vapour pressure at a new temperature, or the enthalpy from two points, as in chemistry courses.

Every result comes with a table, a chart and the working. With a Pro pass, copy the result or download the tables as a CSV, or unlock this one result. Without a pass the free preview shows the boiling point or the vapour pressure in full, with the first rows of each table, the first step of the working and a watermarked chart; other figures hidden.

Common uses

• Knowing how hot your water really boils at home, on a trek or on an expedition
• Adjusting cooking and pressure-cooker times for altitude
• Chemistry homework on vapour pressure, boiling points and the Clausius–Clapeyron equation
• Laboratory work: the vapour pressure of a solvent at room temperature, or its boiling point under reduced pressure
• Understanding why butane, propane and ammonia stay liquid only under pressure

The formulas

• Air pressure (U.S. Standard Atmosphere 1976): in the lowest layer P = 101,325 Pa × (1 − 0.0065 H ÷ 288.15) to the power 5.2559, with the geopotential height H = 6,356,766 Z ÷ (6,356,766 + Z) for an altitude Z in metres; the layers above 11 km use their own gradients.
• Water (IAPWS-IF97, region 4): the saturation temperature from the pressure by the release’s backward equation, which reproduces the release’s check values; at 101.325 kPa it gives 99.974 °C.
• Antoine equation: log₁₀(P/bar) = A − B ÷ (T + C), T in kelvin, with A, B and C for the measured temperature range of each liquid.
• Clausius–Clapeyron: ln(P₂ ÷ P₁) = (ΔHvap ÷ R)(1/T₁ − 1/T₂), R = 8.314462618 J/(mol·K), T in kelvin; water starts from 40.65 kJ/mol at its normal boiling point (the saturated vapour and liquid enthalpies of the NIST Chemistry WebBook’s fluid data, IAPWS-95).
• Pressure cooker: the pressure inside is the air pressure outside plus the gauge pressure.

Cooking at altitude

At altitude water boils sooner but at a lower temperature, and in an open pot it cannot get any hotter: at 3,000 m the boil is at 90 °C. Pasta, rice, beans, lentils and eggs cooked in boiling water therefore take longer, and above about 5,000 m (83 °C) some barely cook at all, which is why climbers and high-altitude kitchens use pressure cookers.

A pressure cooker’s gauge pressure adds to the air pressure outside, so the same 1 bar (about 15 psi) gives 120.4 °C at sea level but only 115.2 °C at 3,000 m. Baking, roasting and frying are not limited by the boiling point in the same way, though water evaporates faster from doughs and batters, which is why high-altitude recipes change the liquid, the leavening and the oven temperature.

Altitude or barometer reading?

The altitude mode uses the standard atmosphere, an average: on a given day the air pressure at a place can be a few percent above or below it, which moves the boiling point by about a degree. For the boiling point right now, enter the station pressure of a barometer at the place. Weather reports give the pressure reduced to sea level (QNH), which at altitude is far above the real one: it would give the sea-level boiling point.

Sources

• U.S. Standard Atmosphere, 1976 (NOAA, NASA and the U.S. Air Force): the defining constants and the layers.
• IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam, section 8 (region 4): the saturation-pressure and saturation-temperature equations and their check values, which this tool reproduces to nine significant figures.
• NIST Chemistry WebBook, Standard Reference Database 69: the Antoine equation parameters, normal boiling points and enthalpies of vaporization (Majer and Svoboda), with the original reference of every data set shown in the working.
• OpenStax, Chemistry 2e, section 10.3 (CC BY 4.0): the Clausius–Clapeyron equation and its worked examples, which are among this tool’s tests.

Limitations

• The altitude mode uses the standard atmosphere; real air pressure varies with the weather by a few percent, moving the boiling point by about a degree.
• Antoine parameters are valid in their measured temperature range: values slightly outside it are extrapolated with a warning, and further outside they are refused.
• Liquids are pure: salt, sugar or other dissolved substances raise the boiling point a little, and mixtures such as fuels boil over a range.
• Clausius–Clapeyron assumes a constant enthalpy of vaporization and an ideal vapour, so it drifts over large temperature ranges.
• Water data run from 0 °C to the critical point (373.946 °C, 22.064 MPa); altitudes from −500 m to 32,000 m.

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