Water boils at a lower temperature at altitude: the numbers and the reasons

Water boils at a lower temperature the higher you are, because the air above it is thinner. At sea level the boiling point is 100.0 °C (212 °F). At 2,000 m (6,562 ft) it is 93.4 °C (200.1 °F), and at 3,048 m (10,000 ft) it is 89.9 °C (193.8 °F).

Why altitude lowers the boiling point

Water boils when its vapour pressure equals the pressure of the air around it. Vapour pressure rises as the water gets hotter, so the boiling point is the temperature at which the vapour can push back against the air. Air pressure falls as you climb, because there is less air above you. A lower air pressure means the water reaches vapour pressure at a lower temperature.

The effect is smooth. Near sea level, pressure falls by about 11% for each 1,000 m, and the boiling point falls by about 3.3 °C for each 1,000 m (100.0 − 96.7 = 3.3).

Air pressure at altitude

The standard atmosphere gives air pressure in hectopascals (hPa) as a function of height. The temperature side of the same climb, about 6.5 °C per 1,000 m, is covered in the temperature drop guide. The altitude tool uses this form:

P = 1013.25 × (1 − 2.25577 × 10⁻⁵ × h)^5.25588   hPa, h in metres

NASA Glenn's model of the troposphere gives a curve fit that is a different equation, with temperature falling with height at 0.00649 °C per metre. At each height checked, from sea level to 8,848 m, the two differ by less than 0.1 kPa, which is 1 hPa. The two are the same standard atmosphere written two ways.

Worked through at 2,000 m, the formula gives 1 − 2.25577 × 10⁻⁵ × 2,000 = 0.954885. Raised to the power 5.25588, that is 0.784557. Multiplied by 1,013.25 hPa, the pressure is 795.0 hPa, or 78.5% of sea-level pressure.

The boiling point from the Antoine equation

The Antoine equation is a semi-empirical fit between vapour pressure and temperature. For water, its coefficients are given with pressure in mmHg and temperature in °C. Wikipedia lists the water constants for the range −20 to 100 °C, and they are the ones used here:

log10(P in mmHg) = 8.07131 − 1730.63 ÷ (233.426 + T in °C)

rearranged:  T = 1730.63 ÷ (8.07131 − log10 P) − 233.426

Pressure in hPa becomes mmHg by multiplying by 0.750062. That conversion is the only step needed to connect the two equations.

log10 596.3 = 2.7754
8.07131 − 2.7754 = 5.2959
1730.63 ÷ 5.2959 = 326.8
326.8 − 233.426 = 93.4 °C = 93.4 × 1.8 + 32 = 200.1 °F

Boiling point by elevation

The table gives the standard-atmosphere pressure and boiling point at a range of heights. The percentage is the pressure as a share of sea-level pressure, which is what sets the boiling point.

Each 1,000 m of climb lowers the boiling point by about 3.3 °C across the table, for example 100.0 − 96.7 = 3.3 °C from sea level to 1,000 m, and 90.0 − 86.7 = 3.3 °C from 3,000 m to 4,000 m.

log10 522.7 = 2.7182
8.07131 − 2.7182 = 5.3531
1730.63 ÷ 5.3531 = 323.3
323.3 − 233.426 = 89.9 °C = 89.9 × 1.8 + 32 = 193.8 °F

The water decisions tool on this site sets a 1-minute rolling boil, or 3 minutes above 1,981.2 m (6,500 ft). At 3,048 m the 3-minute setting applies. For the timing rules, see the guide to how long to boil water.

What a lower boiling point changes

At 2,000 m, water boils 6.6 °C below its sea-level temperature (100.0 − 93.4 = 6.6). Food cooked in water below 100 °C takes longer, so simmering and rehydrating take longer and use more fuel. Pack weight is affected by the same choice, because more fuel means more to carry. For the food side of that trade, see the backpacking food weight guide.

Any cooking time written for sea level assumes a boil at 100.0 °C. At 2,000 m the boil is at 93.4 °C, so an allowance for longer cooking is the sensible starting point. A longer boil does not raise water above its boiling point at the same pressure, so for cooking the extra time is the adjustment that matters. A test at home before the trip shows what the difference means for your own pot and stove. A pressure cooker works the other way round: sealing the pot raises the pressure inside it, and with it the boiling point, which is why high-altitude kitchens use them.

How weather moves the number

The altitude tool takes elevation rather than pressure, so its result is for a standard day. Real air pressure changes with weather systems by a few hectopascals, and sometimes by a few tens of hectopascals. The effect on boiling point is smaller than the effect of height, but it is not zero.

At 2,000 m, the Antoine equation gives 93.4 °C at 795 hPa and 93.7 °C at 805 hPa. A 10 hPa rise therefore moves the boiling point by 0.3 °C (93.7 − 93.4 = 0.3). A 10 hPa fall moves it by the same amount in the other direction.

What the number does not show

The boiling point answers one question: at what temperature does water boil in the air at this height? It does not show whether water is free of germs or chemicals, and it does not show which treatment a source needs. For sources that look cloudy, see the cloudy water guide.