Why water boils at a lower temperature up high
Water boils when its vapor pressure equals the pressure of the air above it. Higher up there is less air above you, so the pressure is lower and water reaches that point at a lower temperature. At sea level water boils at 212 °F (100 °C); in Denver it boils at about 202 °F.
The formulas
- Air pressure from the standard atmosphere: P = 101.325 × (1 − 2.25577 × 10⁻⁵ × h)^5.25588 kPa, with h in meters.
- Boiling point from the Antoine equation for water: T = 1730.63 ÷ (8.07131 − log10(P in mmHg)) − 233.426 °C.
Feet are converted to meters first, and the result is shown in both °F and °C.
Worked example: Denver
- 5,280 ft = 1,609 m.
- P ≈ 83.4 kPa, or 24.64 inHg, about 82% of sea-level pressure.
- Boiling point ≈ 94.7 °C = 202.4 °F, 9.6 °F below the sea-level value.
Boiling point by elevation
| Elevation | Boiling point |
|---|---|
| Sea level | 212.0 °F (100.0 °C) |
| 2,500 ft (762 m) | 207.5 °F (97.5 °C) |
| 5,280 ft (1,609 m), Denver | 202.4 °F (94.7 °C) |
| 7,000 ft (2,134 m), Santa Fe | 199.3 °F (93.0 °C) |
| 10,000 ft (3,048 m) | 193.8 °F (89.9 °C) |
| 14,115 ft (4,302 m), Pikes Peak | 186.2 °F (85.7 °C) |
| 29,032 ft (8,849 m), Everest | 158.5 °F (70.3 °C) |
The rule of thumb is a drop of about 1 °F per 500 ft.
What changes in the kitchen
- Boiling and simmering take longer. Pasta, rice, dried beans and hard-boiled eggs need several extra minutes, because the water is cooler. Turning up the heat only makes the water boil harder, not hotter.
- Pressure cookers help. By raising the pressure inside the pot, they bring the boiling point back to around 250 °F.
- Candy and jam thermometers need adjusting. Subtract the difference between 212 °F and your local boiling point from every temperature in the recipe.
- Baking changes above about 3,000 ft. Reduce baking powder and sugar slightly, add a little liquid and raise the oven temperature by 15 to 25 °F, following tested high-altitude recipes.
Safety note
Lower boiling temperatures do not make water unsafe to drink after boiling: the CDC recommends boiling for 3 minutes instead of 1 above 6,500 ft. For home canning, the USDA requires longer processing times or higher pressures at altitude.