What it does
We live at the bottom of an ocean of air. Its weight presses on everything with about 101 kPa at sea level. Climb a mountain and some of that air is below you, so the air pressure falls with height.
Drag the climber up the mountain. Dots show the air thinning, a mercury barometer falls from 760 mm, and a pot shows the temperature at which water boils. Lower pressure means water boils below 100 °C, so food cooks slowly in the hills. Tick the pressure cooker to push the boiling point back above 100 °C.
It follows Class 8 Science, pressure, winds, storms and cyclones (air presses on us, pressure falls with height), with Class 11 Physics links (the barometer, P = ρgh). Before this, pressure explorer shows what pressure means and liquid pressure shows it growing with depth; for the full atmosphere see standard atmosphere calculator.
How to use it
- Choose a Level. Class 11 shows the formulas with your numbers.
- Drag the climber, use the Height slider (0 to 9,000 m), or Go to a place from a seaside city to Mount Everest.
- Tick Cook in a pressure cooker to add about 100 kPa.
- Try Check yourself at the bottom.
Worked example
At sea level: 101.3 kPa, 760 mm of mercury, water boils at 100 °C.
At 3,500 m, P = 101,325 × (1 − 0.0065 × 3,500 / 288.15)^5.256 = 65.8 kPa, 65% of sea level. Mercury stands at 493 mm and water boils at about 88.3 °C. With the cooker the water is at 165.8 kPa and boils at about 114.3 °C. On Everest (8,849 m) the pressure is 31.4 kPa and water boils at about 70 °C.
FAQ
Why does lower pressure make water boil sooner? Steam bubbles can only grow when their pressure matches the pressure outside. Less outside pressure needs less heat.
Why mercury in barometers? It is so dense that the tube is under 1 m; water would need over 10 m.
Is anything stored? No. Everything runs in your browser.
Where the numbers come from
- International Standard Atmosphere, lowest layer (ICAO Doc 7488; the same as the U.S. Standard Atmosphere 1976 below 11 km): sea level 101,325 Pa and 15 °C, lapse rate 6.5 K/km, with g₀ = 9.80665 m/s², M = 0.0289644 kg/mol and R = 8.314462618 J/(mol K) (CODATA).
- Boiling point from the Clausius–Clapeyron equation through 100 °C at 101.325 kPa, using an average latent heat of 41.5 kJ/mol chosen for 65 to 120 °C; mercury density 13,595.1 kg/m³ at 0 °C. Place heights are rounded, and the 100 kPa of the pressure cooker is a typical rounded value.
- Checked by tests against a step-by-step integration of the hydrostatic equation, the standard value 22,632 Pa at 11 km, 760 mm of mercury at 1 atm, and the Antoine equation for water (within 0.35 °C from 30 to 201 kPa).
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-10.