Water boils at 100°C, right? Not always. On top of Mount Fuji, water boils at about 87 or 88°C. This is because the air pressure there is only about two-thirds of the pressure at sea level.

With less air pushing down, bubbles form more easily. The water boils, but it is not very hot. So cup noodles stay a little hard, and rice may not cook well.

Water boils when the pressure of its vapor becomes equal to the air pressure around it. At sea level, this happens at 100°C. On the summit of Mount Fuji, which is 3,776 meters high, the air pressure is only about two-thirds as high, so water starts boiling at around 87 to 88°C. As a rough guide, the boiling point drops by about 1°C for every 300 meters you climb.

This causes cooking problems: no matter how strong the fire is, the water cannot get hotter than its boiling point, so noodles stay hard and rice may not cook completely. A pressure cooker uses the opposite idea, raising the pressure so that water can reach 110 to 120°C.

Boiling occurs when water's vapor pressure, which rises with temperature, matches the pressure of the surrounding atmosphere; only then can vapor bubbles form throughout the liquid. On the summit of Mount Fuji, at 3,776 meters, the average air pressure is about 640 hectopascals, roughly two-thirds of the sea-level value, so water boils at around 87 to 88°C. Calculations using the modern international standard for water's properties give about 87.5°C at that pressure. The boiling point even shifts slightly with the weather, falling when a low-pressure system arrives.

On Everest, where pressure is about one-third of sea level, it is close to 70°C. The practical consequence is that boiling water on mountains is cooler than people assume: adding more heat merely produces more vapor, so noodles stay firm, rice may remain hard in the center, and boiling water may not sterilize as well as expected. Pressure cookers exploit the reverse effect, raising the internal pressure to reach 110 to 120°C. However, how much longer food must be cooked at altitude cannot be calculated simply, since changes in starch and protein depend on temperature in different ways.