Why rice won't cook properly on Mt Fuji,
but a pressure cooker cooks it faster
We're taught that "water boils at 100°C." But that's only true at sea level. On the summit of Mt Fuji, water boils at about 88°C; inside a pressure cooker, it won't boil until 120°C. The boiling point isn't a property of water — it's set by how hard the surrounding air pushes back.
Rice cooked in a mountain hut somehow stays hard at the core. At high altitude, pasta never quite softens. Hiking guides warn: "rice won't cook properly on tall mountains."
It's not that the flame is too weak. It's not that there isn't enough time. The water is boiling hard — and the rice still doesn't cook.
Meanwhile, back in an ordinary kitchen: a pressure cooker, on the same stove, at the same setting, turns a tough chunk of meat fall-apart tender in a third of the time. Here too, the flame isn't turned up any higher.
These two things are the same principle, used in opposite directions. The key is understanding what boiling actually is.
For a bubble to form, steam has to push the surrounding air out of the way. If it can't push hard enough, the bubble collapses before it forms.
Where the air is thin, up a mountain, pushing it aside is easy, so water boils at a lower temperature. A pressure cooker does the opposite: it makes pushing aside harder, so it raises the boiling temperature.
And what actually decides how well food cooks isn't whether the water is boiling — it's what the actual temperature is. That's the whole story. Let's walk through it.
What boiling actually is — bubbles forming inside water
When you heat water, some of it evaporates gradually from the surface. But that's not boiling. Boiling means bubbles of steam form inside the water and rise up.
Think about what that means. For a bubble to form inside water, it has to push the surrounding water out of the way and make room for itself. And on top of the water sits air, pressing down from above.
So for a bubble to form, the steam needs enough force to push everything around it out of the way. That force gets stronger as temperature rises. The temperature at which it becomes just strong enough to push things aside — that's the boiling point.
So the boiling point isn't decided by water alone. It's decided by a contest with "how hard something is pushing down from above."
If you measure the temperature of a boiling pot, it stays at 100°C no matter how much higher you turn the flame. That's because the extra heat isn't raising the temperature — it's being used to turn water into steam.
In other words, whether it's boiling furiously or simmering gently, the temperature inside the pot is the same. Keeping the heat high doesn't finish the cooking any faster — it just boils the water away sooner. That's why "turn up the heat and it'll be quicker" doesn't work here.
Which means the only way to raise the temperature is to raise the boiling point itself. That's exactly what a pressure cooker does.
Why rice won't cook on Mt Fuji
The higher you go, the less air sits above your head, so the less it pushes down. At the summit of Mt Fuji (altitude 3,776 m), air pressure is said to be roughly two-thirds of sea level.
Since pushing bubbles aside gets easier, they can form even at lower temperatures. The boiling point at the summit is about 88°C. As a rough rule of thumb, the boiling point drops by about 0.3°C for every 100 m of altitude gained.
And here's where cooking comes in. Rice "being cooked" means the starch in the grains absorbs water and transforms into something soft. That change requires a sufficiently high temperature — generally, holding at least 98°C for around 20 minutes is the benchmark.
At 88°C, no matter how long you simmer it, that transformation never fully completes. That's why rice stays hard at the core even though the water is boiling hard. Turning up the flame won't help, either — as long as the boiling point is 88°C, the temperature simply can't rise any higher.
The water itself is just lukewarm to begin with.
What a pressure cooker actually does
A pressure cooker does the opposite. It seals the lid tight and won't let steam escape.
Steam then builds up inside the pot. More steam means a stronger push on the water's surface. A stronger push makes bubbles harder to form. Boiling gets pushed back, and the water's temperature keeps climbing past 100°C.
In a typical household pressure cooker, the inside reaches roughly 2 atmospheres, and the temperature is said to reach about 120°C. The weight or valve on top exists to release excess steam so the pressure doesn't climb any higher than that.
It's only a 20°C difference, but the effect is huge, because the speed at which food changes accelerates faster and faster as temperature rises. As a rough guide, every 10°C rise roughly doubles or triples the rate of change. Between 100°C and 120°C, that's a difference of several times over, by simple multiplication. That's why tough meat turns tender in a third of the time.
- Never open the lid while pressure remains inside. This matters most of all. The contents are above 100°C, and if pressure drops suddenly when you open the lid, the water inside can flash-boil and erupt out. Most accidents happen this way
- Always confirm the pressure has dropped using the indicator or valve — never just by the clock. Go by what the cooker itself shows, not by elapsed time
- Don't overfill it. Foods that foam or swell — beans, pasta, and the like — can clog the valve. Stick to the limits in the manual
- Inspect the valve and gasket. If they're clogged or worn, pressure may not be able to escape. Gaskets have a recommended replacement interval
- Don't leave it unattended while in use. If you hear an unusual sound or see abnormal steam leaking, turn off the heat and let it cool down naturally
The same principle, elsewhere in everyday life
- Instant noodles don't come out right high on a mountain — the water is too lukewarm for the noodles to fully rehydrate. That's why mountain-food products are often designed to rehydrate in cold water or come pre-cooked under pressure.
- Tea tastes lukewarm on an airplane — cabin pressure is kept lower than sea level, so the boiling point drops too.
- A kettle boils fast on a mountaintop, even on a hot day — boiling quickly sounds like a good thing, but the water that results stays at a low temperature.
- Retort-pouch food keeps at room temperature for a long time — large industrial pressurizing equipment heats it to around 120°C for sterilization. Some bacterial spores survive 100°C, and reliably killing them requires that higher temperature.
- Volcanic hot springs vary in temperature by location — deep underground, strong pressure keeps water liquid at temperatures that would boil it away at the surface.
Something you can check in your own kitchen
- Take a needleless syringe (around 10 mL), available at pharmacies, and draw up warm water to about a third full
- Seal the tip firmly with a finger
- With the tip sealed, pull the plunger back sharply (expanding the space inside = weakening the push)
- Bubbles will come gushing up inside the water, while the temperature stays around 40–50°C
It boils without ever going near a flame. That's because you didn't raise the temperature — you lowered the push. You can feel with your own hands that "boils at 100°C" isn't a fixed property of water. Think of it as an extreme version of what's happening at the summit of Mt Fuji. Don't try this with hot water (risk of scalding) — warm water is more than enough to see the effect.
Summary
Rice failing to cook on Mt Fuji and a pressure cooker cooking faster both trace back to one cause: the boiling point isn't a property of water, but is set by how hard the surroundings push on it. And what decides how well food cooks isn't whether the water is boiling, but the actual temperature at the time.
"Boils at 100°C" is a local rule that only applies at sea level.
The boiling point isn't the only thing that shifts with pressure — melting points rise the same way. The most extreme example is at the centre of the Earth, covered in Why isn't the centre of the Earth molten, even though it's as hot as the Sun's surface?
For those who want more — terms, equations, and links to the curriculumLabelled by level, from middle-school science to open research questions
- MSCovered in middle-school science
- HSCovered in introductory high-school chemistry/physics
- HS+Covered in advanced high-school chemistry/physics, or treated as extension/sidebar material in textbooks
- Univ.Not covered in high school — university-level content (thermodynamics, heat transfer engineering)
- ResearchNot yet settled even at university level — an active research question
MSTerms: the vocabulary of boiling and pressure
- Vapour pressure: the pressure a liquid exerts as it tries to become vapour. It rises with temperature. This is the "pushing force" from the main text.
- Boiling point: the temperature at which vapour pressure equals the surrounding pressure. Change the surrounding pressure, and the boiling point changes too.
- Atmospheric pressure: the force with which air pushes down. About 1,013 hPa (1 atm) at sea level. At the summit of Mt Fuji, it's said to be about 640 hPa.
- Latent heat: heat absorbed or released during a change of state. The reason temperature doesn't rise during boiling is that the added heat goes into this instead.
- Gelatinisation: the process by which starch is transformed by water and heat into an easily digestible, soft state. This is what "rice being cooked" actually consists of. It also appears in the article on why bread rises, as the stage where the dough's structure sets.
HSChecking with equations: why a pressure cooker only needs a quarter of the time
A pressure cooker only raises the temperature by about 20°C. How can just 20°C make things cook so much faster? This is something we can work out with a calculation.
A 10°C rise roughly doubles the rate of a reaction
| Temperature rise | units: °C |
| Speed multiplier | ×2 for every 10°C |
This isn't a strict law — it's a rough guide that tends to hold well in everyday temperature ranges. And here's the crucial part: the multiplier compounds by multiplication, not addition. For 20°C, it's not 2 + 2 — it's 2 × 2.
Because it compounds by multiplication, a small rise in temperature can produce a sudden, outsized effect. That's the real secret of the pressure cooker.
| Ordinary pot | 100°C |
| Pressure cooker (~2 atm) | 120°C |
| Temperature difference | 120 − 100 = 20°C |
| How many 10°C steps | 20 ÷ 10 = 2 steps |
| Speed multiplier | 2 × 2 = 4× |
| A stew that normally takes 1 hour | 60 ÷ 4 = 15 minutes |
Down to a quarter of the time. The time savings quoted in pressure-cooker manuals are roughly in this range. "Just a 20°C rise" turning into "4× faster" happens because the multiplier compounds.
Let's apply the same idea backwards. At the summit of Mt Fuji, water boils at about 88°C.
| Difference from sea level | 100 − 88 = 12°C lower |
| How many 10°C steps | 12 ÷ 10 = 1.2 steps |
| Speed is roughly | less than half |
| Rice that takes 20 min at sea level | works out to roughly 20 × 2 = 40 min |
And frustratingly, turning up the flame doesn't fix it. Once the water is boiling, the temperature simply can't climb any further. All the flame can do is boil the water faster — it's air pressure alone that decides the temperature once it's boiling. This is said to be the reason rice won't cook properly on a mountain.
※ Actual rice cooking depends on the temperature and time needed for starch to change, so it doesn't follow a simple multiplier. This is a rough guide only.
Pressure cooker ratings are usually given not as "the total pressure inside" but as "how much higher than atmospheric pressure." Add atmospheric pressure to that figure to get the actual pressure.
| Rated pressure (above atmospheric) | take a model at 100 kPa |
| Atmospheric pressure | about 101 kPa |
| Actual pressure | 101 + 100 = 201 kPa |
| How many atmospheres is that | 201 ÷ 101 ≈ 2.0× |
That's what "about 2 atmospheres" actually means. Read the rated number straight as a multiple of atmospheric pressure, and you'll be off by a full atmosphere. Checking where a unit's zero point sits is exactly the kind of thing that matters in cases like this.
| Sea level (0 m) | 1,013 hPa / 100°C |
| Altitude 1,000 m | ~899 hPa / ~97°C |
| Mt Fuji summit (3,776 m) | ~640 hPa / ~88°C |
| Everest summit (8,848 m) | ~314 hPa / ~70°C |
| Household pressure cooker | ~2 atm / ~120°C |
※ Atmospheric pressure also varies with temperature and weather. Pressure-cooker figures vary by model.
At the summit of Everest, water boils at about 70°C. You can't even make a decent cup of tea. That's why high-altitude expedition food is so often designed not to need heating at all.
HS+The relationship between temperature and pressure, in equation form
The relationship between vapour pressure and temperature is described by the Clausius–Clapeyron equation. In approximate form, it looks like this.
ln(P₂/P₁) = −(ΔHv / R) × (1/T₂ − 1/T₁)
Here ΔHv is the heat of vaporisation (about 40.7 kJ/mol for water), R is the gas constant, and T is absolute temperature. Plug in Mt Fuji's air pressure (about 0.63 atm), and the boiling point comes out to roughly 88°C — matching the real value well.
As for why reactions speed up, that's explained by the Arrhenius equation: k = A·exp(−Ea/RT). Because temperature T sits inside the exponent, even a small rise in T produces a large change in the rate k. The rule of thumb that "a 10°C rise gives 2–3×" is the rough guide that this equation produces.
Univ.Where do bubbles actually come from?
Here's a surprise: heat perfectly clean water in a perfectly clean container, and it won't boil at 100°C at all. A bubble needs some tiny seed to form from in the first place. Try to create a bubble in water with nothing to start from, and surface tension crushes it before it can form.
Real-world boiling happens because scratches or fine grooves in a pot, or gas dissolved in the water, act as footholds for bubbles (heterogeneous nucleation). When those footholds are scarce, water can stay liquid past 100°C (a superheated state). Disturb it in that state, and it boils all at once — this is bumping.
Technology that moves heat by exploiting boiling (boiling heat transfer) is used widely, from power-plant cooling to heat dissipation in electronics. What matters here is the critical heat flux — exceed it, and a film of bubbles blankets the surface, blocking heat transfer and sending the temperature soaring. It's a central quantity in safety design.
ResearchWhat's still not well understood
- Boiling is a familiar phenomenon that's surprisingly hard to predict. Where, when, and how many bubbles form depends heavily on the shape of microscopic scratches on a surface and how wettable it is. No model yet predicts critical heat flux precisely from first principles. In nuclear power and high-performance electronics cooling, engineers still rely partly on experimental data.
- When bumping will occur also can't be predicted in advance. How long a superheated state persists is essentially down to chance, which is why predicting microwave-oven bumping accidents is difficult — countermeasures mostly focus on "making it less likely to happen" rather than eliminating it.
- Boiling behaves differently without gravity. On the ground, buoyancy carries bubbles away; in microgravity, bubbles stay stuck to the surface. Experiments continue aboard the International Space Station, but heat-transport methods suitable for long-term stays in space are still a work in progress.
- Even the molecular-level understanding of "rice cooking" is incomplete. The process by which starch grains absorb water and break down varies in complex ways with temperature, time, moisture content, and rice variety. The parts that affect taste still rely partly on experience and sensory evaluation.
Something you watch happen in a pot every single day is still an active research topic. Familiarity and full understanding are two different things.
Links to the curriculum, by level
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science — states of matter / boiling & melting points / atmospheric pressure | What boiling is, altitude and boiling point, the syringe observation |
| HS | Chemistry basics — states of matter and heat / physics basics — quantity of heat | Latent heat, why temperature doesn't rise during boiling |
| HS | Chemistry — liquid-vapour equilibrium and vapour pressure | Boiling point as the balance between vapour pressure and external pressure |
| HS+ | Chemistry — reaction rates (Arrhenius equation) / thermochemistry | Clausius–Clapeyron equation, 2–3× per 10°C |
| Univ. | Thermodynamics / heat transfer engineering / food engineering | Nucleation, superheating, critical heat flux, pressure sterilisation |
| Research | Boiling heat transfer (unresolved) | Predicting critical heat flux, bumping, boiling under microgravity |
| ― | Home economics — safety | Handling a pressure cooker, never opening it while pressure remains |
- Japan Meteorological Agency explanatory material on altitude and air pressure (standard atmosphere) (気象庁).
- Atkins, P. & de Paula, J., Physical Chemistry (Clausius–Clapeyron equation, vapour pressure and boiling point).
- Carey, V. P., Liquid-Vapor Phase-Change Phenomena (nucleation in boiling, critical heat flux).
- National Consumer Affairs Center of Japan (国民生活センター) and National Institute of Technology and Evaluation (NITE, 製品評価技術基盤機構), case reports and safety advisories on pressure-cooker accidents.
- Research on starch gelatinisation and rice cooking from the Japanese Society for Food Science and Technology (日本食品科学工学会) and related bodies.
- Individual manufacturers' pressure-cooker instruction manuals, on operating pressure, foods not to cook, and maintenance.
※ Figures such as air pressure, boiling points, and pressure-cooker temperatures vary with conditions and model. This article presents commonly used rough guides.
※ This article is a general-audience science explainer. Always follow your product's instruction manual when using a pressure cooker. The figures given here are rough guides for understanding the underlying mechanism, and vary with conditions.