Why rice won't cook properly on Mt Fuji,
but a pressure cooker cooks food faster
We're taught that "water boils at 100°C." But that's only true at sea level. At 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 presses down on it.
Rice cooked in a mountain hut somehow still has a hard core. At high altitude, pasta stays stubbornly firm no matter how long you boil it. Hiking guides even warn that "rice won't cook properly on high mountains."
It's not that the flame is too weak. It's not that there isn't enough time. The water is boiling vigorously — and yet the food still won't cook.
Meanwhile, back in the kitchen: a pressure cooker. Same stove, same flame, yet a tough chunk of meat turns meltingly tender in a third of the usual time. Here too, the flame isn't any stronger.
These two situations are opposite uses of exactly the same principle. The key lies in what "boiling" actually is.
For a bubble to form, water vapour has to push the surrounding air out of the way. If it can't push hard enough, the bubble collapses before it can form.
Where the air is thin, up a mountain, it's easy to push aside, so water boils at a lower temperature. A pressure cooker does the opposite: it makes pushing harder, which raises the boiling temperature.
And what actually determines how well food cooks isn't whether the water is boiling — it's the actual temperature. That's the whole story. Let's go through it step by step.
What boiling actually is ― bubbles forming
When you heat water, it evaporates gradually from the surface. But that isn't boiling. Boiling is when bubbles of vapour form inside the water and rise up.
Think about what that means. For a bubble to form inside the water, it has to push the surrounding water out of the way to make room for itself. And sitting on top of the water is a layer of air, pressing down from above.
So for a bubble to form, the water vapour needs enough force to push that surrounding weight aside. This force grows stronger as temperature rises. The temperature at which it becomes just strong enough to push 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 pressing down from above."
Measure the temperature of a boiling pot, and it stays at 100°C no matter how much you crank up the heat. That's because the extra heat you add doesn't raise the temperature — it goes into turning water into vapour instead.
In other words, whether you boil it at a furious rolling boil or a gentle simmer, the temperature inside the pot is the same. Keeping the flame high doesn't make the dish finish any sooner — it just boils away the water faster. That's why "turn up the heat to cook it faster" doesn't actually work here.
Which means the only way to raise the temperature is to raise the boiling point itself. And 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 — meaning the push gets weaker. At the summit of Mt Fuji (altitude 3,776 m), atmospheric pressure is said to be roughly two-thirds of sea level.
Because it's easier to push aside, bubbles can form at a lower temperature. 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. Cooking rice means the starch in the grains absorbs water and transforms into a soft, digestible form. This change needs a sufficiently high temperature — as a rough guide, holding it above 98°C for about 20 minutes.
At 88°C, no matter how long you simmer it, that transformation never fully completes. Boiling away, yet the rice still has a hard core — that's exactly why. Turning up the flame doesn't help either, because as long as the boiling point is 88°C, the temperature simply won't climb higher.
The water itself just never gets properly hot.
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 postponed, and the water's temperature keeps climbing past 100°C.
In a typical household pressure cooker, the pressure inside reaches roughly 2 atmospheres, and the temperature reaches about 120°C. The weight or valve on top exists to vent off any 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 rate at which food changes speeds up rapidly as temperature rises. As a rule of thumb, 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 on a simple calculation. That's why tough meat turns tender in a third of the usual time.
- Never open the lid while pressure remains inside. This is the single most important rule. The contents are above 100°C, and if you release the lid and the pressure drops suddenly, the water inside will flash-boil instantly and erupt out. Most accidents happen this way
- Always confirm the pressure has dropped using the indicator or valve. Judge by what the cooker shows you, not by elapsed time
- Don't overfill it. Foods that froth or swell, such as beans or pasta, can clog the valve. Stick to the maximum fill line in the instruction manual
- Inspect the valve and gasket. Clogging or wear can stop pressure from releasing properly. Gaskets have a recommended replacement interval
- Never leave it unattended while in use. If you hear unusual noises or see an abnormal steam leak, turn off the heat and let it cool naturally
The same principle, seen elsewhere in daily life
- Instant noodles never come out right on high mountains ― the water is too cool for the noodles to fully rehydrate. That's why many mountaineering foods can be rehydrated with cold water, or come pre-cooked under pressure.
- Tea tastes lukewarm on an aeroplane ― cabin pressure is kept lower than at sea level, so the boiling point is lower too.
- A kettle boils quickly up a mountain even on a hot day ― boiling fast sounds like a good thing, but the water that results stays at a low temperature.
- Retort-pouch foods keep for a long time at room temperature ― because large industrial pressure equipment heat-sterilises them at around 120°C. Some bacterial spores survive 100°C, and reliably killing them off requires that higher temperature.
- Volcanic hot springs vary in temperature by location ― deep underground, strong pressure lets water stay liquid at temperatures that would make it boil at the surface.
Something you can check for yourself in the kitchen
- Draw up lukewarm water to about a third of the way into a needle-less syringe (around 10 mL), available at pharmacies
- Seal the tip firmly with your finger
- Pull the plunger back sharply (this expands the space inside — meaning it weakens the push)
- Bubbles will come bubbling up inside the water. The temperature stays around 40–50°C the whole time
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 actually a property of water. Think of it as an extreme version of what happens at the summit of Mt Fuji. Don't try this with hot water (risk of scalding). Lukewarm water is more than enough to see the effect.
Summary
Rice not cooking properly on Mt Fuji, and a pressure cooker cooking food faster, both come down to the same cause: the boiling point isn't a property of water, it's set by the force pressing down on it. And what determines how well food cooks isn't whether the water is boiling — it's 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 temperatures 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?
Want to go deeper? ― terminology, formulas, and how this connects to the textbooksFrom junior-high science to active research topics, each level is clearly labelled
- JHSCovered in junior-high school science
- HSCovered in high-school "Basic Chemistry" / "Basic Physics"
- HS+Covered in high-school "Chemistry" / "Physics," or treated as advanced/sidebar material in textbooks
- Univ.Not covered in high school — university-level specialist material (thermodynamics, heat transfer engineering)
- ResearchNot even settled "textbook fact" at university level — something researchers are actively investigating right now
JHSTerms: the vocabulary of boiling and pressure
- Vapour pressure: the pressure with which a liquid "wants" to become vapour. It grows larger as temperature rises. This is the "pushing force" from the main text.
- Boiling point: the temperature at which vapour pressure becomes equal to the surrounding pressure. Change the surrounding pressure, and the boiling point changes too.
- Atmospheric pressure: the force with which air pushes down. At sea level, about 1,013 hPa (1 atmosphere). At the summit of Mt Fuji, said to be about 640 hPa.
- Latent heat: heat absorbed or released when a substance changes state. It's why the temperature doesn't rise while boiling — the added heat is being spent on this instead.
- Gelatinisation: when starch is transformed by water and heat into a soft, easily digestible form. This is what "cooking rice" actually consists of. It also appears in the article on why bread rises, as the stage where the dough's structure sets.
HSWorking it out with a formula: why a pressure cooker cuts the time to a quarter
A pressure cooker only raises the temperature by about 20°C. So why does just 20°C make cooking so much faster? This is something we can trace through with a calculation.
A 10°C rise roughly doubles the rate of a reaction
| Temperature rise | Units: °C |
| Rate multiplier | Multiply by 2 for every 10°C |
This isn't a strict law — it's a rule of thumb that happens to fit well in everyday temperature ranges. The important part is that 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 suddenly produce a large effect. That's the real secret behind 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 |
| Rate 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 20°C hotter" turning into "4× faster" is exactly because the multiplier compounds through multiplication.
Let's apply the same idea the other way round. 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 |
| Rate is roughly | less than half |
| A rice cook that takes 20 min at sea level | 20 × 2 = about 40 minutes, by this reckoning |
And frustratingly, turning up the flame doesn't fix it, because once the water is boiling, the temperature simply can't climb any higher. Flame strength can only "bring it to the boil faster" — it's atmospheric pressure alone that decides the temperature once it's boiling. This is thought to be why rice won't cook properly up a mountain.
※ Actual rice cooking also depends on the temperature and time needed for the starch to change, so it isn't determined by a simple multiplier alone. This is a rough estimate.
Pressure-cooker ratings are usually given as "how much higher than atmospheric pressure," not "the absolute pressure." Add atmospheric pressure to that figure to get the actual pressure.
| Rated value (above atmospheric) | Say, a model rated at 100 kPa |
| Atmospheric pressure | ~101 kPa |
| Actual pressure applied | 101 + 100 = 201 kPa |
| How many atmospheres is that | 201 ÷ 101 ≒ 2.0× |
That's where "about 2 atmospheres" comes from. Read the rated figure directly as a multiple of atmospheric pressure, and you'll be off by exactly 1 atmosphere. Checking where the units and reference point actually sit matters in cases exactly 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 values vary by model.
At the summit of Everest, water boils at about 70°C. You can't even brew a decent cup of tea. This is why so much high-altitude mountaineering food is designed to need no cooking at all.
HS+The relationship between temperature and pressure, as a formula
The relationship between vapour pressure and temperature is expressed by the Clausius–Clapeyron equation. In approximate form:
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 atmospheric pressure (about 0.63 atm) and you get a boiling point of roughly 88°C, which matches the observed value well.
The reason for the faster reaction rate is explained by the Arrhenius equation: k = A・exp(−Ea/RT). Because temperature T sits inside the exponent, even a small rise in temperature produces a large change in the rate k. The rule of thumb "2–3× per 10°C rise" is the approximate figure that falls out of this equation.
Univ.Where do bubbles actually come from?
In fact, perfectly pure water in a perfectly clean container won't boil at 100°C. For a bubble to form in the first place, it needs a tiny seed to start from. Trying to create a bubble inside water with nothing to start from is opposed by strong surface tension, which crushes the bubble before it can form.
Real-world boiling happens because scratches, tiny grooves in the pot, and dissolved gas in the water act as footholds for bubbles (heterogeneous nucleation). With too few footholds, water can stay liquid even above 100°C (a superheated state). Disturb it in this state, and it boils violently all at once — this is bumping.
Technology that uses boiling to move heat (boiling heat transfer) is used widely, from power-plant cooling to heat dissipation in electronics. A key quantity here is critical heat flux — exceed it, and bubbles blanket the surface so heat can no longer get through, causing the temperature to spike. It's a central quantity in safety design.
ResearchWhat we still don't fully understand
- Boiling is a familiar phenomenon, yet a hard one to predict. Where, when, and how many bubbles form depends heavily on the shape of microscopic surface scratches and how well the surface wets. No model exists yet that can precisely predict critical heat flux from first principles. Cooling design for nuclear reactors and high-performance electronics still relies partly on experimental data.
- When bumping will occur also can't be predicted in advance. How long a superheated state persists is a matter of chance, which makes microwave-oven bumping accidents hard to foresee — countermeasures mostly focus on making it "less likely to happen" rather than preventing it outright.
- Without gravity, boiling behaves differently. On the ground, buoyancy carries bubbles away; in zero gravity, bubbles stay stuck to the surface. Experiments continue aboard the International Space Station, but heat-transfer methods usable on long-duration space missions are still being worked out.
- Even the molecular-level understanding of "cooking rice" 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 heavily on experience and sensory evaluation.
A phenomenon you watch inside a pot every single day is still, today, a subject of active research. Familiarity and understanding are two different things.
How this connects to the textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science ・ states of matter ／ boiling and melting points ／ atmospheric pressure | What boiling is, altitude and boiling point, the syringe experiment |
| HS | Basic Chemistry ・ states of matter and heat ／ Basic Physics ・ quantity of heat | Latent heat, why temperature doesn't rise while boiling |
| HS | Chemistry ・ liquid–vapour equilibrium and vapour pressure | Boiling point as the balance of vapour pressure and external pressure |
| HS+ | Chemistry ・ reaction rates (Arrhenius equation) ／ thermochemistry | The 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 open it while pressure remains |
- Japan Meteorological Agency explanatory material on altitude and atmospheric pressure (standard atmosphere model).
- 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), on pressure-cooker accident cases and safety warnings.
- 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.
- Instruction manuals from various pressure-cooker manufacturers, on operating pressure, foods not to cook, and maintenance guidance.
※ Figures such as atmospheric pressure, boiling point, and pressure-cooker temperature vary with conditions and equipment. This article presents commonly used approximate values.
※ This article is a general-audience science explainer. Always follow your pressure cooker's own instruction manual when using it. The figures given here are approximate values meant to aid understanding, and will vary with conditions.