In “The Rajma That Defeated My Grandmother”, a pot of water in Leh boils happily at 88 °C and a family’s dinner refuses to cook. The narrator figures out the short version that night. This piece is the long version: what boiling actually is, why the number changes, and where the idea leads.
Before that, it’s worth going back to two details in the story that look like throwaways. The first is the steam. The narrator notices that the steam in Leh is still hot enough to hurt, but doesn’t feel “quite like the furious, skin-stripping hot” of the steam at home in Mumbai. The physics behind it is right: steam coming off water that boils at 88 °C is itself at about 88 °C, twelve degrees cooler than steam from a pot at sea level. But twelve degrees cooler is not safe. Steam and water at 88 °C still scald in well under a second, which is why the narrator’s test is one nobody should repeat. The second is the mug of tea. When the father says the thermometer is broken, the narrator tests it in something whose temperature is roughly known. Scientists call this checking against a reference, and it’s what you should always do before trusting a surprising reading.
At any temperature, some molecules at the surface of water are moving fast enough to escape into the air as vapour. Put a lid on a jar and the vapour builds up until as many molecules are returning as escaping. The pressure that vapour pushes with is called the vapour pressure, and it rises steeply as the water gets hotter. At 20 °C it’s about 2 per cent of normal air pressure. At 100 °C it’s exactly equal to it.
Boiling is what happens when the vapour pressure is high enough to form bubbles inside the liquid, not just escape from the top. A bubble of vapour deep in the pot is being squeezed by the air pressure above, plus a little extra from the water itself. It can only survive and grow if the vapour inside pushes back just as hard. So water boils at the temperature where its vapour pressure matches the pressure around it. At sea level, that’s 100 °C. Change the pressure and you change the boiling point.
Air pressure is simply the weight of the air above you. Climb a mountain and there is less air overhead, so the pressure drops. In Mumbai, at sea level, it’s about 1,013 hectopascals. In Leh, around 3,500 metres up, it’s roughly 650. At the top of Mount Everest it’s about a third of sea-level pressure.
Lower pressure means the water reaches the balance point sooner. Near sea level, the boiling point drops by roughly 1 °C for every 300 metres you climb, though the rate isn’t perfectly constant. That puts Leh at about 88 °C and the summit of Everest at around 70 °C. Don’t mistake that for gentle: water at 70 °C can still scald skin in about a second. It’s dangerous to touch, but hopeless for cooking rice.
Cooking is chemistry, and chemical reactions speed up with temperature. A common rule of thumb is that many reactions run roughly twice as fast for every 10 °C rise. Softening a bean depends on water getting into the starch granules and breaking down the pectin that holds the cell walls together, and both are slow at 88 °C. That’s why rice, dal and beans take much longer in the hills, and why some recipes simply don’t work without help.
The help is the pressure cooker. Sealing the pot traps the steam, which raises the pressure inside, which raises the boiling point. A typical Indian pressure cooker holds around one atmosphere above the outside pressure, which at sea level lets the water reach about 120 °C. The weight on the lid is the clever part: it sits on a small vent and lifts only when the pressure reaches a set value, letting out a burst of steam. That’s the whistle. It acts as a regulator, keeping the pressure, and so the temperature, roughly steady.
It also explains the grandmother’s last line, that “up here, everyone has a cooker”. In high-altitude towns across the Himalayas, pressure cookers are a standard part of the kitchen for exactly this reason. Without one, dal and rajma can take hours, using far more fuel, which is precious when it has to be carried up mountain roads.
The idea is old. In 1679, the French physicist Denis Papin demonstrated his “steam digester” in London, a sealed iron pot with a safety valve, which could soften bones. It’s the direct ancestor of the cooker on most Indian stoves.
The idea also runs backwards. Because the boiling point depends so reliably on pressure, and pressure depends on height, you can measure the temperature at which water boils and work out how high you are. In the nineteenth century, surveyors and explorers carried instruments called hypsometers, which were little more than a small boiler and a very precise thermometer, because they were sturdier than the long mercury barometers otherwise used for the job. Boiling water was, quite literally, a way of measuring mountains.
The same principle works in the other direction too. Lower the pressure enough with a vacuum pump and you can make water boil at room temperature. Chemists use this to remove water from delicate substances without cooking them. Freeze-drying, used for instant coffee, goes a step further: at very low pressure, frozen water turns straight into vapour without ever melting. The number 100 °C turns out not to be a property of water at all. It’s a property of water at the bottom of an ocean of air.