The Matka That Sweats to Stay Cold

Every summer, the first thing Nani does is buy a new matka. She goes to the same potter near the clock tower in Jodhpur, taps every pot with her knuckle and listens like a doctor with a stethoscope, and comes home with one that sounds, to her, correct. The steel bottles in the fridge are colder, she admits, but the water from the matka is the only water that tastes like water, and she will not hear otherwise.

The summer I stayed with her, I noticed something odd. The outside of the pot was always damp, as if it were sweating, and on the hottest afternoons, with the kitchen at thirty-eight degrees, it felt cool to touch. I had a cheap thermometer from school, so I measured. The kitchen: thirty-eight. The water from the matka: twenty-seven. There was no electricity involved, no ice, nothing but a lump of baked mud doing eleven degrees of work.

The clay, it turns out, is full of tiny pores, and water creeps through them to the outside surface. That’s the sweat. There, in the hot, dry air, it evaporates. And evaporating is expensive. Turning a gram of liquid water into vapour takes about 2,260 joules of energy, more than five times what it takes to heat that same gram all the way from freezing to boiling. That energy has to come from somewhere, and the nearest place is the pot and the water inside it.

What I liked most was why that happens, which is about which molecules leave. The molecules in a pot of water are not all moving at the same speed. Some are sluggish, some are fast, and temperature is really a measure of the average. At the surface, only the fastest have enough energy to break free from their neighbours and fly off into the air. Every time one escapes, the water left behind loses one of its most energetic members, and the average drops. The pot is quietly losing its fastest molecules, one at a time, and what remains gets cooler.

Then I went home to Mumbai in July and tried to prove all this to my mother with a matka of our own. The water came out at thirty, against a room at thirty-two. Two degrees. Nani’s pot had beaten her kitchen by eleven.

The difference was the air. In Jodhpur in May, the air is dry, so it has plenty of room for water vapour, and water evaporates off the clay as fast as it seeps through. In Mumbai in the monsoon, the air is already carrying nearly as much water vapour as it can hold. For every molecule that escapes the pot, another one from the damp air lands back on it. Evaporation slows to a crawl, and so does the cooling. The breeze matters too: moving air sweeps the damp layer away from the clay and replaces it with drier air, which is why Nani keeps her matka by the window, on a stand, with a wet cloth tied round its neck.

I told her all this on the phone, very pleased with myself. She listened, and then said that her mother had kept the matka by the window too, and her mother’s mother, and none of them had needed a thermometer to know where the cool air came from. Which is fair. Science, in our family, is mostly finding out why the grandmothers were right.