In “The Matka That Sweats to Stay Cold”, Nani’s clay pot in Jodhpur, on a thirty-eight degree afternoon, kept its water at twenty-seven: eleven degrees cooler than the kitchen. The narrator then tried the same trick in Mumbai in July, and the matka managed two degrees. Same clay, same water, same physics. What changed was the air. And the same thing that limits a matka limits you, because your body cools itself in exactly the same way.
Meteorologists have a number for this limit. Take an ordinary thermometer, wrap its bulb in a wet cotton cloth, and let air blow over it. Water evaporates from the cloth, taking heat with it, and the reading falls. But it doesn’t fall forever. As the bulb gets cooler, the water evaporates more slowly, and eventually it reaches a temperature where the heat flowing in from the warm air exactly balances the heat carried away by evaporation. That temperature is called the wet-bulb temperature. It’s the lowest temperature that evaporating water can cool anything to, in that particular air. A matka, at its very best, approaches it. So does your sweaty skin.
The wet-bulb temperature depends on two things: how hot the air is, and how much water vapour it’s already carrying. Dry air can take up a lot more vapour, so water evaporates into it quickly and the wet bulb drops far below the air temperature. Humid air is close to holding as much water vapour as it can at that temperature, so evaporation is slow, and the wet bulb sits only a little below the air temperature. On a dry afternoon in Rajasthan, at forty-two degrees and fifteen per cent humidity, the wet-bulb temperature is only about twenty-two. On a humid Mumbai day in the monsoon, at thirty-three degrees and eighty per cent humidity, it’s around thirty. The Mumbai day is nine degrees cooler by the ordinary thermometer, and eight degrees worse by the one that matters to your body.
Here’s why it matters so much. Your body is constantly making heat, around a hundred watts even when you’re resting, more when you move. That heat has to leave, or your temperature rises. Normally it flows out of your skin, which is usually at about thirty-five degrees, into cooler air around you. But on a very hot day, when the air is warmer than your skin, heat flows the wrong way: into you. Then you have only one way left to shed heat, the matka’s way. You sweat, and the sweat evaporates, taking heat with it.
Evaporation can only cool your skin towards the wet-bulb temperature. As long as the wet bulb is well below thirty-five, sweating works, which is why people can work, carefully, in the dry heat of a desert at forty-five degrees. But as the wet-bulb temperature climbs towards your skin temperature, evaporation slows, and the sweat simply pours off you, cooling almost nothing. Once the wet bulb reaches skin temperature, no amount of sweating can carry heat away. Your body’s heat has nowhere to go, and your core temperature begins to rise.
In 2010, two climate scientists calculated that a wet-bulb temperature of thirty-five degrees, sustained for hours, would be beyond what any human body could survive, even resting in the shade with a fan. More recent laboratory studies, which tested young, healthy volunteers in heat chambers, found that their bodies began to lose control of their temperature at lower wet-bulb temperatures, closer to thirty-one degrees. For older people, young children, or anyone working hard in the sun, the danger begins lower still.
That’s why weather services increasingly talk about humidity and “feels like” temperatures rather than air temperature alone, and why a humid heatwave can be more dangerous than a hotter, drier one. It’s also why the advice for humid heat is so practical: rest in the shade in the hottest hours, drink water to keep sweating, wear loose, light clothes that let sweat evaporate, and look out for people who can’t do those things easily.
Nani’s matka, sitting by the window with its wet cloth round its neck, was demonstrating all of this every summer. In Jodhpur’s dry air, it could cool almost all the way down to the wet bulb, eleven degrees below the room. In Mumbai’s damp air, the wet bulb was barely below the room temperature, so even a perfect matka had nowhere to go. You are a matka too, sweating to stay cool. And on the most humid days, you share its limits.