Why Your Weird Friends Are Drinking Hot Coffee in the Summer

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It was only 8 a.m. when I stopped for an iced coffee on my way to work yesterday, but I was already melting. So my pre-caffeine lizard brain could not deal when two people ahead of me ordered hot coffee. What is happening? What is wrong with these people? It is 1 million degrees out!

As happens on occasion, I was wrong; summertime hot-coffee fans are probably smarter than me and my fellow iced-coffee lovers. As anyone from a tea-drinking culture in a hot place like India can tell you, hot beverages have some counterintuitive cooling powers, and some recent research is helping scientists understand the physiological mechanics of this phenomenon, said Anthony Bain, a PhD candidate at the Centre for Heart, Lung, and Vascular Health at the University of British Columbia.

He explained in an email to Science of Us:

When we’re hot, we naturally cool our bodies primarily by sweating, or more specifically by having the sweat evaporate from our skin (that’s important!). Our bodies sense changes in tissue temperature by a network of thermosensors located in the skin and in more central parts of our body, which send input to our brain (specifically, the hypothalamus), which then initiates sweating.

When we take in a hot drink, it appears that the thermosensors located in the stomach become overactive, and send strong signals to our hypothalamus that we are hot. In turn, the hypothalamus reacts by initiating an over-compensatory sweating response. So, when this sweat evaporates from our skin, the heat energy we lose due to evaporation exceeds the heat energy gained by drinking the hot drink. In other words, it is because our body overacted to the hot drink that we end up cooler in the end.

But, he says, there’s a slight catch, because it only works if all your sweat evaporates. If you’re already very hot and sweaty, most of that additional sweat generated by the warm beverage won’t have time to evaporate; instead, it’ll drip off your body and onto to the ground, which doesn’t help to cool you off. “So, if you are already very hot and profusely sweating — opt for the cold drink,” Bain said. “If you aren’t already sweating very much, are wearing loose clothing, and it’s not incredibly humid outside — opt for the hot drink.” Case closed on another summertime science mystery.