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When the heat of summer sets in, people flock to the beaches or head to the mountains to cool off. But in some arid parts of the world, on hot, dry days, people cool off by drinking hot beverages.

Which doesn’t make a lot of sense, if you’ve never heard of the practice. If a cup of hot cocoa on a cold, snowy day makes you feel warmer, how could a hot drink also make you feel cooler? But there’s evidence to back up this age-old tradition, if the conditions are just right.

In a study published in 2012, subjects exercised on a stationary bike. And they drank water ranging in temperature from near freezing up to 50 degrees Celsius. After seventy-five minutes, researchers measured the amount of heat stored in subjects’ bodies.

Heat storage was lowest when they drank the warmest water. The key thing to point out here is that the researchers created conditions where sweat could fully evaporate from the subjects’ skin. The relative humidity was low, a fan was blowing, and the participants wore minimal clothing.

In these conditions, the participants lost the largest share of their body heat through evaporative cooling, which is to say, their sweat dissipated heat. When water evaporates, it changes phase from liquid to gas. This phase change requires energy.

More precisely, it draws heat energy from the surrounding environment, which, in this case, includes your skin. Hot beverages stimulate sweating. They activate temperature sensors that most likely reside in your guts.

The sensors send a message to the brain, which then tells the body to sweat more. So even though a hot drink does literally warm your insides, the warming effect is more than offset by a parallel increase in sweat evaporation. In an article published in 2018, which included data from several studies, researchers concluded that likewise, cold beverages have the opposite effect.

They make you sweat less, so they can actually make you warmer. But before reaching for a hot beverage, remember: the cooling effect happens only when sweat can fully evaporate. If you’re somewhere with one hundred percent humidity, a hot drink will just make you hotter.

There’s a reason this isn’t a widespread practice in, like, Florida. And even if you are in an evaporation-friendly environment, you should approach hot beverages with caution. If you’re dehydrated, you can’t really spare the water loss from sweating that a hot beverage would cause.

Also, sweating can still exceed the rate of evaporation, especially if the air is still. If you’re already dripping, adding more sweat won’t help. The amount and type of clothing you’re wearing matters too.

Sweat cools you best when it evaporates from your skin, not your clothes. If you’re generously clothed, like an American football player or a firefighter in full uniform, you should definitely skip the hot drinks. In these cases, it’s better to choose a cold drink because it will absorb heat energy from your body.

There’s also a psychological component: many of us find cold beverages more refreshing. In the end, experts recommended keeping your beverages at whatever temperature you find most palatable. In hot weather or during exercise, the key is to drink enough fluids to match your rate of sweating.

If you enjoy your beverage, you’ll drink more. And that’ll help safeguard against heat-related illness. Thanks for watching this episode of SciShow.

And thanks to the amazing folks who support us on Patreon, you make what we do possible. If you’re interested in helping us make cool science videos, check out patreon.com/scishow. [♪ OUTRO].