Science Puzzle
Why Does Sweating Cool You Down?
On a hot day or during hard exercise, your skin starts to leak sweat, which is mostly water. A fit athlete in the heat can sweat more than a litre an hour. The sweat isn't cold: it comes from inside your body, at about the temperature of your skin.
So how can warm water on your skin cool you down?
The Answer
It isn't the sweat that cools you. It's the sweat evaporating, which carries heat away from your skin.
In liquid water, the molecules are constantly jostling, some moving faster than others. The fastest ones at the surface can break free and escape into the air as water vapour. Every time one of the fastest molecules leaves, the average energy of those left behind drops, so the remaining water, and the skin it's sitting on, gets cooler.
It takes a surprising amount of heat to evaporate water. Evaporating just one litre of sweat removes about 2.4 million joules of heat from your body, roughly the energy needed to bring six litres of cold water to the boil in a kettle.
That's why a breeze helps, by carrying away the damp air so more sweat can evaporate, and why a wet swimming costume feels cold on a windy beach. It's also why humid days feel so uncomfortable: when the air is already full of water vapour, sweat evaporates slowly and drips off without cooling you, leaving you hot and sticky. Dogs, which sweat very little, pant instead, evaporating water from their tongues and airways.
The principle: Cooling by evaporation. When a liquid evaporates, its most energetic molecules escape, taking heat with them and leaving the liquid and its surroundings cooler.