When you're hot, your skin gets damp with sweat, and somehow that makes you feel cooler. The trick is in the leaving, not the wetness. In any film of sweat, some molecules are jiggling faster than the others, and the fastest ones break free and fly off into the air as invisible vapour. As they go, they take extra heat with them, so the sweat left behind is cooler — and it pulls that coolness right out of your skin. Dry, breezy air lets the fast molecules escape easily and keeps the cooling going. Muggy air is already so full of water that they have nowhere to go, so sweat just sits there. Drag the humidity and wind sliders in the simulator and watch your skin temperature drop as the sweat evaporates.
Most people think sweat cools you simply by being wet on your skin. In fact wet skin that never dries does nothing. The cooling happens only when sweat evaporates: the fastest molecules escape as vapour and haul their heat away, about 2,260 joules for every gram that leaves.
What's actually happening
The first surprise is that wet skin alone does nothing to cool you. A damp cloth that never dries just sits there at your temperature. The cooling happens only when the water actually evaporates — turns from liquid into vapour and leaves. To see why, picture the film of sweat as a crowd of molecules, all jiggling at different speeds. Temperature is just the average of that jiggling. The fastest, most energetic molecules are the ones that can break the surface tension and escape into the air. When they leave, they take their above-average energy with them, so the average energy of what stays behind drops. Lower average energy means lower temperature. The liquid cools itself simply by losing its hottest members, and your warm skin keeps topping it up with heat, which is heat leaving your body.
There's a precise price for this. To turn one gram of water into vapour costs about 2,260 joules, the 'latent heat of vaporisation', and that energy is hauled directly out of your skin. That number is enormous: it's more than five times the energy needed to heat that same gram of water from room temperature all the way to boiling. This is why a small amount of evaporating sweat carries off so much heat, and why stepping out of a pool into a breeze gives you that sudden chill. Your body exploits the effect deliberately, pumping out up to a litre or two of sweat an hour in real heat, betting it on evaporation.
But the bet only pays if the vapour can actually get away, and that depends entirely on the air. Evaporation is really a race between molecules leaving the skin and molecules returning to it from the air, and what matters is how much spare room the air has for more water vapour. Dry air is hungry and pulls sweat off fast; humid air is nearly full and barely accepts any. At 100% humidity, sweat can't evaporate at all and your main cooling system fails — which is why a humid 35 °C can be deadlier than a dry 45 °C. Wind helps enormously by sweeping away the thin, saturated layer of air clinging to your skin and replacing it with drier air, so a breeze on a sticky day feels like relief even though it isn't actually any colder. It just lets the leaving continue.
Sweat cools you by leaving, not by being wet, so when humid air blocks evaporation your main cooling system fails and the heat turns dangerous.
- 1Wrap a damp tissue around the bulb of one thermometer and leave a second one dry, then read both — the wet one settles a few degrees lower.
- 2Now fan the wet bulb or blow on it: its reading drops further still, because moving air carries the vapour off and speeds the evaporation.
- 3This is exactly how a 'wet-bulb' thermometer measures humidity — the drier the air, the bigger the gap between the wet and dry readings, because dry air lets more sweat leave.
Common questions
Evaporation is a race between molecules leaving your skin and returning from the air. Humid air is already nearly full of water vapour, so it barely accepts any more and sweat just sits there instead of evaporating.
Wind sweeps away the thin, saturated layer of air clinging to your skin and replaces it with drier air, so the fast molecules can keep escaping. It speeds the evaporation rather than lowering the temperature.
A dry 45 degrees can be survived, but a humid 35 degrees can kill, because once the air is saturated sweat cannot evaporate and the body's main way of shedding heat simply stops working.