Cooling by evaporation
Why Do We Sweat?
Sweat cools you only when it evaporates, because evaporation carries heat away.
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Step 1 of 5
Working muscles produce a lot of heat.
Most of the energy your muscles use ends up as heat. Hard exercise could raise your temperature dangerously within minutes if nothing removed it.
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Step 2 of 5
Your body must stay near 37°C.
Two degrees above is a bad fever. Five is a medical emergency. A few degrees either way disrupts the chemistry you depend on. Temperature control is not comfort — it is survival.
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Step 3 of 5
Evaporation removes heat.
The fastest water molecules escape first and take above-average energy with them. What is left — including your skin — is cooler.
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Step 4 of 5
Only evaporated sweat cools you.
Wind does not cool you. It removes what was in the way. Sweat that drips off does nothing at all. It has to leave as vapour, which is why a breeze helps enormously and dripping does not.
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Step 5 of 5
Humid air is the problem.
Air already full of vapour cannot accept much more, so sweat stays put. That is exactly why humid heat feels so much worse than dry heat.
Sweating over the whole body is unusual among mammals, and it is a large part of why humans are such good long-distance runners.
The short version
Sweat cools you only by evaporating, which is why breeze helps and humid heat is so much harder to bear.
Try it yourself
Wet one arm and stand in front of a fan. The wet arm feels far colder — evaporation, not the water itself.