Sweatta

Meet Sweatta, a drop of sweat who cools you down by carrying heat away as she evaporates.

To turn liquid into vapour, molecules must break free of the attractions holding them in the liquid. That takes energy, the latent heat of vaporisation. The heat needed to evaporate a mass m is Q = mL_v. There is no change of temperature during the change of phase; all of the energy goes into separating the molecules.

Water's latent heat of vaporisation is very large, about 2,256 kJ/kg at 100 °C and somewhat more at skin temperature. Evaporating just 10 grams of sweat takes over 22 kJ from your skin, enough to cool about 6 kg of tissue by 1 °C.

That is why sweating is the body's main way to shed heat during exercise, and why a breeze, which speeds up evaporation, feels cooling. Clay water jugs and evaporative coolers use the same effect.

In humid air, evaporation slows because the air already holds a lot of water vapour, so sweat drips off without cooling as much. That is why humid heat feels more oppressive.