ASK TOM: Why does it feels cold when your skin is wet?

Dear Tom, 

My son, age 10, asks why it feels cold when you are wet. 

–Emily Jedwill


Dear Emily,

That familiar “wet skin chill” is rooted in the rules that explain how heat moves from one substance to another.


Water (in this case, water on our skin) evaporates because some molecules move fast enough to escape as water vapor.


Water molecules within a liquid are constantly jostling and colliding and, as a result of the collisions, some of the molecules acquire above-average speeds — speeds sufficiently great that they break through the water surface — that is, evaporate — taking their above-average energy with them.


On average, those molecules left behind (the water remaining on our skin) now possess less energy and that registers on our skin as a decrease in temperature.