How come wind cools you down, since it’s literally moving atoms, hitting your skin, creating kinetic energy?


How come wind cools you down, since it’s literally moving atoms, hitting your skin, creating kinetic energy?

In: Physics

Phase change Refrigeration. The wind hitting your bare skin increases the rate of evaporation of the moisture on your skin. This causes the moisture on your skin to go from a liquid to a gas. This change in state of matter is an endothermic reaction which means it requires heat energy to be completed. It gets this heat energy from your body which is why you feel cold. Basically it increases the rate that the moisture evaporates from your skin.

If you want to see a low-key version of this spray your skin with rubbing alcohol and blow on it it will feel very cool. This is because rubbing alcohol evaporates very quickly.

Your perception of temperature is largely governed by how quickly your body can shed heat. And how quickly your body can shed heat is related to the temperature difference between your body and the air immediately surrounding your body. The smaller the difference, the less quickly you shed heat and the warmer you feel.

Wind (or any other form of air circulation) moves the air that you’ve warmed away from your body and replaces it with air that has yet to be warmed by your body. This is why wind only makes you feel cooler if the air temperature is, in fact, cooler than your body.

Air that sticks around your skin heats up, until it’s the same temperature as your skin.
Wind that hits your skin heats your skin up a very, very tiny amount.
Wind that hits the air next to your skin blows it away, replacing it with new air, which is much cooler than your skin.

So on net, wind cools you down, because it keeps your skin in contact with cooler air for longer. (If the air temperature is hotter than your skin, this stops working unless you’re also sweating, since sweating will pull heat away by evaporating, and the wind *also* pulls the water away, giving new sweat more dry air to evaporate into.)

Most of the time, the wind is lower temperature than you are. So, although it has kinetic energy, it’s more likely to *get* kinetic energy from you, not give it.

When the wind is hotter than you, you still get evaporation cooling, like /u/slaax976 discusses in their comment.

If the wind gets fast enough then the directed kinetic energy in it can become a bigger contributor than the random thermal motion…that’s why things heat up when you go *really* fast (like supersonic). This is called “total temperature” or “stagnation temperature” and it can be hundreds or thousands of degrees higher than the ambient temperature (“static temperature”). But it’s so fast that it’s effect is negligible at normal wind speeds that we experience.

it’s the same principle as using running water to thaw food. The rapidly flowing molecules increases your exposure to total molecules and thus can exchange heat quicker.

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