Why Sweat Cools You Only When It Evaporates

On a hot day, a wet shirt can seem like proof that your body is cooling down. Yet producing sweat is only part of the process. The main cooling occurs when that liquid turns into water vapor and enters the air.

Your body continually produces heat as it uses energy. Working muscles generate extra heat, which is why walking uphill can make you warm even on a cool day. When your brain detects a rise in body temperature, it can signal tiny glands in your skin to release sweat: mostly water with a small amount of salt.

Turning liquid water into gas requires energy. As sweat evaporates from your skin, it draws some of that energy from your body as heat, helping you cool down. Sweat that drips onto the ground does not provide the same cooling benefit because it leaves your skin before it can evaporate there.

The surrounding air makes a considerable difference. In dry air, sweat can usually evaporate more readily. Humid air already contains a lot of water vapor, so evaporation is slower. You may therefore feel very wet yet remain uncomfortably hot. Moving air can help by carrying away the damp air close to your skin and replacing it with air that allows more evaporation.

Visible sweat is consequently not a straightforward measure of cooling, effort, or fitness. People sweat differently, while clothing, weather, and activity all affect what happens to that moisture. Sweat may also evaporate before you notice it.

The useful distinction is between producing sweat and losing heat. Your body supplies the water, but conditions around your skin help determine how effectively that water can cool you.