What evaporation does to body temperature

Short answer: When sweat evaporates it draws heat out of the skin, and that is the body's main way of cooling itself. A fabric that lets moisture evaporate supports the process. A fabric that holds on to moisture can slow it down instead.

Evaporation takes energy

Every gram of sweat that evaporates removes around two thousand joules of heat from the skin. That is the energy needed to turn liquid into vapour, and it is taken from the body. So sweat only cools once it leaves the skin. An open fabric like a bamboo jogging suit feels cooler than a dense one because the vapour can escape.

Sweat alone does not cool

It is not the sweat itself that cools you but its evaporation. Sweat that drips off or is soaked up by a layer without evaporating removes almost no heat. That is why you can sweat a lot without cooling down when conditions are wrong. The body works hard, but the heat stays, and you feel both warm and clammy.

The fabric can slow or help

A fabric that holds moisture close to the skin does not let it evaporate, and the cooling does not happen. A fabric that moves moisture to the surface lets it evaporate from there and cools better. The difference is clearest in layers worn close to the body all day, like bamboo panties, where moisture otherwise tends to linger.

Air exchange is the other half

Evaporation stops once the air around the fabric is saturated with moisture. Fresh air is needed before sweat can keep evaporating. That is why a loose fit often cools better than a tight one, because air can move between fabric and skin. The same applies to your feet and bamboo socks for women.

Humid weather makes it harder

On a humid day the air is already close to saturated. Sweat struggles to evaporate, and the body loses its best way to cool down. You sweat more but do not get cooler. That is why a humid summer day can feel much hotter than a dry day with the same temperature on the thermometer.

When cooling comes at the wrong time

Evaporation also cools you when you do not need it. A wet top keeps drawing heat from the skin after you stop moving. In wind it happens even faster. That is why you can feel cold shortly after being warm. Change into a dry layer once the activity is over.

How much sweat we are talking about

During moderate activity, half a glass to a full glass of fluid evaporates in an hour. That does not sound like much, but it is enough to remove a fair amount of heat. The fluid does need replacing, so remember to drink after being active. Thirst is a sign the body has already used some of its reserves.

How to help your body

Choose light, open layers when you are going to move. Leave room for air between fabric and skin. Take off a layer before you get too warm, not afterwards. And change into dry clothes when you are done. Small choices like these make it easier for your body to keep a comfortable temperature all day.

Frequently asked questions

Why is it worse in humid weather

Because the air is already almost saturated, so sweat cannot evaporate. The body sweats more, but the cooling gets weaker.

Does a wet top cool you

Yes, but it also cools you when you do not want it to, for example in wind or after you have stopped moving.

How much sweat evaporates in an hour

During moderate activity, half a glass to a full glass of fluid. The amount rises in warm weather and when you move faster.