Sweat: Wiping Versus Evaporation

When I get sweaty, I wipe it off with a rag or my gym shirt. This never brings a cold sensation the way a breeze does when I’m wet. Why is that? If the sweat leaving the body carries away heat, then removing with a cloth should do the same as doing it with air. But this is apparently not the case. Why does evaporation carry away the heat when soaking up the sweat a cloth or towel fail to?

It’s not that sweat removes heat from your body. Your sweat evaporating is what removes the heat from your body. Wiping it away loses the important evaporation cooling.

It takes 540 calories to evaporate a gram of water. Mopping it off with a towel, just about nothing.

Oops that might not have been completely clear. When you wipe it off with a towel you are just moving the sweat. When the sweat evaporates it is changing from a liquid to a gas state. It is that state change that takes the heat away.

In order to evaporate, the molecules must attain escape velocity. This means they have to get heat from somewhere. That somewhere is your skin. Think of each molecule as a rocket and the heat of your body as rocket fuel. As each rocket leaves, it takes a little rocket fuel with it.

But what if the towel weighs 3000 pounds? Wiping it off with that’d take a few calories.

if you can lift that towel, you don’t need the gym…:smiley:

As the sweat evaporates, the hotter molecules are drawn away, transferring some of the heat energy into the atmosphere. The rest of the sweat is still on your skin, though, and still being heated. Presumably if you wipe off the sweat, while you will remove some heat with it, you are just resetting the process.

As relative humidity rises, evaporation slows down, which is why dry heat feels cooler.

It’s not just heat being transferred as the sweat leaves - the change of state from liquid to gas requires an energy input that does not raise the temperature of the resulting gas above that of the liquid from which it evaporated - it’s called latent heat of evaporation.

To give you an idea of the amount of heat that can be absorbed in a change of state. It takes 1 BTU to change one pound of water fron 60 degrees to 61 degrees. But it takes 970 BTUs to change one pound of water at 212 to steam.

I’ve noticed wiping the sweat off produces a cooling sensation. Just a guess but wiping sweat off leaves a thin film of sweat behind on the skin, which I believe would have less surface tension, making evaporation easier.

In a way yes, but that energy, taken from within your body, is converted into moving it plus heat, so in the end you will only get hotter producing even more sweat.

Does this mean that a cloud of water vapor at 60 degrees has more energy than a pool of water at 60 degrees?

Assuming the mass is the same (so the cloud will be much bigger than the pool), I think so

way, way more.

Seat of the pants estimate:
The electrical element on your kitchen stove is, what, 500 watts? And it probably takes a couple of hours to completely boil away a gallon of water, so 1 kilowatt-hour. Multiply by about 15,000 gallons for a decent pool.

For a more exact number, multiply the mass of water in the pool (56,780 kg) by the heat of vaporization for water (2257 kJ/kg), and that’s how big the energy difference is (128,152,460 kJ).

Want a feel for that number? Take your 350-horsepower V-8 engine, and run it wide-open for 136 hours straight; that’s how much more energy a 57,780-kg cloud of water vapor has than an equal mass of liquid water.

So why does evaporative cooling work? Any collection of molecules/atoms has a distribution of thermal energy levels. The average is represented by the bulk temperature, but some of the molecules have more energy (hotter), and some have less energy (cooler). In the case of a liquid (such as the sweat on your skin), some of the hottest individual molecules will have enough energy to become vapor. When they do this, the colder molecules are left behind: the average energy level (temperature) of the remaining molecules of sweat on your skin drops, i.e. the liquid sweat left behind is actually colder. This colder collection of sweat pulls heat from your body, raising the average energy level, and the hottest molecules of water continue to jump to vapor form.

When you wipe the sweat away with a towel, you are wiping away equal quantities of hotter and colder water molecules; you don’t particularly change the temperature of the sweat left behind on your skin.

Why does evaporative cooling NOT work in humid environs? The same energy distribution concept is at work in the water vapor molecules in the air: although the vapor molecules are, on average, at the same temperature as the air, some of the vapor molecules have low enough energy to spontaneously condense into liquid form on the nearest nucleation site or free liquid surface. At 100% relative humidity, we have equilibrium: the rate at which molecules of water evaporate from your skin is equal to the rate at which water vapor molecules are condensing, and no net evaporative cooling effect takes place.

This last paragraph helps explain why a breeze really improves things. If you sweat while standing still in quiescent air, you develop a high-humidity microclimate right next to your skin. A breeze helps move that humid layer of air away from you, bringing in fresh dry air to help hasten the evaporation process.

Actually a cloud is not water in the gaseous state, so no.

I also notice that having wiped sweat off (in truth leaving a thinner layer of sweat on my skin) I cool off better than if the larger amount of sweat is left dripping. Is the surface tension hypothesis correct? An alternative hypothesis is that evaporative cooling is just one means of cooling and that we also radiate heat off - more than a thin layer of sweat may function as an insulating layer preventing that radiant heat loss while not increasing the evaporative loss form that achieved by a thin layer of sweat.

And since 2 ounces of scotch has only 130 calories, I should be able to sit around in the sauna, drinking scotch over ice, and lose weight like mad!!

P.S. I tried this and it didn’t work.

Two ounces of scotch has 130 Calories, with a capital “C”. It’s 130,000 calories. You could try to make it up in volume though.

I don’t believe surface tension appreciably affects evaporation rate.

At the temperatures we’re talking about, the magnitude of radiative heat transfer is negligible compared to evaporative and convective cooling.

Please let me know when you open your gym. I would like to join. :slight_smile: