I always assumed the way sweat cooled us was how the water on the outside of our skin cools off and in turn cools the skin down however I don’t get why the air that cools the sweat cant cool our bodies especially if our skin can heat that sweat just as fast or faster than the air can cool it.
In: Biology
As the sweat evaporates it takes some of your body heat with it. That’s why humidity sucks. Humidity makes it so that the sweat doesn’t have anywhere to go. It’s like trying to clean up a spill with a wet paper towel. The towel, like the air, can’t take in any more moisture.
Let’s say you have a class of kids with an average grade of B. Most have B. Some have A. Some have C. Now, let’s say all the A kids get moved to some special gifted child class. What’s the class average now? Probably a B-. Did the other kids in the class get dumber? No. The smart ones were just removed, lowering the average.
Temperature is due to the random motion of molecules and atoms. Basically, the higher their average motion, the higher the temperature.
Your skin is a solid. You can only cool when air molecules hit you and fly away from the collision with more energy than they came in with. This carries away your thermal energy, cooling you. This does happen, as you are colder that the air, so the average collision gives energy to the air. But it not very fast at moving this heat. Air is not very dense, and the rare collisions don’t take that much energy.
Water is a liquid that is quite volatile, that is it evaporates easily. It’s the same temperature as you on your skin, and this corresponds to a certain average energy for the water molecules. Now, this is just an average. Some have more energy, some have less. Now back to water being volatile. Some of the higher energy ones have enough energy to break away from their bonds to other water molecules. They evaporate into a gas.
What does the high energy water molecules leaving mean? Your average dropped. Just like the A kids leaving, the most energetic water molecules leaving leaves a lower average behind, that is cooler. Ypur body then transfer heat to this now colder water, cooling you. And water is dense as a liquid, and a lot of energy is lost when the water evaporates, so this is very good at cooling you.
All A/C and fridges work the same. We just do it with some gas other than water that is even more volatile. That is, boils at a colder temperature and evaporates more easily. We then use a compressor to undo it so we can do it again. A/C is sweating in a closed loop.
The water on your skin doesn’t just cool off by touching the air, it evaporates. Water requires a lot of energy to turn from a liquid into a gas, and it takes some of that energy from your skin, reducing its temperature.
The process at work here is called evaporative cooling. The water in sweat evaporates, and evaporating water takes energy, quite a bit of energy it turns out, equivalent to about half that water’s weight in TNT. Heat from the water itself and its surroundings goes into turning liquid water into water vapor. The result is a stronger cooling effect than you can get from a temperature difference alone. You can even cool an object to below ambient temperature in this way.
Sweat doesn’t always cool our bodies- sweating is only effective when equilibrium vapor pressure holds or favors condensation. Basically, if the environmental air is depleted of moisture, when the sweat from your body hits the dry environmental air, the sweat from your body experiences net evaporation. This is because the water vapor molecules have less mass than diatomic oxygen and nitrogen (primary constituents of dry air). In this specific case, a person can be unaware that they’re even sweating and if that person continues physical exertion, it will lead to heat exhaustion because their perspiration is immediately being wicked away from their skin. Now, if a person is sweating and their surrounding environmental air has a water vapor pressure in equilibrium or greater than equilibrium, that person’s sweat will enter an environment where it contains too much water vapor molecules. To try to reach equilibrium vapor pressure, the water vapor in your sweat will condense on your skin, changing phase into liquid water. When water vapor condenses, energy (heat) is released which lowers the temperature of the water molecules on your skin, essentially working to cool you down by releasing heat into the air that surrounds you and simultaneously cooling you in the process.
In dry climates like Nevada, Arizona, and Utah where there’s little humidity in the air, one option for cooling a house is an evaporative cooler. They operate by having a big metal box on top of the roof typically and in the four sides are a material that can absorb water. A fan pulls air through the wet pads and down into the house. With the humid air being blown into the house, that moist air evaporating is what cools the house. Funny enough, it works best with your windows open compared to a traditional AC unit that runs with the windows closed
Lick the back of your hand, now gently blow on it. That’s how sweat cools you down, evaporation.
short answer: evaporation.
experiment you can try at home: get a dropper or q-tip. then get a cup of water and some rubbing alcohol (higher concentration the better, go with 90%+ vs. 50%). now put a drop of water on the back of your hand or forearm while keeping your hand/forearm very still. note how warm/cool it feels. now note how long it takes to evaporate. repeat with a drop of alcohol. bonus: repeat with a drop of hand sanitizer (main ingredient is ethanol; rubbing alcohol is isopropyl alcohol–similar to each other, but different).
you’ll note the alcohol feels much cooler even though it came from a room temperature bottle.
your sweat works the same way. for molecules of a liquid to make the jump to becoming a gas takes energy. if it’s the drops of water or alcohol as well as sweat, it can get that energy from your skin’s surface. so each little drop takes a bit of energy to jump to being gas molecules (evaporation) and so that spot on your skin cools off just a bit.
repeat this by a larger surface area of sweaty skin and/or add a breeze which helps in that evaporation process and you can really feel that cooling off effect.
another way to visualize it is to put lots of marshmallows on a large shallow baking pan. jiggle them around slowly–not enough to knock them off the pan. that’s kind of like the movement of the molecules in a liquid. if you were to add more shaking, you’d see some marshmallows probably leave the pan. if you jiggle and shake a lot, you’ll see even more marshmallows leave the pan. see? it takes energy for those “molecules” to leave the pan and have enough movement to transition to the “gas phase.”
How do you feel coming out of the shower? A bit chilly? The water evaporates, chilling you down.
The fact that you can sweat with your entire skin makes you cool. A dog ie. can only sweat through is tongue, this is why any human can easily outpace more or less any other animal. A horse is quicker than you, but you can keep going for days, the horse can not. It will get overheated and collapse and then you are still able to throw a spear into it.
This is probably a lot of the reason that humans, as an animal, have been able to succeed as well as we have.
After you get out of the shower, your wet skin starts to get cold right? The water makes you feel cold. Sweat makes your skin wet like water does to get you cooled down.
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