In school we’re all taught that the function of perspiration is to cool the body. Okay, fine; that makes sense. But if that’s true, why does wiping it off make you feel cooler? Shouldn’t it cool me more effectively if I just leave it there?
I’m sure there’s a good physics-based explanation for this …
My WAG, when you’re really sweaty, the droplets are too thick to evaportae quickly. When you wipe off sweat, you’re not wiping skin completely dry but actually leaving a thinner layer of sweat on the skin that evaportaes more quickly and makes you feel cooler.
Hmmmm.
“Sweating itself does not cool the body - evaporation of the sweat cools the body. One often sees athletes use a towel to wipe sweat from his or her body. While it helps keep sweat from stinging the eyes or keeps hands dry for a better grip, this practice actually decreases the cooling effect of sweat because the sweat does not have a chance to evaporate.” (www.hmc.psu.edu/ufc/resources/news/heatdisorders.doc)
So, wiping sweat is actually counter-productive to cooling, as you said; perhaps it “feels good” because the sensation of slick beady droplets clinging to us gives us the willies, plus sweat makes your hands slippery, stings eyes, etc?
EmmaJane
Your link appears to be private so I can’t tell if this is your cite for this.
But I have trouble with your explanation. My personal experience is that, if I have a lot of sweat on me then I am producing it much faster than it can evaporate. So the sweat sits on my skin. The sweat has contains heat which is removed with the sweat on my towel so wiping it away removes some heat from the surface of my skin. (maybe the act of wiping my skin produces more heat from muscle use?) As soon as the towel wipes the sweat away I produce more sweat which pulls more heat from my body. The evaporation now occurs closer to my skin and I feel cooler.
It seems to me that if you have drops of sweat on you, you are sweating more than the evaporation can deal with. In that case, you are merely removing the excess and won’t decrease the evaporation and its cooling effect.
Does wiping it off make you feel cooler? I’ve never noticed this.
It’s a relief to wipe it off your face since you’re keeping it out of your eyes and mouth, but that’s about it. I don’t see people constantly wiping sweat off other parts of the body unless they’re purposefully trying to dry off.
Maybe it’s a clothes thing. I don’t think sweat works very well to cool the body when you have a layer of cloth covering the sweat. It may actually feel cooler to dry the skin in this case.
I think this is correct. And I think wiping actually increases the evaporation - because it allows the sweat to evaporate from the warm skin (or even below the skin - I don’t think sweat needs to spread out onto the skin before it can evaporate.) Whereas if the sweat is beading on your skin, it’s only evaporating from the surface of the already cool droplets.
As sweat evaporates, it leaves salt behind, which increases the salt content, and so reduces the vapor pressure of the sweat still on the skin. The decreased vapor pressure decreases the rate of heat dissipation. Since the water evaporates, and the salt doesn’t, the salt content of the liquid on the skin keeps increasing. Wiping off this (relativly) concentrated brine allows subsequently produced sweat to evaporate faster and/or at a lower temperature, increasing the heat removal rate. Further, due to reduced evaporation, the brine is closer to body temperature than “clean” sweat, removing it from the skin instantly removes a bit of heat.
Brine removal is a requirement for efficiency in industrial evaporative cooling applications.