Are we all just cold ones?
Last week here in New York City, temperatures were over 90°F for a few days in a row. It was humid, I was sweaty, it was gross. It was fun but as a bonus, it made me wonder how and why we perspire.
I think many of us know the general idea. Our body releases sweat through its sweat glands, which then sits on the surface of our skin and slowly evaporates. This evaporation is actually a form of energy release for the body—converting liquid water into water vapor. It’s quite energy intensive—to push one water molecule into the air, you have to give it enough energy to break away from the surface of the water on our skin. [To be preciese, it takes 540 calories of energy to evaporate one gram of water; one water molecule has a mass of 0.00000000000000000000002991507 g; therefore it takes 0.0000000000000000000161541378 (1.61541378×10-20) calories of energy to get one measly water molecule to evaporate.] Once that happens, it is kicked out into the atmosphere along with the energy. And since the average kinetic energy a system is defined as its temperature, our body is cooled one tiny bit.
So yah, totally awesome, right? Sure, whatever you say…
Anyway, that bit is pretty well covered in introductory physics classes and physical trainer training courses… but my question is this: when it is hotter outside than it is on our skin, does water not evaporate? Does our skin start to act like a cold can of cheap beer and pull water from the air onto its surface? Does water condensate onto our skin?
The answer is “yes” but also “no, not really.” So yeah, question answered. See you next time…
Seriously though, water is evaporating because of the science reasons in the first couple of paragraphs above, but there is also condensation. Always, there is condensation. Never forget.
BUT (that’s a big but!) we only perceive the average effect though—see above where I define temperature as the "average" kinetic energy of a system (I’m tricky like that). So in normal outside situations we experience the evaporation of perspiration and barely even notice the condensation that is happening (never forget!).
The same is true for when the temperature is greater than body temperature (for humans that is about 35°C [98.6°F]), and when the humidity is really high—90% or more—the net effect is evaporation. So there you go. The law of averages wins again.
One exception is what I have dubbed the "Blues Brothers Situation, or the BBS" (in a sauna), where our skin will actually become a cheap can of cold beer and gain liquid on the surface of our skin. But you’re also sweating. It’s just that the BBS is the ultimate battleground for evaporation and condensation. Who wins is ultimately up to our body. You either leave the sauna and evaporation wins or you stay in that sauna forever and our skin gets all wrinkly like when I’ve been in the bath for an hour and condensation wins the (average for the) day.
While researching this idea, I came across this cool piece of science: these tree frogs "chill out" in trees during dry seasons and use condensation to collect water to hydrate themselves. Cool idea (I’m gonna get my comedy license revoked for that one…).