THE KINETIC THEORY OF SWEAT

As we enjoy this bit of toasty weather, do you ever stop to ask yourself why we sweat, and how it is that sweating cools us off? 

The reality is, sweating actually doesn’t cool us off – at least not directly. If the concept isn’t clear, spend a few days during a summer in Louisiana.  You’ll sweat like a pig and not cool off at all. You see, sweating itself doesn’t cool you off; we cool off only when sweat evaporates from our skin.   

To get this, there’s just a bit of basic physics involved.  I’ll try to keep it to a minimum, so please bear with me.

What we sense as temperature is actually the average kinetic (motion) energy of the molecules in our environment. As things heat up, the molecules speed up – and their average kinetic energy increases.  The key here is ‘average’.  There are billions of molecules in a single droplet of sweat, and their velocity (and thus, kinetic energy) ranges widely. 

One way to visualize this is to imagine a pool table with bazillions of teeny balls bouncing around. Whenever one of these balls hits a cushion or another ball, it transfers a bit of its energy to it. The larger the balls, or the faster they’re going, the more energy it transfers. That’s not a perfect analogy for kinetic energy, but it’s close enough for our purposes. On our pool table, some of the balls are moving slowly, some at a medium rate, and some really fast. Remember, that temperature is the AVERAGE of these. Every so often, one of the faster pool balls flies off the table (evaporates). Because it’s the fastest balls that are most likely to fly off the table, the AVERAGE speed of the balls that remain on the table is slower – that is to say that the average kinetic energy is lower, and the temperature decreases. 

Essentially, this is what happens when your sweat evaporates.  The fastest, hottest molecules of sweat evaporate, leaving the slower cooler ones next to your skin ... 'Ahhh!' 

So why doesn’t sweat work as well in high humidity? Well, as air picks up more and more water vapor, the process begins to work in reverse, with water vapor condensing onto our skin, heating it up.  As the air finally becomes saturated and can hold no more; what we call 100% humidity. At that point, a water molecule is as precisely as likely to condense into the droplet of sweat as one is to evaporate from it.  Since condensation has the opposite effect as evaporation, there is a balance between the cooling effect of evaporation, and the heating effect of condensation.  Sweat all you want – it’s going to just sit there on your body.  

As a bonus, the body reacts to the failure of this cooling mechanism by ... you guessed it, sweating even more.  You end up drenched in sweat that isn't doing you a bit of good.  

The technical term for this balance of heating and cooling is called equilibrium, but on a hot, humid day, you might know it better as Hell.