Edit: Thank you all for the detailed answers! You guys are awesome.
In: 2953
Most people think that you have some really cold ice, and you add a lot of heat energy to it, and then it gets to melting temp of 0ºC, and you add a tiny bit more then it melts into 0ºC water, then you add a bunch more and it gets to 100ºC, then a tiny bit more to convert it to steam, and it flies off.
That’s a completely wrong picture. Here’s how it really works.
Say you have some really cold ice and you heat it until it becomes 100ºC water. What does the heat energy input look like?
Well, you add some amount of energy to get it to 0ºC. How much is the “tiny bit” of energy required to get this 0ºC ice to turn into 0ºC water? In other words, how much heat are we adding not to raise the temperature, but just to break apart the water molecules?
That “tiny bit” turns out to be the same amount of energy as it takes to get the water from 0ºC to 80ºC—four-fifths of the way to boiling!
Okay, so now we get it to 100ºC water. How much energy does it take to convert that to 100ªC steam, that is, just to break the water molecules apart into a gas without raising the temperature? It turns out this “tiny bit” of energy is enough to take that amount of 0ºC water to 100ºC *six times*, or six times the water from 0ºC to 100ºC, if you prefer to think of it that way.
Why am i telling you all of this? Because when you sweat, and water evaporates off of your skin, it is taking away that amount of heat energy from you … the same amount of energy it would take to heat six times that amount of water from 0ºC to 100ºC. That is how much it’s cooling you off.
Now, let’s say that you are in perfectly dry air, and every time you lose a bit of water, it flies off and is replaced with perfectly dry air. This is an ideal cooling environment, and however much you sweat is how much heat you’ll lose.
But now we start adding humidity to the air. We add enough such that the amount you sweat is replaced by water condensing out of the air onto you. This means the humidity is somewhere between 0% and 100%, but not at either extreme. In this environment, you cannot shed *any* heat from sweating. (That’s not quite true since the water could be evaporating and condensing at different temperatures, but it should be clear that that’s a second order effect in terms of how much that affects heat transfer.)
What if we keep adding humidity after that point? In this environment, we’re going past equilibrium. Now, more water is condensing on you than you’re losing. Not only can you not sweat, but the environment is adding heat energy to you.
Latest Answers