Why does it get hotter (feel hotter) in a sauna when one pours water over the hot stones?
Wikipedia says that the water condenses onto the skin, but the actual air humidity is so low that I doubt anything is condensing there. The water (sweat) evaporates from skin instead.
Best Answer
It actually gets a bit complicated, since several effects are involved:
Evaporating water does require heat, which comes primarily from the hot stones. So throwing water on the stones does cool them down. (This is where the claim one occasionally hears, that "throwing water on the stones makes the sauna colder", comes from. Technically it's true, if one considers the total heat content of the sauna as a whole. But since most of that heat is in the stones, and since you don't sit on the stones, that's pretty much completely irrelevant to how hot the part of the sauna that you do sit in gets, or feels.)
On the other hand, throwing water on the stones also significantly increases the heat transfer rate from the stones to the air: the evaporation produces a lot of hot steam, which will rise and mix with the ambient air in the sauna. So it is possible for the air temperature in the sauna to increase, even as the stones are cooled down.
Also, the introduction of steam obviously increases the humidity of the air, which will increase the rate of water precipitation on skin, and/or decrease the rate of sweat evaporation. (The relative importance of these two effects will depend on the baseline humidity of the air, which can vary quite a lot. My gut feeling, based on experience, is that in all but the driest of saunas condensation probably dominates, simply because human skin is so much cooler than the air.) In either case, the effect will be to transfer more heat to the skin, and thus to make the air feel hotter.
Finally, as the hot steam rises off the stones, it will push hot air around the sauna in front of it. While this increase in air movement is slight and transient, it probably does have a noticeable effect: as the hot air flows past the people in the sauna, it will act to disperse the layer of cooler air that forms over the skin, and thus increases heat transfer to the skin. (If you don't believe me, try blowing some air over your skin in a sufficiently hot sauna. It burns.)
The upshot is that throwing water on the stones increases heat transfer, both from the stones to the air and from the air to your skin. As long as the stones stay hot enough to supply that heat, the net effect will be that you feel hotter.
However, if you throw too much water on the stones, it's possible to "kill the stones" by cooling them close to or even below the boiling point of water. At that point, throwing more water is useless, and all you can do is add more firewood or turn up the thermostat and wait for the stones to heat up again. Or, if you manage to do this in a smoke sauna, go wash yourself and get dressed up because the sauna is over for the night.