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So when you're in hot water and start to move or something the steam feels really hot even though I read its temperature is about 50 degrees Celsius in thermal spas. So why does it feel that warm?

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  • $\begingroup$ It's not clear to me exactly what you are talking about. What is a "thermal spa"? $\endgroup$ – JMac Dec 10 '18 at 18:18
  • $\begingroup$ because water (in this case, steam) conducts heat much better than air. You wouldn't put your hand in a pot of boiling (100 C) water, but you can put your hand in a hot (200C) oven to pull pots in/out. $\endgroup$ – Mohammad Athar Dec 10 '18 at 20:14
  • $\begingroup$ If you're talking about a hot tub (full of water, that you sit in) I'm pretty sure they are legally mandated to not exceed 105F or 40C. $\endgroup$ – DaveC426913 Dec 11 '18 at 1:36
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50 degrees is hot. Room temperature is about 20 degrees. Body temperature is 37 degrees. In addition, air with a lot of steam in it has much higher heat capacity and heat transference than normal air, so a sauna at 50 degrees will feel much hotter than normal air at 50 degrees, much like water at 50 degrees will feel much hotter than air at 50 degrees. Plus the high humidity will prevent your skin from engaging in evaporative cooling.

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    $\begingroup$ This similar to why a stone floor feels colder than a carpeted floor when though they (initially) are the same temperature... your feet are able to warm the carpeted floor easier. $\endgroup$ – BobtheMagicMoose Dec 10 '18 at 19:29
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    $\begingroup$ @BobtheMagicMoose Or, conversely, the stone floor is able to take and spread out your body heat much better than the carpet. $\endgroup$ – JMac Dec 10 '18 at 20:30
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Your body acts like a lightbulb and is constantly producing heat. Normally when the environment is colder than you - your body cools down by radiating out this heat. Basically, the rate of "cooling" due to heat escaping you is high enough that you maintain a constant temperature.

But when you are an enviornment that's hotter than your body temperature, you can no longer radiate heat to keep a constant temperature. Instead this human "light bulb" heat has nowhere to go and you body very quickly increases in temperature.

This is why you're only supposed to stay in saunas for a short amount of time.

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In fact, you can not feel temperature, but just loss of heat. Try it on your own: In your room is probably something made of metal and some thing made of wood (those two things are definitely at the same temperature if the are at rest for some time without (heat) radiation absorbed). Metal conducts (heat and electricity as well) much better than wood, so you lose much more heat when having contact with metal than with wood. The same is true for air. If the air is full of stream it can conduct the heat from the air better to your body as air with less steam does.

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The reason it feels so hot is that the water is being circulated, and, as a result, the convective heat transfer rate to your skin is much greater than if the water were stagnant. Basically, the heat transfer coefficient between your skin and the circulating water is high. So your skin temperature gets hotter. The same thing happens when you put your hand into the cold water flowing out of your kitchen spigot. This cold water us actually pretty close to room temperature, but it feels much colder because of the enhanced heat transfer rate.

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So when you're in hot water and start to move or something the steam feels really hot even though I read its temperature is about 50 degrees Celsius in thermal spas. So why does it feel that warm?

What exactly is 50 C? I doubt very seriously that it is the temperature of the water you are in. Classic studies in the 1940s by Henrique and Moritz indicate a 2nd or 3rd degree burn would occur on human skin in about 4 minutes if exposed to 51 C.

In any case, if you move around in hot water it will always feel “hotter” than if you remained still. It would be the same if you moved around in steamy air. This is because the relative motion of your skin with the steam or water increases the convective heat transfer rate to your skin from the water or steam that is at higher temperature than the skin. When you take a bath in hot water, it feels hot when you first get in. After a while, if you remain still, you get used to it. But if you try moving your arm around the skin on your arm will feel hotter.

The situation is similar to that relating to the wind chill factor, in which the heat transfer rate out of your skin to the colder surrounding air is greater with wind than without wind, making your skin feel colder.

Hope this helps.

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