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When the humidity in the air is high, we sweat more and feel it's hotter than when the humidity is lower.

So why don't we feel it's hotter when we go inside water, where the water content is much higher than in the air, than when we're not inside the water? Is it just because it's liquid and not a gas?

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    $\begingroup$ Please be somewhat more specific in your question. $\endgroup$ – David White May 22 '16 at 19:59
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    $\begingroup$ If you are submerged in 85 degree water you will feel hot indeed. $\endgroup$ – Joshua May 23 '16 at 0:48
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    $\begingroup$ It does not always feel warmer when humidity increases. Cold and damp feels much colder than cold and dry. $\endgroup$ – Jan Hudec May 23 '16 at 6:48
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    $\begingroup$ @Joshua you mean Celsius? 👀 $\endgroup$ – Vim May 23 '16 at 7:54
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    $\begingroup$ I sat in a hot tub early this morning and I assure you I felt considerably hotter in the tub than in the air surrounding it! Your question is lacking important information like the temperature of the air, the temperature of the water, the relative humidity of the air, and so on. $\endgroup$ – Eric Lippert May 23 '16 at 16:48
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You feel cold when heat is flowing from you to the surroundings, your body tries to burn more energy to keep up your temperature, so you shiver.

Water conducts heat much more effectively than air (more than 100x as well) so even with water at the same temperature as air you will lose a lot more heat and feel cold.

When your body is too hot it losses energy most efficiently by sweating. It releases water which evaporates, the energy needed for the water to go from liquid to gas comes from your skin which is then cooled.

In humid conditions it is harder for the water to evaporate (because there is already a lot of gaseous water in the air) so you can't cool as efficiently and so feel hotter.

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  • $\begingroup$ I see. So what you are saying is that inside water the body doesn't need to sweat much because the water cools it. In a humid air, the body needs to sweat but can't do it efficiently because of the water vapors in the air. $\endgroup$ – Ronen Festinger May 23 '16 at 1:53
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    $\begingroup$ @RonenFestinger The first part you said is incorrect. We feel cold in water because the water is directly touching the skin, so heat conducts very quickly. $\endgroup$ – Nayuki May 23 '16 at 5:37
  • $\begingroup$ Yes, what I mean is in water you cool by sweating less efficiently than in humid air. But the reason you don't feel hot is because you don't really need to cool by sweating because the water itself conducts your heat away. $\endgroup$ – Ronen Festinger May 23 '16 at 15:44
  • $\begingroup$ And humid air conducts heat better than dry air, but makes cooling by sweating less efficient. $\endgroup$ – Ronen Festinger May 23 '16 at 15:50
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    $\begingroup$ @RonenFestinger In water, sweating does not help you at all because cooling by sweating is based on evaporation of sweat, which naturally doesn't happen underwater. The difference in heat conduction /convection coefficients between humid and dry air is negligible compared to the heat conduction /convection coefficient of water. $\endgroup$ – Sanchises May 23 '16 at 17:27
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When you feel hot, you perspire so as to benefit by evaporative cooling. As the relative humidity gets closer to 100%, the sweat cannot evaporate and evaporative cooling becomes less effective.

Liquid water is a much better conductor of heat than air (even humid air) is, so if the water is even a few degrees cooler than your body, you feel cold because the water is efficiently conducting your body heat away. A room at 60°F (15°C) feels slightly chilly; in 60° water, you will die in a few hours.

The same is true of metal. On a very hot or very cold day, touch a metal pole and compare it to the feeling of a wood post: it will feel much, much hotter or colder respectively, because metal is a much better conductor of heat than wood.

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    $\begingroup$ "very cold day, touch a metal pole". Um. People who live in places where "very cold" means very cold don't touch metal poles if they can avoid it. So if you travel from a temperate location to a very cold location, don't be tempted to try this out. Very cold metal is kind of sticky if you touch it with skin: it conducts heat well enough to freeze any moisture that touches it ;-) $\endgroup$ – Steve Jessop May 23 '16 at 18:09
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    $\begingroup$ I live in San Francisco, where "very cold" is 50°F (10°C) and "very hot" is 95°F (32°C). If you have the misfortune to live somewhere less temperate, yes, don't make it worse for yourself. $\endgroup$ – Malvolio May 23 '16 at 18:37
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You are describing two different mechanisms of cooling the human body:

1) When we sweat our body produces fluids that tranfer heat from us through their evaporation (fluids gets the heat from our body and evaporate).

One can easily understand that the more the surrounding enviroment has a highly humidity value , the more this heat- transfer gets difficult to occur ,so we feel more hot.

This is the reason that the air-condition units regulate both temperature and humidity.See also :https://en.wikipedia.org/wiki/Air_conditioning

2) On the other hand in the second case when we are in the water the heat-tranfer (if the water is cooler) occurs via convection - it has nothing to do with the humidity.

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  • $\begingroup$ This was a lesson for me.Next time I will be more careful for the 'timing' and the 'way' I am posting my answers. $\endgroup$ – user98038 May 23 '16 at 10:12
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    $\begingroup$ When we are in the water, isn't the heat transfer by conduction and not convection? $\endgroup$ – Soundar Rajan May 23 '16 at 12:21
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There's an important difference between liquid water and water vapor. The difference is 539 calories per gram to be exact. That's why being in humid air makes you hotter - the water vapor has all this latent heat it can release upon you by condensing. Equivalently, it prevents your sweat from evaporating and absorbing this energy.

Liquid water doesn't have this problem.

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