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Properties of air vs water

  1. Feb 19, 2009 #1
    This is just asilly question that somebody asked and it was fun because a dozen different intelligent men hemmed and hawed about it and gave some truly silly answers. I kind of want to see how this board does with it.

    If I am standing around and the temperature of the air is 90 degrees, then I feel hot and sweat. If I take a shower and the temerature of the water is 90 degrees, I feel cool and shiver. Why?
     
  2. jcsd
  3. Feb 19, 2009 #2

    mathman

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    Part of the answer may be that the shower water on your skin is evaporating, which has a cooling effect.
     
  4. Feb 19, 2009 #3
    From http://en.wikipedia.org/wiki/Water_(molecule)#Heat_capacity_and_heats_of_vaporization_and_fusion":
    "Water has the second highest specific heat capacity of any known chemical compound"

    This means it takes more heat energy from your body to raise the temperature of water to equal your body temperature, compared to air. Also water's thermal conductivity is higher than air's (0.6 vs 0.025 W/(m·K)), which means heat moves easier to water. In simpler terms water can absorb more heat from you, and do it faster than air, thus leaving you with less heat, and feeling colder.

    Edit: Two more factors just popped into my head. First at these temperatures water is liquid and air is a gas. Therefore, water will be more dense than air, meaning there will simply be more of it close to you. Secondly air will be fairly static, whereas water will be pulled by gravity and only in contact for a few seconds before going down the drain (along with the heat you gave it), and being replaced with new water not yet warmed by you.

    So we have a combination of water having a higher heat capacity, higher thermal conductivity, higher density, and being constantly moving. There is more of the water, which absorbs heat faster, and absorbs more before reaching equilibrium, and it is constantly being replaced by fresh unwarmed water. It's a wonder we don't get hypothermia in the shower.
     
    Last edited by a moderator: Apr 24, 2017
  5. Feb 19, 2009 #4

    russ_watters

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    Well that's where the OP leads us: we would get hypothermia in the shower if the water temperature were much cooler than body temperature. That's why you want it warmer!
     
  6. Feb 19, 2009 #5

    rcgldr

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    Skin temperature is less than body temperature, and the skin releases heat into the air as part of internal temperature regulation. Since air has a much lower rate of heat capacity than water, the comfort zone for air is lower wider than it is for water.

    http://www.wonderquest.com/skin-temperature.htm
     
  7. Feb 20, 2009 #6

    GT1

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    The amount of heat your body loses to the environment is Q=H*A*deltaT, where H is the convection heat transfer coefficient, A is the surface area, and deltaT is the temperature difference between your skin temperature and the water/air temperature. For the same temperature difference you will lose much more heat in water then in air because the heat transfer coefficient in water is much much bigger then in air. If your skin temperature is 90 and the water/air temperature is also 90, deltaT=0 and you won't lose heat (by convection).
     
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