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J C asked in Science & MathematicsEngineering · 10 years ago

thermal absorption and conductivity of moist air.?

The old saying of a cold damp day chills you to the bone, seems true.

How does moist or humid air behave? Is this saying true and why. I looked things up on the net and came up with all kinds of technical equations that were very specialized, so much so it would take hours for even a scientific person to get a grasp on it. So please help.

My ideas are that the solution to this saying is two part.

First the thermal conductivity of the moist air vs dry?

Second, can the moist air absorb more heat without raising in temperature as quickly as dry air?

The question is complex unless you are in the field, but with the cold damp weather today here in LA it sure feels like the old saying is true.

Thanks in advance

Chilled to the bone,

JC

6 Answers

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  • 10 years ago
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    Well, it does depend on what you mean by "moist air". Moist air has three parameters needed to specify its state (bulk pressure, dry bulb temperature, and relative humidity).

    I will assume you mean 100% relative humidity air at 20 Celsius and 1 bar pressure, and comparing to dry air (zero humidity) at the same temperature and pressure.

    First the thermal conductivity of the moist air vs dry? (looked up in Engineering Equation Solver)

    Neutral temperature day:

    k_humid = 0.0252 [W/m-K]

    k_dry = 0.02514 [W/m-K]

    And just for comparison, let's check 30 Celsius and 10 Celsius:

    Hot day:

    k_humid = 0.026 [W/m-K]

    k_dry = 0.02588 [W/m-K]

    Cold day:

    k_humid = 0.02442 [W/m-K]

    k_dry = 0.02439 [W/m-K]

    Looks like thermal conductivity of moist air vs dry air isn't what is the problem.

    Now let's discuss the heat capacity. I will discuss mass-basis specific heat c, and volumetric heat capacity rho*c. Both specific heats are isobaric specific heats

    Cold day:

    c_humid = 1.021 [kJ/kg-C]

    c_dry = 1.006 [kJ/kg-C]

    rho_humid*c_humid = 1.241 [kJ/m^3-C]

    rho_dry*c1_dry = 1.238 [kJ/m^3-C]

    Neutral temperature day:

    c_humid= 1.034 [kJ/kg-C]

    c_dry = 1.007 [kJ/kg-C]

    rho_humid*c_humid = 1.201 [kJ/m^3-C]

    rho_dry*c1_dry = 1.196 [kJ/m^3-C]

    Hot day:

    c_humid= 1.058 [kJ/kg-C]

    c_dry = 1.007 [kJ/kg-C]

    rho_humid*c_humid = 1.164 [kJ/m^3-C]

    rho_dry*c1_dry = 1.157 [kJ/m^3-C]

    Conclusion:

    For all conditions of air investigated, there isn't any substantial variation in any of the thermal properties.

    Why then does a cold-damp day chills you to the bone?

    Not because of the humidity blended in the air, it is because of the CONDENSED liquid water which it leaves behind on surfaces.

    Condensed liquid water has much higher specific heat capacity than any condition of air. Also, liquids are MUCH better conductors of heat than gasses, especially liquid water.

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  • 4 years ago

    Thermal Conductivity Of Air

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  • cagle
    Lv 4
    3 years ago

    Conductivity Of Air

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  • 4 years ago

    Thank you Gintable for doing the homework on this.

    I like your numbers, but find it difficult to support your conclusion: if the humidity is below 100% and you re warmer than the environment, there is no reason for water to condense on you.

    Comparing mass-basis specific heat capacities may be the error. In a mixed gas, thermal energy will be distributed equally (on average) between the different molecular species. Water molecules (MW 18) are lighter than nitrogen (MW 28) or oxygen (MW 32) molecules and because each water molecule has three atoms, it has some additional modes of vibration. This allows the MOLAR heat capacity of water molecules to be quite a bit higher than those of oxygen and nitrogen. When water vapour is added to air the water molecules take the place of some of the oxygen and nitrogen, keeping the total number of molecules in give volume the same. This means that the thermal capacity of a sample of moist air will be higher than a similar sample of dry air under the same conditions of temperature and pressure.

    However, at the Freezing point, even 100% humid air contains less than 1% of water vapour, so I m doubtful that this effect is large enough to notice.

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  • Irv S
    Lv 7
    10 years ago

    Simply put:

    Those water molecules in the moist air are lighter than the gas molecules. (O2, N2, etc.)

    Remember your kinetic theory of gasses, and that in kinetic theory, in an elastic collision,

    the lighter particle carries off more energy?

    Well those lighter water molecules carry off more heat than the normal air molecule.

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  • 4 years ago

    Humidity is concentration of water in the air. Since water is one of the greatest conductors of heat, the answer would be yes.

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