Does humidity impact density and how?

In summary, the humidity in the air affects the density by making it lower than that of dry air at constant dry bulb temperature. This is because the more moisture present, the less dense the air becomes. The density of standard air is 0.075lb/cu.ft, while the density of saturated steam (or water vapor) at 60F is only 0.0008292lb/cu.ft. However, this data is for saturated steam and not for 100% humidity air. The saturation pressure of water vapor at 60F is 0.256398psia, which is significantly lower than the standard atmospheric pressure of 14.7psia. This results in a much lower density of 0.000
  • #1
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Does the humidity in the air affect the density?
and how does it affect?
thanks
 
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  • #2
The density of moist air is always less than that of dry air at constant dry bulb temperature. The more moisture the less density of the mixture.

The density of standard air is 0.075lb/cu.ft where as the density of saturated steam(or water vapor) at 60F is mere 0.0008292lb/cu.ft.
 
  • #3
The numbers that quark gives don't look right. Water has mol. wt. of 18, wile O2 and N2 have wts. of 32 and 28. I suspect that at a given temp. and pressure, humid air would have a slightly lower density than dry air, but nowhere near that much
 
  • #4
Saturated Steam @ 60°F:

Temperature (T) 15.56 C 60.00 F
Pressure (P) 0.0177 bar 0.256 psi
Density Saturated Liquid (f) 998.97 kg/m3 62.364 lb/ft3
Saturated Vapor (g) 0.013272 kg/m3 0.000829 lb/ft3
Specific Volume Saturated Liquid (vf) 0.0010010 m3/kg 0.016035 ft3/lb
Saturated Vapor (vg) 75.344 1206.9
Internal Energy Saturated Liquid (uf) 65.31 kJ/kg 28.08 Btu/lb
Evaporated (ufg) 2331.6 1002.4
Saturated Vapor (ug) 2396.7 1030.4
Enthalpy Saturated Liquid (hf) 65.31 kJ/kg 28.08 Btu/lb
Evaporated (hfg) 2464.6 1059.6
Saturated Vapor (hg) 2530.0 1087.7
Entropy Saturated Liquid (sf) 0.23258 kJ/kg-K
(mayer) 0.05555 Btu/lb-R
Evaporated (sfg) 8.5360 2.0388
Saturated Vapor (sg) 8.7684 2.0943
 
  • #5
Mathman,

To be frank, I was indeed puzzled by the hint of your post, initially, for quite sometime. Good steam properties calculator like steamtab which is based on IAPWS formulae should never lie, so it became my responsibility to check for a possible solution. This is what comes to my mind.

The specific volume of any gas, at STP, can easily be calculated by Avagadro's law if we know the molecular weight of the gas. Molecular weight of water is 18, as you rightly said, and average molecular weight of air is 29.

So, one lb mole of water vapor occupies 379/18 = 21.05 cu.ft at STP. So the density is about 1/21.05 = 0.0475lb/cu.ft. One lb mole of air occupies 379/29 = 13.07 cu.ft at STP. So the density is about 1/13.07 = 0.0765lb/cu.ft. So what you said should be right.

But the key word here is STP(14.696psia and 60F - a general standard, though there are many variations to the definition of STP). The saturation pressure of water vapor, at 60F, is 0.256398psia. So, you have to expand the water vapor from 14.696psia to 0.256398psia(isothermally) and the ratio is about 57.31. So the density should be 0.0475/57.31 = 0.0008288lb/cu.ft. This is close to the value I specified above and I think my procedure is fair if not accurate.

Thanks for pointing out.

Regards,
 
  • #6
I think the data you are discussiing is for steam, which is not the same as for 100% humidity air. I believe the original question was about the latter.
 
  • #7
quark said:
The density of moist air is always less than that of dry air at constant dry bulb temperature. The more moisture the less density of the mixture.

The density of standard air is 0.075lb/cu.ft where as the density of saturated steam(or water vapor) at 60F is mere 0.0008292lb/cu.ft.

mathman said:
I think the data you are discussiing is for steam, which is not the same as for 100% humidity air.
In a sense, you are both right.

The key is 'saturated' steam, or water vapor. At 60F the sat pressure is 0.2563 psia, compared to 14.7 psia, or 1 atm.

If one were to look at the property of 'saturated' steam at 1 atm (14.7 psia), which would have a temperature of 212F, then the density is 0.037 lbm/ft3.

The partial pressure of water vapor in air is very low - near the sat. pressure IIRC. It's been so long since I've looked at this stuff - my memory is a bit 'foggy'. :biggrin:

Here's a basic discussion on humidity - http://www.usatoday.com/weather/wrelhum.htm
 
  • #8
There are two types of saturation when we deal with moist air. One is air saturation with water and the other is water vapor(or steam) saturation at the given partial pressure corresponding to the dry bulb temperature. Though air is not saturated with water vapor at any RH below 100%, water vapor is at saturated state as the partial pressure of water vapor is its saturation pressure at the given temperature. So the above process is valid and infact RH calculation based on the ratio of partial pressure of vapor at the given temperature to the partial pressure of vapor when air is fully saturated is done by the above method.
 

What is humidity and how does it affect density?

Humidity is the amount of water vapor in the air. It affects density because water vapor is less dense than dry air, so when there is more humidity, the air is less dense.

How does high humidity affect the density of air?

High humidity causes the air to be less dense because there is more water vapor present, which has a lower density than dry air.

What is the relationship between humidity and air density?

The higher the humidity, the lower the air density, and vice versa. This is because water vapor takes up space in the air, making it less dense.

How does humidity affect the density of other substances?

Humidity can affect the density of other substances, such as liquids, by altering their evaporation rates. This can lead to changes in the density of the substance as a whole.

Why is it important to consider humidity when measuring density?

It is important to consider humidity when measuring density because it can significantly impact the accuracy of the measurement. Humidity affects the density of the substance being measured, as well as the surrounding air, which can lead to errors in the measurement if not taken into account.

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