Title
Because humid air has more in it?
At a constant pressure and temperature, a given volume of gas will always have the same number of molecules, regardless of what particular chemical species those molecules are. See Ideal gas law - Wikipedia
If air is humid, more of those molecules will be water molecules than in dry air, but the total number of molecules will be the same.
Water molecules weigh less than oxygen and nitrogen molecules.
So, humid air, with proportionally more light water molecules displacing the heavier oxygen and nitrogen molecules, is lighter.
Humid air isn’t just air with water in it. It’s air mixed with water vapor. And water vapor is a gas like any other–nitrogen, oxygen, etc.
To a first approximation, all gases have the same volume for a given number of molecules (for a fixed temperature and pressure). So, dense gases have heavy molecules, and low-density gases have light molecules.
Water is of course H2O, which has a molecular weight of 1+1+16=18. Air is mostly nitrogen (N2), which has a weight of 14+14=28. Most of the remaining is oxygen (O2), which has a weight of 16+16=32.
Therefore, water vapor is less dense than the other constituents of air, and mixing some in lowers the overall density, in the same way that mixing in helium would also lower the density.
Ach–ninjad! Even our paragraph breakdown of the principles is almost identical! At least I had numbers…
Because water vapor has a lower molecular weight than the other molecules in dry air, but according to the standard gas laws, takes up the same “space”. For example, a cubic meter of gas at Standard Temperature and Pressure contains 2.65×10^25 molecules. This is true regardless of what type of molecules they are; only that they are in gaseous state at STP. That’s due to Avogadro’s Law.
If the volume is constant, and the number of molecules is constant, the only variable left that affects density is mass… in this case, molecular weight. The average molecular weight of a typical air molecule works out to approximately 29 grams per mole. Water, being composed of a lot of hydrogen and a little oxygen, is a lot lighter per molecule–only 18 grams per mole. And since each water molecule displaces an air molecule in a standard quantity of molecules in a given volume of gas at STP, adding water vapor to dry gas makes it weigh less per volume: less dense.
First of all, the ideal gas law does not apply here. This is a real world application. The real answer is on the psychrometric chart which gives the specific volume of air at any temp. Specific volume is the reciprocal of density. It’s been awhile for me as I’ve been away from HVAC calcs. However, a simple consult of the psychrometric chart tells all.
Of course the ideal gas law applies here. We aren’t talking about very cold or very dense systems here. The ideal gas law is quite accurate for typical atmospheric conditions.
The psychrometric chart has good descriptive power, but has no *explanatory *power. The ideal gas law tells us (in part) why the chart says what it does.
OK, how far off is the Ideal Gas Law variation of density of air Vs. humidity?
According to Wikipedia, the error is less than 0.2% (-10 to +50 C).