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Name: Bob
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Question:
I am puzzled. More humid air is lighter because of the hydrogen molecules. Water absorbs more heat, but why would humid air absorb more heat at the same temperature if it has less mass? This does not seem consistent with the kinetic theory of gases.


Replies:
Bob,

Air is composed mainly of 78% nitrogen (molecular weight 28) and 21% oxygen (molecular weight 32), with another 1% of other gases. Water vapor (molecular weight 18) is considerably lighter and when it displaces nitrogen and oxygen in the air, makes the air density lower (lighter, as you have said, than dry air at the same temperature).

The result of the absorption of energy is a higher energy state of the molecules, reflected only in increased temperature and pressure if the volume of the air is held constant. However, in the atmosphere there are no fixed boundaries to keep the air volume the same, so when the temperature of the air increases, it wants to expand (although this is slowed by the shear mass and weight of air around it), thus reducing the molecular weight of the air in the original volume. The temperature of all of the molecules increases as water vapor radiates some of its absorbed energy to neighboring nitrogen and oxygen molecules.

If your volume of air has water vapor in it, it will warm more rapidly than dry air, since water vapor is a greenhouse gas that has the capacity to absorb more energy than nitrogen and oxygen. The amount of water in the air is always small compared to the amount of nitrogen and oxygen, so a humid volume of air, though being less dense than a dry volume of air, is not an impediment to the absorption of energy by the water vapor. The only mass of any molecule in the air that is important is that of the water vapor itself. The more water vapor, the more absorption of energy is likely, irrespective of the mass of other constituents of the air.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


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