Why is Air Colder and Less Dense at Higher Altitudes?

[Russell5150]

In atmospheric physics, there are two kinds of density.

One is mass density, that is, kilograms per cubic meter. The mass density of a volume of air depends upon its composition, its temperature, and its pressure. Colder humid air is less dense than slightly warmer dry air at the same pressure, because the mass of a vapor molecule is less than the mass of a dry air molecule. However, air at high elevations is less dense than air at lower elevations because the number density is less.

Now you wait a minute here.
If cold wet air is heavier, thicker, and can easily displace warm dry air, How can you say that it is less dense?

 

[klimatos]

Now you wait a minute here.
If cold wet air is heavier, thicker, and can easily displace warm dry air, How can you say that it is less dense?

I said cooler humid air is less dense than "slightly" warmer dry air. Humid air is also less dense than dry air at the same temperature. This is because the average water molecule is much lighter than the average dry air molecule: 2.99E-26 kilograms versus 4.81E-26 kilograms.

 

[klimatos]

Now you wait a minute here.
If cold wet air is heavier, thicker, and can easily displace warm dry air, How can you say that it is less dense?

I said nothing about "thicker" (whatever that means) or being able to "easily displace warm dry air". It is usually the denser air mass that displaces the less dense--other things being equal.

If by "thicker" you mean having more molecules per unit volume, you are mistaken. At the same temperature and pressure moist air does not have more molecules per unit volume than dry air. It has exactly the same number. That's Avogadro's Law.

 

[Russell5150]

I said nothing about "thicker" (whatever that means) or being able to "easily displace warm dry air". It is usually the denser air mass that displaces the less dense--other things being equal.

If by "thicker" you mean having more molecules per unit volume, you are mistaken. At the same temperature and pressure moist air does not have more molecules per unit volume than dry air. It has exactly the same number. That's Avogadro's Law.

My geophysics instructor explained that a moving cold air mass will displace and lift warmer air because the warm air is lighter and less dense than cold air. I guess I mean thicker in the sense that it has more molecules (air) in the same volume.
If you want to test this put your empty soda can into the freezer and watch it crush in on itself.