What is least dense, hot and dry air or humid air?

Question

I was told humid air is less dense than hot dry air. Why is this? I thought the heat would make the gas disperse and therefore less dense, and that saturating it with water would cause greater density than not having anything in it at all.

This is the explanation I was given:

When lighter molecules of water, humidity, are added to dry air, the air will become less dense.

You guys are wrong:

http://www.theweatherprediction.com/habyhints/260/

Answer 1

Who told you that? Humid air is full of water vapor, molecules of H2O,

Answer 2

Humid air in bathroom is less dense than dry air on the outside provided that their tempertures are equal.

Pressures are equal, temperatures are equal, therefore equal volumes contain equal number of molecules:
n = PV/RT

But molar mass of water is 18 gram, whereas average molar mass of air is somewhere between 28 of nitrogen and 32 of oxygen. Replacing part of molecules of air with lighter molecules of water makes humid air less dense.

Answer 3

Density Rho = Mg/V; where M is the mass contained in a volume V and g is acceleration due to gravity. Rho is the so-called weight density. Mass density rho = Rho/g = M/V

Now, which do you think is the most dense: air without water in it or air with water in it? Let's work this a bit. If we have a liter = V of water and another liter = v of just air and put the V on one side of a balance beam and the v on the other side, both equidistance from the pivot point, which side would fall because it's heavier (more massive)?

Sure, the side filled with water. And there you are. As you add water to the volume, that volume becomes heavier, even if the water is mist. Therefore Rho = Mg/V is denser with water in the air than without water in the air.

So, as the other answer so delicately put it...you were told wrong.

Answer 4

Your initial assumption was correct. Someone is gravely mistaken, or they are leading you down the primrose path.

Answer 5

Hi. You were told wrong I fear.