Why sound wave travel faster in humid air than in dry air?

A:The answer is humid air is less dense.
Q:But why humid air is less dense?
A:Because water molecules have less mass than the average air molecules do.
It will happen if and only if the pressure is unchanged. Given that dry and humid air are both under the same pressure then we have smaller density for humid air. But when a place is saturate with water vapour the pressure increases (i.e for the vapour pressure of the water).
I am thinking of a closed container half filled with water. At first the air over the water surface is dry but as time passes water molecules from the surface escapes and eventually the air becomes saturated. So the density of the air should increase when air is saturated by water vapour.
This is the part which I can't understand.
Please help me.

You missed one point. Molecules in water are less tightly packed than in air.

The same argument could be applied to water/ice. Ice is perceived to be denser than water, as ice is solid, but we all know ice floats in water suggesting it is less dense.

Humid air is less dense because the average atomic distance between molecules in air is much shorter than the average atomic distance between molecules in water. This makes air denser, same as it makes water denser than ice.

If you think of physics phenomena in terms of atoms and molecules, then they all makes sense. I used to boil a plastic bag of water by putting the fire to the plastic directly, much to the amazement of others. You may wish to try that yourself and try explaning in terms of molecules why the plastic bag doesn't just burn out. It's fun.

â™* the statement is wrong;
humid air means mist; mist means molecular air + droplets of water;
density of mist � is greater than that of dry air;
♥ theory of sound says: speed of the wave v ~1/√�,
thence the greater � the less v;

The velocity of propagation of sound in gas is

c = √ [γ p / �]


Where γ = 1.4. p is the pressure and � is the density.

But � = μ/v where μ is the mass of a mole of a gas and v is its volume.

From pv/T is a constant

p μ / �T is a constant.

p /� = a constant * T/ μ

Therefore, c = √ [ a T / μ] where a is a constant.

The velocity of propagation of sound in gas is proportional to the square root of the temperature and does not depend upon the pressure of gas.

Its dependence upon the molecular weight is interesting.

In hydrogen, c = 1263 m/s and in air 331m/s.

Lesser the molecular weight greater is the velocity.
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You have stated that

“I am thinking of a closed container half filled with water. At first the air over the water surface is dry but as time passes water molecules from the surface escapes and eventually the air becomes saturated. So the density of the air should increase when air is saturated by water vapour�.

When the air becomes saturated, no more water molecules evaporate. In a closed container, when water evaporates, volume of water decreases where as the volume of air increases.

And there fore, your observations regarding pressure have no correlation to your question.
As stated above the velocity of sound does not depend upon pressure but it depends upon the molecular weight.