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

[Ahmed Abdullah]

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.
But 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 up 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 (for the volume is unchanged and there is more molecules).
This is the part which I can't understand.
Please help me.

 

[Ahmed Abdullah]

I know that humid air is less dense than dry air.
So it turns out that the atmospheric pressure always remains constant. Volume change in such a way to keep the pressure constant.
Right?

Does it not also implies that the atmospheric pressure does not change with temperature?

 

[Ahmed Abdullah]

Can someone knowledgeable enlighten me?

 

[kiblex]

"Why sound wave travels faster in humid air than in dry air?"

Is this really true? Isnt the speed of sound in relation only with temperature?

You can calculate the speed of sound: acoustic velocity(a)=sqrt(ratio of specific heat(gamma)*gas constant(R)*temperature(T))

If we have a day with humid air and a temperature of 288K and we have another day with dry air but with a temperature on 300K. The speed of sound will be higher the day with dry air and higher temperature? Or am I wrong here?

 

[Astronuc]

The speed of sound changes with the temperature and a little bit with the humidity, but not with the air pressure.

http://www.sengpielaudio.com/calculator-airpressure.htm

Humidity has a small, but measurable effect on sound speed. Sound travels slightly (0.1%-0.6%) faster in humid air.

http://en.wikipedia.org/wiki/Speed_of_sound

Sound Speed in Gases
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe3.html

The mean molecular mass of air drops slightly with a little moisture since H₂O, M=18, is lighter than N₂, M=28, O₂, M=32, but the amount of moisture is relatively low.

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe.html#c4

For properties of air, see this page about 2/3's of the way down -
Illustration of Variation of Air Properties with Temperature (1 Atmosphere Pressure)
http://www.uigi.com/psychrometry.html

 

[Danger]

Astro, you never cease to amaze me with the things that you can pull out of your beard on a moment's notice.

 

[Ahmed Abdullah]

I know that humid air is less dense than dry air.
So it turns out that the atmospheric pressure always remains constant. Volume change in such a way to keep the pressure constant.
Right?

Does it not also implies that the atmospheric pressure does not change with temperature?

How pressure of our atmosphere changes with varying temperature?
Thnx

 

[Astronuc]

How pressure of our atmosphere changes with varying temperature?
Thnx

The density of an unconfined (unconstrained) gas decreases with temperature, and the pressure is simply the integral of $\rho(T(z))\,g\,dz$ from the point of interest to the top of the atmosphere.