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

1. Jun 16, 2007

### 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.

Last edited: Jun 16, 2007
2. Jun 16, 2007

### 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?

Last edited: Jun 17, 2007
3. Jun 16, 2007

### Ahmed Abdullah

Can someone knowledgable enlighten me?

Last edited: Jun 17, 2007
4. Jun 16, 2007

### 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?

5. Jun 16, 2007

### Astronuc

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

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

Illustration of Variation of Air Properties with Temperature (1 Atmosphere Pressure)
http://www.uigi.com/psychrometry.html

6. Jun 16, 2007

### Danger

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

7. Jun 17, 2007

### Ahmed Abdullah

How pressure of our atmosphere changes with varying temperature?
Thnx

8. Jun 17, 2007

### Astronuc

Staff Emeritus
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.