How to relate speed of sound with relative humidity? I am exploring the idea of measuring the humidity of a space using sound waves, however I am having trouble finding a mathematical relationship between the speed of sound and the humidity level. 
$c_{air} = 331.3 \sqrt{1 + \frac{T}{273.15}}$   but this is for dry air (0%RH)
How can I factor the effects of humidity into this relationship?
 A: The speed of sound in a gas is:
$$ c = \sqrt{\gamma R T}$$
where $\gamma = c_p/c_v$ is the ratio of specific heats, $R$ is the specific gas constant and $T$ is temperature. Both $\gamma$ and $R$ depend on the composition of the gas, which includes humidity in air.
The specific heats are $c_p = 1.005+1.82H$ (see this answer) where $H$ is the absolute humidty and $c_v = c_p - R$. Finally, $R = R_{univ}/M_{gas}$ where $M_{gas}$ is the molecular weight of the gas (which depends on humidity). 
To get it all in terms of relative humidity is just an exercise in unit conversion. 
A: Speed of sound in a gas is given by the equation:
$$ c = \sqrt{\gamma R T}$$
where $\gamma = c_p/c_v$ ( $c_p$ and $c_v$ are specific heats), $R$ is the gas constant, and $T$ is temperature. The specific heat of a gas changes with humidity, so varying these will vary your calculated speed of sound.
This page has a calculator as well as a great explanation of how their formula works.
Hope this helps!
A: you can simply use the formula as
Velocity = 331.4 + 0.6*Temperature + 0.0124*Relative_Humidity
Temperature is in Celsius Degrees
Relative Humidity can be measured by sensors in %age
