# Air Density: How to calculate Relative Humidity

1. Mar 12, 2010

### Pooven

Hi there,

I'm trying to calculate air density; I noticed on http://en.wikipedia.org/wiki/Density#Density_of_air" that there is a table that makes it seem that air density is a function of temperature... I suppose I could safely say that temperature has a significant effect on air density? What error could one expect if they had to use measurements from this table?

Anyway, I know that humidity plays an important role in air density and there's a nice formula on http://en.wikipedia.org/wiki/Density_of_air#Water_vapor": ρhumid air = pd/Rd.T + pv/Rv.T.

This seems like it's the only way to calculate the density of humid air. I have temperature, absolute humidity and ambient pressure of the system (all in the appropriate SI unit). This is insufficient information to calculate the relative humidity as specified on http://en.wikipedia.org/wiki/Relative_humidity" [Broken].

I understand that vapour pressure readings can be taken from steam tables to calculate the saturated vapour pressure. But then everything gets really blurry. But before I get into that... I saw this example from http://www.cheresources.com/questions/thermodynamics-41.html" that suggests that I can calculate the relative humidity from the absolute humidity... I haven't come across anything else that explicitly linked the two so I'd like to know if the answer is sound. The equation given was:

Humidityabsolute=Humiditysaturation * RelativeHumidity/100