Air Density Formula ```Name: Scott Status: N/A Age: N/A Location: N/A Country: N/A Date: N/A ``` Question: Hi there, Do you know a formula that I could use to calculate air density using the 3 inputs from the instruments I have: Temperture in Celsius? Rh - relative humidity? Barometric pressure ( from my barometer in inches of Mercury)? I would like to put the formula in my programmable calculator to track changes in daily Air density. I know you have to allow for saturation pressure and water vapour pressure but I'm not sure how to go about it. Replies: There are a number of websites that treat the properties of the atmosphere. They vary in complexity, because the atmosphere is a dynamic mixture of gases that is not constant. Some of the treatments get rather involved because of the intended applications of engine design and aircraft behavior. The most direct source I found for your needs is: http://www.aeromech.usyd.edu.au/aero/atmos/atmos.html It may require some "number crunching" on your part to write an algorithm for a programmable calculator but I do not think it is out of reach. Vince Calder Scott, It requires some equations, but it can be done. First we need to convert your pressure (P) into kPa (kiloPascals): P(kPa) = P(in. Hg)*3.3864 Next we need to determine the vapor pressure (VP) in kPa. To do this we first calculate the saturation vapor pressure (SVP) in kPa from the temperature (T): SVP = 0.61121*e^[17.67*T/(T+243.5)], where e means exponential. Then we can calculate VP from RH and SVP: VP = RH*SVP/100 Now we calculate the mixing ratio (XM): XM = 0.622*VP/(P-VP) Finally, we calculate moist air density (RHO): RHO = [P*(1+XM)]/[0.28703*(T+273.16)*(1+1.16078*XM)], where the T+273.16 puts the temperature into Kelvin degrees. So, if T=26 deg. C, P=29.23 in. Hg (99 kPa), and RH=40%, we get SVP=3.3615 kPa, VP=1.345 kPa, XM=0.008564, and RHO=1.1514 kg m^-3 (or 1.1514 x 10^-3 g cm^-3). David R. Cook Climate Research Section Environmental Science Division Argonne National Laboratory Click here to return to the Weather Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs