To use this page, you need to provide the calculator with three parameters: air temperature, air pressure, and dew point. You should be able to look up these three parameters for your region from most weather web sites.
Provide estimates by modifying the default values in the editable fields in the calculator. As you complete an update to a field, the calculated air density will update itself. Also, be sure to move your cursor over the graphs, since they are interactive. I've tested this page on Safari, Chrome, and Firefox. The graphs work on IE9, but not on earlier versions of IE. Upgrade, or download Chrome; it's a wonderful browser!
Units: metric imperial

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First, let's calculate the pressure of water vapor in the air. The pressure of water vapor is equal to the saturation pressure of water vapor at the dew point temperature. In other words, given the dew point temperature (in degrees Celsius), we need a way to calculate the saturation vapor pressure E_{s} (hPa) at that dew point temperature. (Another name for a hectoPascal is a millibar, so 1 hPa = 1 mb.) A very accurate equation for calculating E_{s} was developed by Herman Wobus:
E_{s} (hPa) = e_{so} / p^{8} (1)where:
e_{so} = 6.1078
p = c_{0} + T (c_{1} + T (c_{2} + T (c_{3} + T (c_{4} + T (c_{5} + T (c_{6} + T (c_{7} + T (c_{8} + T (c_{9}) ) ) ) ) ) ) )
T = air temperature (degrees Celsius)
c_{0} = 0.99999683
c_{1} = 0.90826951 · 10^{2}
c_{2} = 0.78736169 · 10^{4}
c_{3} = 0.61117958 · 10^{6}
c_{4} = 0.43884187 · 10^{8}
c_{5} = 0.29883885 · 10^{10}
c_{6} = 0.21874425 · 10^{12}
c_{7} = 0.17892321 · 10^{14}
c_{8} = 0.11112018 · 10^{16}
c_{9} = 0.30994571 · 10^{19}
Given this, the pressure of water vapor P_{v} is found by using the dew point temperature T_{dewpoint} (C) as T in equation (1). So, we get:
P_{v} (hPa) = E_{s} at T_{dewpoint} (2)
Next, we need to calculate the pressure of dry air P_{d}, given the measured air pressure P from a weather report and the water vapor pressure P_{v} calculated from equation (2). The measured air pressure P is the sum of the pressures of dry air P_{d} and water vapor P_{v}. Rearranging, we get:
P_{d} (hPa) = P (hPa)  P_{v} (hPa) (3)Now that we know P_{v} and P_{d}, we're ready to calculate the air density Rho (kg/m^{3}). The density is:
Rho (kg/m^{3}) = (P_{d} / (R_{d} · T_{k})) + (P_{v} / (R_{v} · T_{k})) (4)where:
P_{v} (hPa) is from equation (2)
P_{d} (hPa) is from equation (3)
R_{v} is 461.4964
R_{d} is 287.0531
T_{k} is measured temperature in degrees Kelvin, i.e., measured temperature T (Celsius) + 273.15
Credits go to Richard Shelquist's page on an introduction to air density for the details behind these equations!