A calculation of relative humidity with a change of temperature using JavaScript. Also calculates the dew point and the vapor pressure.

Enter the first temperature and relative humidity, along with the second temperature, below. Press the calculate button, and the relative humidity at the second temperature is displayed. The absolute humidity (measured in mm Hg) is also displayed, along with the dew point (the temperature at which the relative humidity is 100%).

The calculation is performed using a form of the Clausius-Clapeyron equation:

P = P_o e ^{-H / R T}

where P is the saturated water vapor pressure, P_o is a pre-exponential factor (1.11 x 10⁹ mm Hg, equivalent to the vapor pressure of water at infinite temperature), H is the enthalpy of evaporation (43.8 kJ/mol), R is the gas constant (8.314 J/Kmol) and T is the absolute temperature (in K). P_o and H were chosen so that the equation fits two data points exactly; 23.76 mm Hg at 25 C and 31.80 mm Hg at 30 C.

Knowing the saturated water vapor pressure, which is only dependent upon temperature, the absolute water vapor pressure is calculated directly using the relative humidity. To calculate the relative humidity at a second temperature, the saturated water vapor pressure is again calculated, and the original absolute vapor is divided by this number. To calculate the dewpoint, the Clausius-Clapeyron equation is solved for temperature, and the absolute humidity
is used as the independent variable.

Thanks to Psyke Phaeton for correcting the value of the pre-exponential factor in the text above (although the Javascript value was fine).

And no, those guys up there are not Clausius and Clapeyron.