The humidity level is 80% and T=303Kelvin. So I can use the equation RH = Pw/Pws to calculate the Pw = Partial pressure of water vapor, by finding Pws with the following equation: Pws=e^(77.3450+0057T-7235/T)/T^8.2 = 4200Pa, where T is in Kelvin. So Pw=0.8*4200Pa=3360Pa I can find Pa by using two equations for the density of dry air: pa=0.0035*Pa/T, where pa = density of dry air I rearrange this into Pa=paT/0.0035 Now I calculate the density of dry air by using the ideal gas law: pa= P / Rspecific*T which gives me: 101325 Pascal / 287.058*303 Kelvin = 1.1644Kg/m^3 So Pa=1.1644*303/0.0035=100803Pa or 100.8 KPa. But this seems like too much. Because the equation for evaporation rate is W = (95+0.425*V)*(Pw-Pa)/Y And Pw-Pa in this case gives me a negative number. So what am I doing wrong?