Calculated for 2m above ground
tc2m = temperature 2m above ground [°C]
rh2m = relative humidity 2m above ground [%]

'define dewp2m = tc2m-((14.55+0.114*tc2m)*(1-0.01*rh2m)+pow((2.5+0.007*tc2m)
                 *(1-0.01*rh2m),3)+(15.9+0.117*tc2m)*pow((1-0.01*rh2m),14))'