tc850mb = temperature at 850hPa [°C]
rh850mb = relative humidity at 850hPa [%]

'define dewp850mb = tc850mb-((14.55+0.114*tc850mb)*(1-0.01*rh850mb)+pow((2.5+0.007*tc850mb)
                    *(1-0.01*rh850mb),3)+(15.9+0.117*tc850mb)*pow((1-0.01*rh850mb),14))'
'define vapr850mb = 6.112*exp((17.67*dewp850mb)/(dewp850mb+243.5))'
'define e850mb    = vapr850mb*1.001+(850-100)/900*0.0034'
'define w850mb    = 0.62197*(e850mb/(850-e850mb))'
'define te850mb   = tmp850mb+(2260000*w850mb/1004)'
'define ept850mb  = (te850mb*pow((1000/850),(287/1004)))-273.16'