subroutine asknewet (e,Tm,lambda,zwd)

c% This subroutine determines the zenith wet delay based on the
c% equation 22 by Aske and Nordius (1987) 
c%
c% c Reference:
c% Askne and Nordius, Estimation of tropospheric delay for microwaves from
c% surface weather data, Radio Science, Vol 22(3): 379-386, 1987.
c%
c% input parameters:
c%
c% e:      water vapor pressure in hPa 
c% Tm:     mean temperature in Kelvin
c% lambda: water vapor lapse rate (see definition in Askne and Nordius 1987)
c% 
c% output parameters:
c%
c% zwd:  zenith wet delay in meter 
c%
c% Example 1 :
c%
c% e =  10.9621 hPa
c% Tm = 273.8720
c% lambda = 2.8071
c%
c% output:
c% zwd = 0.1176 m
c%
c% Johannes Boehm, 3 August 2013
C% Johannes Boehm, 24 December 2014, converted to Fortran
c% ---

      implicit double precision (a-h,o-z)
      
      double precision k1,k2,k2p,k3,lambda

      !% coefficients
      k1  = 77.604d0                     !% K/hPa
      k2 = 64.79d0                       !% K/hPa
      k2p = k2 - k1*18.0152d0/28.9644d0  !% K/hPa
      k3  = 377600.d0                    !% KK/hPa

      !% mean gravity in m/s**2
      gm = 9.80665d0
      !% molar mass of dry air in kg/mol
      dMtr = 28.965d-3
      !% universal gas constant in J/K/mol
      R = 8.3143d0

      !% specific gas constant for dry consituents
      Rd = R/dMtr 

      zwd = 1.0d-6*(k2p + k3/Tm)*Rd/(lambda + 1.d0)/gm*e
      
      end subroutine