c=======================================================================

      subroutine c6dta(sig,v) 2
c **  molecular scattering

c     sig=0.
c     if(v.le.3000.) return
c     sig=v**4/(9.26799e+18-1.07123e+09*v**2)
c
c calculate molecular rayleigh scattering coefficient
c using approximation of shettle et al., 1980 (appl. opt., 2873-4)
c with the depolarization = 0.0279 instead of 0.035
c input: v = frequency in wavenumbers (cm-1)
c output: sig = molecular scattering coefficient (km-1)
c               for temperature = 273 k & pressure = 1 atm.

      data rscfac/1./
 
      sig = rscfac*v**4/(9.38076e+18 - 1.08426e+09 * v ** 2)

      return

c. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

      entry modrsc(xrsc)

c purpose:  multiply rayleigh scattering 
c optical depth by xrsc for sensitivity studies
c
      rscfac=xrsc
      return
      end