function taucloud(sza,az,sat,pt,rs,rad)
c
c author:               ed celarier
c procedure name: 	taucloud
c procedure type: 	fortran function
c version date:		27.vi.97
c
c purpose:
c   calculates the putative cloud optical thickness for a particular
c   viewing geometry, and for a terrain of a certain pressure and
c   reflectivity, given the toms-measured radiance in one of the 
c   reflectivity channels.
c
c calling parameters:
c   name	type	i/o	units	description
c   --------	----	---	-----	--------------
c   taucloud	r8	o		calculated putative cloud optical
c					thickness
c   sza		r8	i	deg	solar zenith angle
c   az		r8	i	deg	solar-fov-satellite azimuth
c   sat		r8	i	deg	satellite zenith angle
c   pt		r8	i	atm	terrain surface pressure
c   rs		r8	i	frac	surface reflectivity
c   rad		r8	i	i/f	toms-measured radiance
c
c external routines required:
c	radlow		calculates pairs (tcl, radiance) for low tcl.
c	radhigh		calculates pairs (tcl, radiance) for high tcl.
c       lagrangeco      calculates order and coefficients for lagrange
c                       polynomial interpolation.
c
c notes & caveats:
c
c revision history:
c
c============================================================================
      implicit none
      integer*4 ntcl_l
      parameter(ntcl_l=5)
      integer*4  ntcl_h
      parameter(ntcl_h=6)
c     -- calling parameters
      real*8 taucloud,sza,az,sat,pt,rs,rad
c     -- local variables
      real*8 outrad(0:ntcl_l+ntcl_h-1), tcl(0:ntcl_l+ntcl_h-1)
      real*8 outrad_l(0:ntcl_l-1), outrad_h(0:ntcl_h-1)
      real*8 outtcl_l(0:ntcl_l-1), outtcl_h(0:ntcl_h-1)
      equivalence(outrad_l(0), outrad(0)) 
      equivalence(outrad_h(0), outrad(ntcl_l))
      equivalence(outtcl_l(0), tcl(0)) 
      equivalence(outtcl_h(0), tcl(ntcl_l))
      integer*4 i0, deg, nout_l, nout_h, i, inrange
      real*8 coef(0:3), sum
      logical success
c
      call radlow(rs,sza,az,sat,pt,outrad_l,outtcl_l,nout_l,success)
      if(.not.success) return
      if (rad .le. outrad_l(nout_l-1)) then
         if (rad .le. outrad_l(0)) then
            taucloud= 0.d0
            return
         endif
         call lagrangeco(rad,outrad,nout_l, i0,deg,coef,inrange)
      else
         call radhigh(sza,az,sat,rs,outrad_h,outtcl_h,nout_h,success)
         if (.not.success) return
         if (rad .gt. outrad_h(nout_h-1)) then 
            taucloud= 100.d0
            return
         endif
         call lagrangeco(rad,outrad,nout_l+nout_h,i0,deg,coef,inrange)
      endif
      sum=0.d0
      do i=0, deg 
         sum=sum+coef(i)*tcl(i0+i)
      end do     
      taucloud= sum
      return
      end