t_18 :: (real ~ "1" on
         (i = 1 .. nlon + 1 by
          j = 1 .. nlat + 1 by
          k = 2 .. nlev
         )
        )


CSE Eliminated

t_18 := log(varidx(p, [i, j, k]) / varidx(p, [i, j, k + 1]))