B727 FLC CONCERN REGARDING WX INFO RPT OF VISIBILITY PRIOR TO DEP.

Narrative

THE WX HAD BEEN LOWERING SINCE OUR ARR FROM OMA. AS WE PREPARED OUR PAPERWORK FOR DEP; THE ATIS RPTED 1/4 SM AND FZFG. WE ANTI-ICED AND STARTED ENGS. AS WE TAXIED TO RWY 31R; A NEW ATIS CALLED THE VISIBILITY 1/2 SM AND FZFG. AS WE APCHED THE RWY THE TWR RPTED TWR VISIBILITY TO BE LESS THAN 1/4 MI. WE ASKED THE TWR IF THIS WAS OFFICIAL RWY VISIBILITY; BUT WERE TOLD IT WAS TWR VISIBILITY. WE NEEDED 1/4 MI RWY VISIBILITY FOR TKOF; SO WE TOLD THE TWR WE WOULD WAIT A FEW MINS AS THEY SAID A NEW ATIS WAS COMING UP SOON. A MIN OR SO LATER; A B757 LANDED ON RWY 31R. WE ASKED THE TWR WHEN THE NEW ATIS WOULD BE UP AND THEY SAID THE VISIBILITY IS STILL RPTED AT 1/2 MI. WE SAW THE B757 LAND AND ROLL OUT IN FRONT OF US AND THE VISIBILITY DOWN THE RWY WAS GOOD (THE FOG HAD FORMED A LOW CEILING ABOVE THE SURFACE). SO WITH THAT; WE ACCEPTED TKOF CLRNC AND MADE AN UNEVENTFUL TKOF. MY CONCERN IS WITH THE GRAY AREA THERE SEEMS TO BE BTWN OFFICIAL RPTED VISIBILITY AND 'TWR VISIBILITY' WHICH CAN VARY GREATLY ESPECIALLY IN FOGGY CONDITIONS AND IN SITS WHERE THERE IS A TALL TWR FAR FROM THE RWY OR WX RPTING FACILITY.