FLC OF A DC9-30 LANDED DURING MINIMUM LNDG WX AS OBSERVED BY THE FLC DURING APCH AND LNDG.

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WHILE ON DSCNT TO LAND AT MSN; ATIS RPTED ILS RWY 18 THE APCH IN US AND THE TWR CLOSED. GREEN BAY FSS AND ZAU RPTED WX AS WINDS CALM AND 1/4 MI VISIBILITY WITH FREEZING FOG. WHILE ON DSCNT PRIOR TO RADAR VECTOR TO FINAL; ATC RPTED THE TWR WX OBSERVER SAID IT LOOKED BETTER FOR US IF WE EXECUTED THE ILS RWY 36 APCH INSTEAD OF ILS RWY 18. DURING DSCNT APCH; VECTOR TO FINAL AND FINAL DSCNT FROM 10000 FT MSL TO SURFACE AND 15 MI FROM ARPT; WE COULD SEE THE APCH LIGHTS; RWY LIGHTS; CTRLINE LIGHTS AT MSN. AFTER LNDG; THE PATCHY FOG OBSCURED SOME TXWYS FROM 1/4 - 1/2 MI VISIBILITY. THE TWR OPENED AFTER WE LANDED AND RPTED THE VISIBILITY UP TO 1/2 MI. WE DID THE ILS RWY 36 APCH UNDER GOOD VMC WITH THE LAST RPT OF VISIBILITY AT 1/4 MI. THE REQUIRED VISIBILITY FOR THE APCH IS 1/2 MI. WE HAD BETTER THAN 7+ MI DURING ALL PHASES; DSCNT; APCH AND LNDG. THE QUESTION AFTER WE LANDED WAS 'WERE WE LEGAL TO LAND WHEN THE LAST RPT GIVEN TO ATC WAS 1/4 MI VISIBILITY?' CONTRIBUTING FACTORS: THE LACK OF TRUE WX RPTING OF CONDITIONS FOR RWY 36 VERSUS WHAT THE PERSON AT THE TWR LOCATION IS SEEING OR RPTING. WE; THE CREW; NOT BEING ABLE TO ASK FOR A VISUAL APCH BECAUSE ATC HAS TO OPERATE UNDER THE WX RPT GIVEN AND UNABLE TO APPROVE SUCH A REQUEST. CORRECTIVE ACTIONS: HAVE AN AWOS RPT DURING HRS THE TWR IS NOT IN OP. ALLOW TO CONTINUE THE APCH AND DSND BELOW FAF ALT WHEN THE LNDG SURFACE OR ENVIRONMENT CAN BE SEEN AT THE FAF ALT AND INFLT VISIBILITY IS GREATER THAN 7 MI REF FAR PART 121-651.