DURING A NIGHT OP AN MD11 CARGO FO QUESTIONS THE LEGITIMACY OF THE FLT'S APCH AND LNDG DURING A RAPIDLY DETERIORATING VISIBILITY CONDITION AT THE DEP END OF RWY AT PHL; PA.

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UPON ARR IN THE PHILADELPHIA AREA; ATC KEPT US AT 10000 FT OVERHEAD THE RWY DUE TO TFC. ARPT WAS IN SIGHT AND IT WAS CLR BUT HAD FOG W OF THE AREA. AT MID FIELD; WE WERE GIVEN A R TURN TO START DSCNT TO 3000 FT AND A BOX PATTERN FOR THE ILS RWY 27L. ON DOWNWIND; THE VISIBILITY DROPPED TO 2 1/2 MI WITH FOG. ON FINAL; A THIN LAYER WAS FLOWN THROUGH; BUT THE ARPT LIGHTS; ESPECIALLY RWY 27R; WERE IN SIGHT. APCH CTL DID NOT HAND US OFF TO TWR UNTIL INSIDE THE FAF AND TWR CLRED US TO LAND. AT 500 FT; THE RWY WAS CLRLY VISIBLE WITH THE W END SEEMINGLY PARTIALLY OBSCURED. THE LNDG WAS NORMAL; HOWEVER; DURING ROLLOUT THE VISIBILITY DROPPED. THE RWY SURFACE; LIGHTS AND MARKINGS WERE ALWAYS VISIBLE; BUT CONTINUED TO DETERIORATE UPON TAXI IN. TAXI AND PARKING WERE NORMAL AND NO UNUSUAL EVENTS OCCURRED. I RPT THIS WITH THE FOLLOWING THOUGHTS: ALTHOUGH VISIBILITY WAS ADEQUATE DURING APCH AND LNDG; FOG THAT BLOWS IN QUICKLY IS A SERIOUS HAZARD TO ACFT OP. IT WOULD BE HELPFUL TO RECEIVE VISIBILITY VALUES FROM VARIOUS FIELD POS AND TXWYS IN FOG PRONE AREAS. THERE ARE MANY TIMES IT CAN BE IFR ON THE RAMP AND VFR ON THE RWY. DURING THIS LNDG; IF THE FOG WAS CLOSER TO MID FIELD AND WHILE IN THE FLARE; IT COULD HAVE BEEN A WORSE SIT TO TRY A REJECTED LNDG. AFTER OUR LNDG; THE ARPT WAS 'TURNED AROUND' AND TKOFS MADE WITH A TAILWIND (LESS THAN 5 KTS) FOR THE UNLIKELY REJECTED TKOF.