FOG BANK MOVES OVER TWR RESULTING IN VISIBILITY BELOW TKOF MINIMUMS. RWY VISIBILITY GREATER THAN 1 MILE.

Narrative

UPON LEAVING THE GATE AT PHL I NOTICED A FOG BANK DEVELOPING RAPIDLY ON THE S END OF THE FIELD WHERE THE TWR IS LOCATED. THEY RPTED 1/4 MI VIS; WHICH IS THE TKOF MINIMUM (ACR) FOR RWY 9L. TFC WAS DEPARTING OFF RWY 9L. WE WERE #3 IN LINE WHEN THE TWR SAID THE VIS WAS NOW 1/16 MI AT RWY 9L (WE OBSERVED BETTER THAN 1 MI AS THE FOG WAS ONLY ON THE S SIDE OF THE FIELD). THEY THEN CLRED ANOTHER ACR TO TKOF; NO MENTION WAS MADE OF LEGALITY BY THE TWR. THEY THEN CLRED THE NEXT CARRIER INTO POS AND FOR TKOF. NOW WE WERE #2. THE CARRIER AHEAD OF ME REQUESTED RWY 9R AND WAS TOLD TO HOLD POS. I WAS THEN CLRED FOR TKOF. AT ALL TIMES WE HAD BETTER THAN 1 MI VIS; HOWEVER TWR VIS WAS RPTED AS 1/16 MI. I FEEL IF THIS WAS NOT A LEGAL DEP; THEN THE TWR SHOULD NOT HAVE CLRED US TO DEPART WITH NO MENTION OF TKOF MINIMUMS REQUIRED. AT NO TIME WERE WE ASKED WHAT WE REQUIRED AND THE CARRIER THAT REQUESTED RWY 9R WAS GIVEN AN INDEFINITE DELAY TO GET THERE. I FEEL THAT IN THE INTEREST OF SAFETY AND LEGALITY THAT TWR CTLRS SHOULD WORK WITH FLT CREWS IN PROTECTING EACH OTHER FROM SUCH MISTAKES. SUPPLEMENTAL INFO FROM ACN 98996: IT WAS EVIDENT FROM OUR POS THAT A LIGHT WIND WAS BLOWING A HEAVY WALL OF FOG OFF THE DELAWARE RIVER FROM THE SE. BEING THAT THE TWR IS LOCATED APPROX 2600' TO THE S OF RWY 9L; AND ONLY APPROX 600' N OF THE DELAWARE RIVER; IT WAS ENSHROUDED IN THE FOG FROM THE DELAWARE RIVER WHILE THE ACTIVE RWY REMAINED IN THE CLR. TWR CALLED TWR VIS 1/16 MI. I ESTIMATED THE VIS TO BE 1 1/4 MI TO THE SSE AND TO THE E. THE VIS IN ALL OTHER QUADRANTS WAS CLEAR. EXPECTED THE TWR TO INFORM US OF WHAT WE ALREADY KNEW AND TO EXPECT AN INDEFINITE DELAY; BUT INSTEAD ASKED US IF WE HAD ACCESS TO THE RWY AND TO POS AND HOLD. AFTER A BRIEF DISCUSSION THE CAPT AND MYSELF AGREED THAT THE VIS FOR RWY 9L WAS FAR ABOVE MINIMUMS AND THAT THE TKOF WOULD BE COMPLETED W/O ANY COMPROMISE TO SAFETY.