MD80 FO DEPARTS BELOW TKOF MINIMUMS.

Narrative

DEPARTING JFK; THE FOG THAT HAD EARLIER RESTR THE ARPT APPEARED TO BE GONE. ATIS RPTED SEVERAL MI VISIBILITY AND NO CEILING. AS WE TAXIED; WE NOTICED A FEW WISPS OF FOG BUT THE VISIBILITY APPEARED GOOD AND TKOF AND LNDG OPS WERE BEING CONDUCTED ON SEVERAL RWYS. THE ACFT AHEAD OF US WAS CLRED FOR TKOF AND DEPARTED. I BELIEVE THERE WAS NO MENTION OF RESTRS TO VISIBILITY TO THAT ACFT. WE COULD SEE THAT ACFT CLBING OUT ON THE DEP AS WE MOVED INTO POS ON THE RWY. OUR TKOF CLRNC; ROUGHLY 1 MIN LATER; WAS SIMILAR TO THE FOLLOWING: '(CALL SIGN); RVR XXXX; TOUCHDOWN XXXX; MID XXXX; ROLLOUT; CLRED FOR TKOF RWY 31L. THE FIRST 2 RVR VALUES WERE IN THE 3000-5000 FT RANGE. I DID NOT CLRLY HEAR THE ROLLOUT RVR DUE TO A RADIO DISTORTION OR A DISTR. THE DYNAMICS OF THE SIT CAUSED ME TO ASSUME THAT THE RVR VALUES WERE GIVEN AS AN ADVISORY ONLY. I HAD NO REASON TO SUSPECT THAT RVR WAS NEAR MINIMUMS OR EVEN RESTRICTING. 'DYNAMICS' MEANING; WE HAD HEARD NO PREVIOUS RPTS OF RESTR VISIBILITY; THE ARPT APPEARED TO BE OPERATING UNRESTR; WE HAD THE PREVIOUS ACFT IN SIGHT; AND THE MANNER IN WHICH THE CTLR GAVE US CLRNC FOR TKOF IN THE SAME BREATH AS THE RVR RPT. DURING TKOF; THE VISIBILITY WAS GOOD UNTIL APCHING ROTATION; THEN DECREASED RAPIDLY. 100 FT AGL WE WERE IN VMC. I DO NOT KNOW IF THE ROLLOUT RVR WAS ABOVE THE MINIMUMS AND AM CONCERNED BY OUR LACK OF AWARENESS OF THE SIT. CONTRIBUTING FACTORS: THE PREVIOUSLY MAINTAINED 'APPEARANCES' OF GOOD WX; THE CTLR'S METHOD OF DELIVERY OF THE RVR VALUES WITH TKOF CLRNC; THE CAPT'S DELAY OF FINAL CHKLISTS UNTIL ENTERING THE RWY CAUSED A DIVISION OF ATTN BTWN PREPARATION FOR TKOF AND RADIO. PLTS ARE NOT GIVEN TIME TO ABSORB SUBTLE INFO. THERE IS AN EXPECTATION THAT THE ACFT WILL BEGIN MOVEMENT AS THE TKOF CLRNC IS RECEIVED. THEREFORE; WHEN WX CHANGES; THE CTLR MUST MAKE MORE EFFORT TO ENSURE THE PLT HAS RECEIVED THE INFO. THE WX SHOULD BE GIVEN IN A SEPARATE XMISSION; NOT WITH A CLRNC. A 'CHK WX MINIMUMS' CALL SIMILAR TO THE 'CHK DENSITY ALT' CALLOUT MAY BE HELPFUL.