B727 CREW TOOK OFF BELOW TKOF MINIMUMS. CAPT USED PREVAILING VISIBILITY RATHER THAN THE CTLING RVR.

Narrative

TAXIING FOR TKOF AT MSP RWY 22 AT INTXN E. VISIBILITY WAS DECREASING AND VARIABLE FROM THE ORIGINAL ATIS VISIBILITY OF 2 MI. FOLLOWING 3 OTHER AIRLINE DEPS; WE WERE CLRED INTO POS. REQUIRING 1/4 MI VISIBILITY; WE REQUESTED THE CURRENT VISIBILITY. WE WERE GIVEN 1) ATIS VISIBILITY 1/4 MI; THEN 2) RVR TOUCHDOWN ZONE 1000 FT THEN 3) VISIBILITY 1/4 MI. USING THE HIGH INTENSITY RWY LIGHTS AS A REF; WE ESTIMATED APPROX 2000 FT VISIBILITY AS A CHK TO THE FINAL 1/4 MI RPT GIVEN. WHAT WE WERE GIVEN SEEMED TO BE A CHRONOLOGY OF VISIBILITY CHOICES LEADING UP TO THE FINAL 1/4 MI RPT. ATIS RPTS ARE NOT AT THE MOMENT; RVR RPTS ARE CONTINUALLY CHANGING (APPROPRIATE AT INTXN?); THEN WE WERE GIVEN VISIBILITY. IN VARIABLE CONDITIONS WE NEED ONE CURRENT; APPLICABLE RPT OF THE LOWEST CONDITIONS. SUPPLEMENTAL INFO FROM ACN 405266: WE REQUESTED VISIBILITY FROM TWR AS WE WERE CLRED ONTO THE RWY. TWR GAVE TOUCHDOWN ZONE RVR OF 1000 FT AND 1/4 MI VISIBILITY. THINKING THAT THE RVR MAY BE ERRONEOUS; WE COUNTED THE RWY LIGHTS TO MAKE SURE WE HAD 1/4 MI VISIBILITY. WE ESTIMATED VISIBILITY TO BE 2000 FT. WE FELT THAT SINCE TWR HAD GIVEN US 1/4 MI; AND WE VERIFIED AT LEAST THAT MUCH WITH THE RWY LIGHTS; WE DEPARTED.