ACR TKOF AT CEDAR RAPIDS; IA; WITH LESS THAN THE REQUIRED VISIBILITY.

Narrative

AS WE STARTED OUR ENGS AND BEGAN OUR TAXI OUT; FOG RAPIDLY MOVED IN ON THE ARPT. AT THE BEGINNING OF OUR TAXI; RVR WAS OVER 6000 FT; BUT 90 SECONDS LATER IT DROPPED TO 1400 FT. THE CAPT AND I WERE ON THE LAST LEG OF AN OVERNIGHT; HAVING GOTTEN ABOUT 3 HRS OF SLEEP THE NIGHT BEFORE; AND THE CAPT WAS RUSHING TO TAKE OFF BEFORE THE FOG GOT TOO THICK. TWR ADVISED US THE RVR HAD FALLEN FROM 1600 FT TO 1400 FT; AND I TOLD THE CAPT I THOUGHT WE NEEDED 1600 FT RVR TO TAKE OFF (THE CHART SAID WE NEEDED 1600 FT). THE CAPT SAID WE COULD DEPART WITH LESS THAN 1600 FT IF WE HAD 'ADEQUATE VISUAL REFS.' I EXPLAINED I WASN'T EXACTLY SURE WE NEEDED 1600 FT SINCE I HAD HAD A SIMILAR CONVERSATION WITH ANOTHER CAPT JUST DAYS EARLIER; AND THAT CAPT WAS ALSO UNDER THE IMPRESSION WE COULD DEPART WITH VISIBILITY LESS THAN 1/4 MI IF WE HAD 'ADEQUATE VISUAL REFS.' THE CAPT I WAS FLYING WITH ON THIS OCCASION SAID HE WAS 'ABSOLUTELY SURE' WE COULD TAKE OFF WITH RVR 1400 FT; AND SINCE I WASN'T AS SURE; I MERELY EXPLAINED I WASN'T SURE AND WASN'T GOING TO STOP HIM FROM TAKING OFF. THE CAPT REAFFIRMED WITH ME HE WAS TOTALLY POSITIVE WE WERE LEGAL TO DEPART; SO WE TOOK OFF. WHILE ENRTE; I NOTICED THE CAPT FIDGETING NERVOUSLY BEFORE HE FINALLY BEGAN ASKING ME AGAIN IF I THOUGHT IT WAS LEGAL TO TAKE OFF. APPARENTLY; HE WASN'T AS SURE AS HE SAID HE WAS WHILE WE WERE ON THE GND. AFTER ARR AT OUR DEST; WE LOOKED IN OUR MANUAL AND FOUND WE NEEDED CTRLINE LIGHTS TO DEPART WITH LESS THAN 1600 FT RVR -- THE RWY FROM WHICH WE DEPARTED HAD NO CTRLINE LIGHTS. FROM THIS I LEARNED THAT WHENEVER I'M THE SLIGHTEST BIT UNSURE OF ANYTHING; I SHOULD STOP AND INSIST WE LOOK UP THE REG. IT WOULD HELP TO HAVE THE REGS EASILY ACCESSIBLE TO US IN OUR SMALL COCKPIT. HOWEVER; OUR JUDGEMENT WAS ALSO CLOUDED DUE TO OUR LACK OF SLEEP.