RPTR DID NOT RECOGNIZE GND FOG AT NIGHT AND CANCELED IFR AND WENT IMC ON SHORT FINAL.

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THE WX WAS SEVERE CLR; 20 MI VISIBILITY. I HAD THE BEACON IN SIGHT (PRE DAWN); AWOS RPTING 200 FT OVCST VISIBILITY 1/2 MI. I WAS TALKING TO BNA APCH. EVERYWHERE WITHIN 75 NM WAS CLR; VISIBILITY UNRESTR. WITH THE BEACON IN SIGHT AT 15 MI; I ALSO HAD THE REIL AND THE RWY LIGHTS. I THOUGHT FOR SURE THE AWOS MACHINE HAD GONE NUTTY. I CANCELED IFR; PROCEEDED TO THE FIELD VFR. OVERFLEW THE FIELD. THERE WAS A THIN LAYER OF HAZE OVER THE ARPT. I HAD BEEN AIRBORNE ALMOST 7 HRS 15 MINS AND WAS TIRED BUT WAS ABSOLUTELY SURE THE AWOS WAS MISTAKEN THE FIELD GOOD VFR. DOWNWIND TO BASE AOK; FINAL APCH ALL WAS OK; EVERYTHING STILL IN SIGHT. THEN ALL WENT BLACK; ALL VISUAL CONTACT LOST. BACK ON INSTS FOR A SECOND AT 300- 400 FT AGL. LIGHTS IN SIGHT BUT VERY HAZY. I INITIATED A GAR WITH LOSS OF ALL VMC ON CLB FOR 200 FT OR SO. THEN VMC 20 MI VISIBILITY AND ARPT ENVIRONMENT IN SIGHT. I NOTIFIED BNA THAT I WOULD PROCEED NE 15 MI TO MNV VFR AND ADVISED THEM OF WHAT HAPPENED. HE REMARKED 'YOU WERE CLRED VFR APCH.' LATER THAT MORNING A MAN AT MNV ADVISED ME 'IN THE FALL ARPT A IS CLR; 10 MI DOWN ARPT B SOCKED IN ZERO ZERO. THE MOUNTAIN ARPTS CAN BE TRICKY.' FROM NOW ON I WILL HEED THE AWOS TAPE NO MATTER WHAT IT LOOKS LIKE FROM THE AIR DURING NIGHT OPS.