C172 PLT ENCOUNTERS WX DURING INITIAL CLB AND IS VFR IN IMC.

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MCCOLLUM'S AWOS RPTED VFR CONDITIONS WITH 7 MI VISIBILITY AND THE LOWEST CEILING A BROKEN LAYER AT 12000 FT. HOWEVER; I ENCOUNTERED A THIN LAYER AT 1500 FT MSL AS I MADE MY TURN FROM RWY HDG TO R XWIND. ONE MIN I COULD SEE FINE -- AND THE NEXT I COULDN'T. REALIZING I WAS IN A CLOUD; I LOWERED THE NOSE AND DSNDED OUT OF IT. NOW; THOUGH; I WAS BELOW PATTERN ALT; AND LESS THAN 500 FT ABOVE GND. THE LAYER APPEARED TO EXTEND FOR SOME DISTANCE AROUND THE ARPT. I DIDN'T KNOW WHAT TO DO. REGARDLESS OF WHAT I DID; I'D BE IN VIOLATION OF EITHER THE CLOUD OR OBSTACLE CLRNC MINIMUMS. LOOKING UP; I COULD SEE THE RPTED CLOUDS HIGH ABOVE THE LAYER; SO I DECIDED IT WAS THIN ENOUGH TO POP UP THROUGH. ONCE THROUGH; IT WAS ABOVE 15 FT THICK -- I HAD PLENTY OF FORWARD VISIBILITY AND COULD EASILY STAY ABOVE THE LAYER; THROUGH WHICH I COULD STILL SEE ENOUGH OF THE GND TO MAINTAIN REF. I STILL DON'T KNOW WHAT I SHOULD HAVE DONE. THE LAYER WAS NOT RPTED ON AWOS AND WAS INVISIBLE FROM THE GND; SO I DIDN'T KNOW IT WAS THERE UNTIL I WAS IN IT. WHILE POPPING UP THROUGH THE LAYER WAS AGAINST CLOUD MINIMUM REGS; SO WOULD STAYING BELOW IT HAVE BEEN -- THE LAYER WAS 500 FT ABOVE THE GND; SO STAYING 500 FT BELOW IT WAS NOT AN OPTION. IN ADDITION; THE REGS FOR MSA'S DICTATE THAT I HAD TO FLY A MINIMUM OF 500 FT ABOVE THE GND (PROBABLY 1000 FT AS KENNESAW IS A CONGESTED AREA); WHICH WOULD HAVE PUT ME IN THE CLOUD. I THINK I BROKE THE FEWEST RULES BY GOING UP; BUT WOULD HAVE BEEN SAFER TO STAY JUST BELOW IT -- THUS MAINTAINING FORWARD VISIBILITY -- AND RETURN TO THE ARPT TO WAIT FOR IT TO EITHER LIFT OR MOVE OUT OF THE AREA.