NON INST RATED C172 PLT TAKES OFF INTO IMC CONDITIONS WITH NO FLT PLAN FILED. ATC COMES TO THE RESCUE.

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DEPARTED FROM MERRITT ISLAND ARPT AT APPROX XA00. SHORTLY AFTER TKOF; VISIBILITY WAS LOST DUE TO CLOUDS; AND I WAS FORCED TO FLY BY MY INSTS FOR A SHORT TIME UNTIL VFR WAS REGAINED AT APPROX 1800 FT MSL. I THEN CONTACTED ORLANDO APCH AND WAS XFERRED TO DAYTON APCH FOR INSTRUCTION. WHEN VERO BEACH ARPT OPENED WITH VFR CONDITIONS; I WAS XFERRED TO ZMA TO APCH AND LAND AT VERO BEACH. I HAD EARLIER CALLED 1-800-WXBRIEF FOR OFFICIAL WX BUT FAILED TO SPEAK TO A LIVE BRIEFER. THE WX HAD NOT YET BEEN UPDATED AND WAS MERELY A RECORDING. I DID NOT FILE A FLT PLAN BECAUSE I UNDERSTOOD MY RENTAL AGREEMENT TO REQUIRE A FLT PLAN FOR XCOUNTRY FLTS (50 NM RADIUS I ASSUMED) AND VERO BEACH; MY ORIGINAL DEST; IS NOT 50 NM FROM MERRITT ISLAND ARPT. I ALSO DID NOT GAIN INFO ABOUT THE WX (AS USUAL) FROM THE ARPT DUE TO IT BEING CLOSED BECAUSE OF HOW EARLY I DEPARTED. I SAW THE SKY; BUT FAILED TO SEE THE CLOUD LAYER DUE TO IT BEING DARK AND THE THICKNESS AND AREA IT COVERED; I SAW NO MOVEMENT OF THE CLOUDS; WHICH LED ME TO BELIEVE IT WAS A CLR SKY. IN THE FUTURE I CAN AVOID THIS MISTAKE BY ALWAYS SPEAKING TO A LIVE WX BRIEFER AND FILING A FLT PLAN REGARDLESS OF THE LENGTH OF THE FLT LEG. HAD I SPOKE TO A BRIEFER; I NEVER WOULD HAVE LEFT THE GND THAT DAY.