A CH200 PVT; NON IFR RATED PLT GOES IMC IN VFR FLT DURING A CLB TO ON TOP CONDITIONS WHEN TRAPPED BY SUDDEN CLOUD FORMATIONS ON A VFR FLT NEAR TO AND DIVERTING TO AMW; IA.

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PRIOR TO DEP FROM FAIRFIELD; IA; I HAD CHKED THE AWOS; NOAA WX RADIO; AND WX CHANNEL; BUT NEGLECTED TO CHK WITH FSS. WX WAS CLR WITH NO FOG FORECASTED WITHIN MY 106 MI RTE. WE DEFINITELY HAD VFR IN FAIRFIELD WHEN I DEPARTED. AS BEST AS I COULD REMEMBER; CEILING WAS 4500 FT. I DEPARTED AROUND XA00 WITH A COURSE TOWARDS WINTERSET (3Y3) TO UNDERCAST S OF DES MOINES AIRSPACE WITH INTENTION OF TURNING N FROM THERE TOWARDS MY FINAL DEST AT THE PVT AIRSTRIP IN DE SOTO (IA51). VISIBILITY AND CEILINGS WERE GOOD UNTIL I GOT WITHIN ABOUT 5 MI OF WINTERSET. MY RADIO WAS TUNED TO 117.5 -- VOR DES MOINES FOR ALTERNATE NAV VERIFICATION. AND I COULD BARELY MAKE OUT A VOICE ON THE CHANNEL WITH SOME RECORDED ADVISORY ABOUT RIME ICE AND PERHAPS I COULD MAKE OUT TOP BUT WASN'T SURE AND I WASN'T OVER AN ARPT. THEN OFF IN THE DISTANCE TO THE N AND W WHERE I WAS HEADED; THINGS LOOKED A LITTLE HAZY. THEN I NOTICED THE HAZY CONDITION SEEMED TO BE MOVING TOWARDS ME A LOT FASTER THAN I WAS MOVING TOWARDS IT. AND I SUDDENLY FOUND MYSELF IN HAZE WITH POOR GND VISIBILITY. I TURNED 180 DEGS; MAINTAINING 3200 FT MSL. BUT THE CLR AIRSPACE I HAD JUST FLOWN THROUGH WAS NOW HAZE ALSO. THE HAZE ALMOST INSTANTLY SEEMED TO TURN TO FOG AND THE GND DISAPPEARED. I WAS 2000 FT AGL; BUT I WAS AFRAID TO DSND WITHOUT VISIBILITY. I CLBED TO ABOUT 6000 FT MSL AND WAS THEN ABOVE THE CLOUD DECK AND I COULDN'T SEE A HOLE IN ANY DIRECTION. I MADE AN EMER CALL ON 121.5 AND RECEIVED HELP FROM FORT DODGE FSS AND PRIORITY HANDLING FROM DES MOINES APCH. ALSO; A PVT ACFT FLEW UP FROM AMW TO HELP AS AN ESCORT. I WAS TOLD TO FLY W WHERE THERE WAS A HOLE IN THE CLOUDS 5-10 MI WIDE. I DSNDED THROUGH THE HOLE AND FLEW TO A LNDG AT AMES ARPT (AMW). I MET ANOTHER PLT ON THE GND WHO HAD ALSO GOTTEN CAUGHT IN THE SAME FOG; BUT HE SAID HE HAD GOTTEN OUT OF IT BY DSNDING. BUT I DIDN'T FEEL THAT WAS AN OPTION AT THE TIME. I RECEIVED PHONE CALLS ON THE GND FROM DES MOINES APCH AND FORT DODGE FSS WHILE AT AMES AND WAS TOLD THAT THIS FORM SHOULD BE FILED.