GA SMA FAILED TO FOLLOW IFR INST DEP PROC LEAVING PVU A NIGHT WITH IFR CLRNC.

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RECEIVED WX BRIEF FROM CEDAR CITY FSS BY PHONE ABOUT 15 MINS PRIOR TO DEP. FILED IFR FLT PLAN FROM PROVO; UT; (PVU) TO SALT LAKE APCH AND DEPARTED PVU. CLRNC WAS TO FFU VIA PUBLISH DEP PROC; CLB TO 10000 FT FOLLOWED BY TRANSPONDER CODE; VOID TIME; AND FREQ FOR SALT LAKE APCH. I WAS TIRED AND IN RETROSPECT SEE THAT I MISSED THE IMPORTANCE OF THE PUBLISHED DEP PROC PART OF THE CLRNC. I DEPARTED RWY 31 AND TURNED DIRECT TO FFU VOR. I WAS CLBING TO 10000; BUT DUE TO DOWN DRAFTS FROM THE WIND OVER THE MOUNTAINS; I WAS NOT ABLE TO CLB ABOVE 7500 FT FOR SOME TIME. APCH RADAR IDENTED ME AND SAID THAT I WAS N OF COURSE AND NOT AT 10000 FT AND ASKED MY INTENTIONS. AT THIS TIME I WAS CLR OF CLOUDS AND COULD SEE THE CITIES OF AMERICAN FORK AND OREM; (APPROX 8-10 MI). I WAS ALSO EXPERIENCING MODERATE UP AND DOWN DRAFTS (UP TO 500 FPM DOWN AT FULL PWR AND MAX RATE OF CLB). I DECIDED TO CANCEL IFR; BECAUSE OF THE RPT FROM PVU AWOS OF CLR BELOW 12000 AND BECAUSE I HAD NOT ENCOUNTERED CLOUDS UNTIL VERY CLOSE TO FFU ON CLB OUT; I CANCELLED IFR AND TURNED TOWARDS PROVO. I WAS HEADED SE; LOOKING UP THE FREQ FOR PROVO VOR (PVU) OR FOR THE LOC WHEN I INADVERTENTLY ENTERED A CLOUD LAYER AT ABOUT 8000 FT MSL. I WAS QUITE SURPRISED AND EXPERIENCED VERTIGO FOR A SHORT REF TO INSTS AND CALLED APCH TO TELL THEM THAT I WAS IN CLOUDS AND REQUESTED VECTORS AND AN ILS APCH TO PROVO. WHILE ON VECTORS TO THE APCH; CONTINUED TO ENCOUNTER OCCASIONAL UP AND DOWN DRAFTS THAT MADE IT DIFFICULT TO MAINTAIN ASSIGNED ALT. ILS APCH AND LNDG AT PROVO WERE UNEVENTFUL. CONSIDERATIONS: I AM GOING TO PAY A LOT MORE ATTN TO MY PHYSICAL STATE AND REALIZE HOW FATIGUE CAN AFFECT MY PERCEPTIONS AND JUDGEMENT. IN RETROSPECT; THE PROBLEM WAS MISUNDERSTANDING THE CLRNC AND NOT FOLLOWING THE DEP PROC. I SHOULD HAVE ASKED INSTEAD OF ASSUMING THAT IT MEANT TO FLY DIRECT TO THE VOR. ADDITIONALLY I SHOULD HAVE ASKED FOR THE ILS INSTEAD OF CANCELLING IFR. (ALTHOUGH THERE WERE NO INDICATIONS AT THE TIME THAT I WOULD HIT CLOUDS AT THAT ALT WHILE HEADED BACK TO PVU). THE AWOS SYS HAS LIMITATIONS AND BASED ON THIS AND OTHER EXPERIENCES; I DO NOT FEEL THAT RPTS ARE ALWAYS ACCURATE IN REGARDS TO CLOUD COVER AND HEIGHTS.