INEXPERIENCED COMMERCIAL PLT ON A CFI CERTIFICATION FLT INADVERTENTLY ENTERS IMC.

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I ARRIVED AT ZZZ ARPT AT XA00 FOR AN XB00 CFI RATING FLT. AS THE STUDENT; I COMPUTED THE WT AND BAL FOR THE FLT AND CHKED THE LCL WX. THE CURRENT METAR WAS SHOWING 8 MI WITH CLR SKIES. THE TAF SHOWED A TEMPORARY CONDITION OF 2 MI IN SMOKE; THAT WAS TO END AT XB00. AFTER XB00 IT WAS PREDICTED TO BE GREATER THAN 6 MI AND CLR. THE INSTRUCTOR AND I LISTENED TO THE ATIS AFTER STARTING THE ENG. IT HAD THE VISIBILITY AS 5 MI IN HAZE WITH CLR SKIES. WE DECIDED TO CONTINUE SINCE IT WAS STILL VMC. WE WERE GIVEN TAXI INSTRUCTIONS TO DEPART FROM RWY 23 AT INTXN F. AFTER COMPLETING THE RUN-UP WE WERE GIVEN A TKOF CLRNC WITH A L TURN SBOUND AFTER DEP. AT THIS TIME THE WX STILL APPEARED TO BE GOOD VFR. AFTER TKOF; AT APPROX 500-600 FT; THE VISIBILITY DROPPED DRASTICALLY; FROM GREATER THAN 5 MI TO LESS THAN 1 MI. IT WAS NECESSARY THAT WE RETURN TO THE ARPT AND LAND. I CALLED ZZZ TWR; AND ADVISED THEM THAT THE VISIBILITY HAD DROPPED DRASTICALLY AND WE NEEDED TO RETURN AND LAND. HE IMMEDIATELY GAVE US LNDG CLRNC; THIS TIME FOR RWY 32. AT THIS TIME WE WERE APPROX 1.5 DME S OF THE ARPT; AND AFTER TURNING TOWARDS IT; I WAS UNABLE TO SEE THE RWYS. I SEARCHED FOR A SHORT WHILE AND DETERMINED THAT WE PROBABLY WOULD NOT SEE IT AT THE ALT WE WERE AT. I CALLED THE TWR AGAIN; AND INFORMED THE CTLR THAT WE COULD NOT SEE THE ARPT AND NEEDED A LCL IFR. HE TOLD US TO FLY OUR PRESENT HDG (030 DEGS) AND TO CLB TO 2000 FT MSL; AND ASKED US THE CURRENT FLT CONDITIONS. WE WERE THEN GIVEN A FREQ CHANGE TO ZZZ DEP. ONCE CONTACT WAS MADE WITH THE DEP CTLR; WE GOT A DISCRETE BEACON CODE; AND WERE EVENTUALLY VECTORED ONTO THE LOC FOR THE ILS TO RWY 32. WE BROKE OUT AT APPROX 400 FT AGL; WITH FLT VISIBILITY BEING LESS THAN 1 MI. WE MADE AN UNEVENTFUL LNDG AND WENT IMMEDIATELY BACK TO THE RAMP. I THINK ONE THING I SHOULD HAVE DONE BUT DIDN'T WAS TO MAKE A CALL TO AN FSS BRIEFER. A BRIEFER MIGHT HAVE GIVEN ME SOME MORE INFO THAT COULD POSSIBLY HAVE CHANGED OUR DECISION TO GO ON THE FLT. ANOTHER THING THAT COULD HAVE BEEN DONE WAS TO ASK FOR A LCL IFR IMMEDIATELY AFTER DISCOVERING THAT THE FLT VISIBILITY WAS BELOW VFR MINIMUMS INSTEAD OF LOOKING FOR THE FIELD. IN THE FUTURE I KNOW TO ALWAYS GET AS DETAILED A WX BRIEF AS POSSIBLE; AND TO MAKE SURE I ALWAYS HAVE APCH CHARTS WITH ME.