INSTRUCTOR AND STUDENT FLEW IN IMC WITHOUT IFR CLRNC IN CLASS E AIRSPACE.

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AT ABOUT XA00 LCL; FSS WX BRIEFING INDICATED A CEILING COULD BE EXPECTED AT 1000 FT AGL THROUGH XF00 LCL THEN BECOMING 2000 FT AGL AT XF00. BY XF00 LCL; CEILINGS WERE FORECAST TO BE 2500 FT AGL. AFTER XF00; VISIBILITY WAS EXPECTED TO BE NO LESS THAN 5 MI. BY THE TIME MY STUDENT AND I WERE READY TO PREFLT AT XF00; ATIS WAS RPTING A CEILING OF 1300 FT BROKEN AND 2900 FT OVCST WITH VISIBILITY AT LEAST 5 MI. CLOUD COVERAGE AT THE LOWER LEVEL APPEARED TO BE BREAKING UP INTO ABOUT 3 1/10 MI SCATTERED; ESPECIALLY UPWIND TO THE NE. BEFORE TKOF; I ASKED ANOTHER INSTRUCTOR JUST TAXIING IN ABOUT THE CLOUD COVERAGE AND HE RPTED AN END OF THE LOWER LAYER TO THE NE. I THOUGHT THIS TO BE THE FORECASTED CLRING OF THE LOWER LAYER AND DECIDED TO DEPART VFR WITH MY STUDENT TO THE NE. JUST TO BE PREPARED; HOWEVER; I CHOSE TO FLY AN IFR EQUIPPED C152 WITH CURRENT INSPECTIONS; VERIFIED MY OWN CURRENCY; AND TOOK CURRENT CHARTS ALONG JUST IN CASE. APPROX 5 MI N OF DAY AT 2500 FT MSL; WE ENCOUNTERED VERY LIGHT RAIN WHICH REDUCED VISIBILITY FROM AN ESTIMATED 7 MI TO APPROX 4 MI. A FEW SCATTERED CLOUDS; WHAT I BELIEVED TO BE THE REMAINDER OF THE LOWER DECK; APPEARED AHEAD; SO WE CLBED TO 3500 FT MSL. AT THAT POINT; THE LOWER LAYER BECAME BROKEN AND IT WAS VERY DIFFICULT TO PICK OUT THE UPPER DECK CLOUDS BECAUSE THEY BLENDED IN SO WELL WITH THE OVERALL LIGHT GRAY SKY COLOR. AT THAT POINT; WE WERE STILL VFR; BUT THE CONDITIONS WERE DETERIORATING RAPIDLY. I CALLED DAY DEP AND REQUESTED A TURN S BACK TO DAY. THE CTLR REQUESTED A L TURN TO 180 DEGS AND A DSCNT TO 2500 FT MSL. DURING THE MANEUVER; A CLOUD APPEARED FROM THE MURK AND I WAS ABLE TO AVOID IT. WE LOST VMC FOR APPROX 20 SECONDS BUT THEN RESUMED VFR NAV. EVENTUALLY WE REQUESTED A LCL IFR AND RETURNED TO DAY WITHOUT FURTHER INCIDENT. NEXT TIME I'LL BE MORE CONSERVATIVE REGARDING THE WX AND TURN BACK OR PICK UP AN IFR CLRNC SOONER.