GA PLT ENTERED FORECAST AND RPTED ICING CONDITIONS WITH C172 NEAR SUW; WI.

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UPON PICKING UP MY ACFT AFTER IT RECEIVED ITS ANNUAL INSPECTION; I FAILED TO CHK TO SEE THAT THE DEFROSTER VENT WAS OPEN FOR THE WINDSCREEN. SINCE I NEVER CLOSE IT; I ASSUMED THAT IT WOULD BE OPEN. AFTER CLBING FROM 1200 FT AGL TO 4000 FT MSL IN THE CLOUDS; MY WINDSCREEN FOGGED UP AND I HAD DIFFICULTY TO SEE ANYTHING FORWARD OF MY POS. AT THIS POINT I KNEW IT WOULD BE DIFFICULT TO DETERMINE WHETHER OR NOT I WAS PICKING ANY ICE UP OR NOT. I KNEW THE TOPS OF THE CLOUDS WERE PROBABLY LESS THAN 2000 FT ABOVE ME; SO I ASKED FOR HIGHER AND MANEUVERED MY ACFT TOWARD WHAT LOOKED TO BE A HOLE IN THE LAYER ABOVE. MY POOR FORWARD VISIBILITY LED ME TO FLY INTO A CLOUD AND PICK UP WHAT LOOKED TO BE LIGHT RIME ICE. I WAS ONLY IN THE ICING CONDITIONS FOR 15 SECONDS. ONCE I SAW THE ICE; I IMMEDIATELY BROKE OUT ON TOP AND LEVELED OUT LATER ON AN 8000 FT MSL. I NEVER PUT MYSELF IN DANGER; BECAUSE I KNEW IF I ENCOUNTERED ANY ICE; THAT I WOULD BE ABLE TO GET OUT OF IT AT ONCE. ONCE IN LEVEL FLT; I BEGAN TO LOOK AROUND A BIT MORE AND SAW THAT THE DEFROSTER VENT WAS CLOSED. I THEN OPENED IT UP FULLY AND THE WINDSCREEN IMMEDIATELY CLRED UP. THERE WAS NO ICE LEFT ON THE ACFT BY THIS TIME. THE REST OF THE FLT WAS ROUTINE. I BELIEVE MY FAILURE TO OPEN THE DEFROSTER COULD HAVE LED TO A DANGEROUS SIT IF THERE HAD BEEN MORE ICING IN THE CLOUDS. THERE WERE 2 PIREPS OF LIGHT ICE 40 NM N OF MY POS; BUT ONE WAS OVER 1 1/2 HRS OLD. I KNEW THERE WAS FORECASTED ICE ABOVE 4000 FT FREEZING LEVEL. SINCE THERE WAS NOT KNOWN ICE ALONG MY RTE; I ASKED FOR AND RECEIVED A HIGHER ALT; WHICH I KNEW WOULD PUT ME ABOVE THE CLOUDS. IF I HAD NOT BEEN ABLE TO LEAVE THE ICING CONDITIONS IMMEDIATELY; THE FOGGY WINDSCREEN MAY HAVE BEEN A CONTRIBUTING FACTOR TO A DANGEROUS SIT. MY JUDGEMENT TO ASSUME ITS POS AS BEING OPEN WAS AN OVERSIGHT ON MY PART. I LIMITED MY ABILITY TO SEE FORWARD AND ALSO MY ABILITY TO DETERMINE IF I WAS PICKING UP ANY ICE. I BELIEVE THIS RPT COULD BENEFIT OTHER HIGHER TIME PLTS. I BELIEVE MYSELF TO BE A CAREFUL SAFETY ORIENTED PLT. HOWEVER; ON THIS OCCASION; I CHKED AND DOUBLECHKED EVERYTHING -- OR SO I THOUGHT.