FLT INSTRUCTOR LANDS AT ARPT WITH INACCURATE AWOS WX INFO. UNAUTH CTL ZONE ENTRY.

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WHILE ENRTE TO THE DANVILLE ARPT I MONITORED DANVILLE AWOS. I HAD PREVIOUSLY RECEIVED A PREFLT FSS BRIEFING AND WAS INFORMED THAT DANVILLE HAD NO WX RPTING AND THAT THE NEAREST RPTED TERMINAL WAS CHAMPAIGN WHICH WAS RPTING AND FORECASTING VFR CONDITIONS. THE AWOS WAS RPTING CEILING 1100 FT BROKEN; 2500 OVCST AND 7 MI VISIBILITY. THEN CONTACTED UNICOM FOR TA'S. HOWEVER; APPROX 8 MI W OF THE ARPT; I ENCOUNTERED A CEILING OF ABOUT 1000 FT AND VISIBILITIES 3 MI IN RAIN AND FOG. I CONTINUED THE APCH IN WHAT APPEARED TO BE MARGINAL VFR AND AFTER LNDG; WAS TOLD BY ARPT FBO PERSONNEL THAT THE AWOS WAS CONTINUING TO RPT ABOVE VFR MINIMUM CONDITIONS. I DEPARTED SHORTLY THEREAFTER AND THE RAIN HAD INCREASED IN INTENSITY WHILE DEPARTING THE ARPT AREA. I UNDERSTAND THAT THE SENSOR'S LOCATION ONLY RPTS CONDITIONS OVER THE APCH END OF THE RWY. HOWEVER; THERE CAN BE A SUBSTANTIAL DIFFERENCE IN PREVAILING ARPT AREA CONDITIONS FROM WHAT THE AWOS IS SENSING. SINCE THIS ARPT IS A NOT-TWRED ARPT WITH A PART- TIME CTL ZONE; PLTS RELYING ON AWOS INFO COULD ENCOUNTER IMC WITHIN THE CTL ZONE WHILE IT IS IN EFFECT AND WITHOUT A SPECIAL VFR CLRNC THAT THE PLT WOULD HAVE OTHERWISE REQUESTED PRIOR TO ENTERING THE CLR ZONE. I HAVE ALSO SEEN DIFFERENCES IN WHAT AWOS AT ANOTHER ARPT (IKK) GREATER KANKAKEE HAS RPTED VERSUS PREVAILING AREA CONDITIONS. THE AWOS APPEARS TO GENERALLY RPT BETTER CONDITIONS THAN THE ACTUAL CONDITIONS THAT THE PLT MAY ENCOUNTER. I WOULD RECOMMEND THAT; UNTIL FUTURE AWOS PROVIDE WX PRECIPITATION INFO THAT AWOS BROADCAST END WITH A STATEMENT STRESSING THAT AWOS DOES NOT PROVIDE ARPT AREA ACTUAL CONDITIONS BUT ONLY CONDITIONS OVER THE APCH END OF A RWY. MANY PLTS; I BELIEVE; ASSUME SINCE BEING FAMILIAR WITH ATIS BROADCASTS INFO; THAT AWOS IS; IF NOT SIMILAR; THE VERY SAME THING; AND DON'T REALLY DISTINGUISH ANY DIFFERENCES BTWN THE SOURCES OF THE RPTED CONDITIONS -- HUMAN VERSUS COMPUTER GENERATED INFO.