GROB 102 GLIDER COLLIDED WITH TREES 6 MI S OF THE APT. THE PLT WAS SOARING 2 MI S OF THE APT AT 5900 FT MSL AND 6200 FT MSL IN 'WEAK LIFT.' HE ELECTED TO FLY FURTHER S IN THE VIC OF A RIDGELINE WITH AN ELEV APPROX 6000 FT MSL. AGAIN THE PLT EXPERIENCED 'WEAK LIFT' COND THE PLT WAS ABLE TO MAINTAIN BETWEEN 5900 FT MSL AND 6100 FT MSL AND KEEP OPEN FIELDS WITHIN GLIDING DIST. THE PLT HAD MADE SEVERAL PASSES IN THE VIC OF THE RIDGELINE. ON THE 3RD PASS A STRONG DOWNDRAFT AND THE ACFT LOST 125 FT OF ALT AND 15 KTS OF A/S. THE PLT MAINTAINED CONTROL OF THE ACFT AND COLLIDED WITH TREES SHORT OF THE OPEN FIELDS. THE PLT FLT LOG INDICATED HE HAD FLOWN FROM THE APT NINE TIMES SINCE OCT 1984 ONCE IN THE PREVIOUS 90 DAYS. HRLY WX RPTS BY FSS LOC 30 MI S OF APT WINDS 240 DEG AT 6 KTS 20 MIN PRIOR TO THE ACC AND CALM WINDS 40 MIN AFTER THE ACC. THE PLT IND THAT HE OBTAINED HIS WX FROM A PLT OBS PRIOR TO TAKEOFF AND NO FAA WX RPT OR BRIEFINGS WERE OBTAINED.