On August 30, 2003, at 1416 Pacific daylight time, a non-powered PZL Swidnik PW-5 glider, N105PS, impacted trees after a loss of thermal lift near Yreka, California. The pilot/owner was operating the glider under the provisions of 14 CFR Part 91. The private pilot, the sole occupant, sustained minor injuries; the glider sustained substantial damage. The local flight departed Siskiyou County Airport, Montague, California, about 1330. Day visual meteorological conditions prevailed, and no flight plan had been filed. The primary wreckage was at 41 degrees 43 minutes north latitude and 122 degrees 46 minutes west longitude. In a telephone interview, the pilot reported that he was released from a tow airplane near mountains at 5,600 feet mean sea level (msl). He climbed, using thermals, to 7,500 feet msl and began to maneuver toward a ridge. The glider was caught in a downdraft and descended at a rapid pace. He turned the glider to face into the ridge while losing altitude. As he came closer to the treetops on the mountain, he lifted the nose, which resulted in a loss of airspeed. The glider stalled into the trees. After the accident, the pilot radioed for help and successfully met up with a rescue team. The pilot speculated that the downdrafts were from a wave effect from mountains that were just south of the accident location. During the pilot's glider training, his intructors taught him to always turn away from ridges when "thermaling," in efforts to avoid downdrafts. In a written statement, the pilot observed that the glider damage consisted of the right wing being torn off, the tail boom broken, and the left wing broken at the aileron. There were no mechanical malfunctions reported.