THE GLIDER INSTRUCTOR STATED THAT, DURING THE TURN FROM BASE TO FINAL APPROACH, HE ENCOUNTERED A DOWN DRAFT. HE STATED THAT HE COULD NOT ARREST THE AIRCRAFT DESCENT PRIOR TO IMPACTING THE TREES ADJACENT TO THE RUNWAY. THE TEMPERATURE WAS 33 DEGREES CELSIUS, AND THE DENSITY ALTITUDE OF THE AIRPORT AT THE TIME OF THE ACCIDENT WAS 2,762 FEET.