THE GLIDERS COLLIDED AT APPROXIMATELY 6,000 FEET MSL WHILE CLIMBING IN A THERMAL AREA. IN THIS PARTICULAR AREA, APPROXIMATELY 20 GLIDERS WERE MANEUVERING ATTEMPTING TO ATTAIN OPTIMUM LIFT. THIS RESULTED IN OVERLAPPING FLIGHT PATHS. THE PILOT OF JAPANESE REGISTRY JA2413 OBSERVED FINNISH REGISTRY OH825 AND TWO OTHER GLIDERS APPROACHING FROM HIS LEFT AND BELOW HIS ALTITUDE. HE LOST SIGHT OF THE GLIDERS. SHORTLY THEREAFTER, OH825 STRUCK HIS AFT FUSELAGE ON THE LEFT SIDE, WHICH RESULTED IN THE SEPARATION OF HIS EMPENNAGE. PAINT TRANSFER FROM THE RIGHT SIDE CANOPY TRACK AREA OF OH825 WAS FOUND ON THE LEFT SIDE OF JA2413 AT THE 7 O'CLOCK POSITION WHEN VIEWING JA2413 ALONG ITS LONGITUDINAL AXIS FROM THE REAR. ADDITIONAL PAINT TRANSFERS WERE LOCATED AT THE 8 O'CLOCK, 9 O'CLOCK AND 10 O'CLOCK POSITIONS. OH825 CONTINUED GLIDING, BUT IN AN ERRATIC MANNER, EVENTUALLY COLLIDING WITH THE TERRAIN. THE COCKPIT AREA OF OH825 SUSTAINED THE IN-FLIGHT COLLISION DAMAGE. THE PILOT OF JA2413 PARACHUTED TO SAFETY.