According to the LET Super Blanik L23 flight instructor, he and a student pilot were towed to 2,700 feet above the airport, where the student pilot performed a series of maneuvers. Afterwards, the student pilot descended the glider toward a midfield downwind entry, about 1,500 feet above the airport. While descending, turbulence from an approaching rain shower increased, so the flight instructor took the controls. The flight instructor flew the remainder of the downwind leg, followed by a base leg, at 70 knots with full spoilers deployed since the glider would not descend. Turning onto final, while heading into the wind, the glider experienced an "extremely strong" sink rate, from which, the flight instructor could not recover. The glider subsequently landed hard about 100 feet short of the runway causing substantial damage. Shortly after the landing, light rain fell on the airport, and there was thunder in the distance. Ten minutes later, there was lightning about 2 miles southeast of the airport, and the winds dropped and remained at less than 10 knots the rest of the afternoon. The flight instructor reported over 9,600 hours of flight time with 3,700 hours as a pilot in command of gliders.