The certified flight instructor (CFI) was providing instruction to a student pilot in a glider. After a routine local instructional flight, they returned to the departure airport for landing. They flew a standard traffic pattern to runway 08R (dirt glider landing area) and the wind was from approximately 100 degrees at 12 knots, gusting to about 20 knots. The student was flying the glider, and the CFI was providing verbal instructions to him. The glider encountered a high sink rate, the CFI took the controls, and the glider ground looped. Post accident examination showed that the tail boom was fractured.