During a cross country flight the glider encountered an area of sink. The pilot was unable to find any updraughts and selected a paddock in which to land. As the glider was on short final, it suddenly lost height and collided with the boundary fence of the paddock. The pilot was inexperienced on type and also lacked recent experience of outlandings. The paddock selected gave a shorter landing distance than desirable and the pilot aimed for a lower than normal height to cross the boundary fence. When sudden sink was encountered in the hot, turbulent conditions the pilot was unable to prevent colliding with the fence.