The owner-pilot departed his home airport in the tailwheel-equipped airplane for a personal local flight. He spent about 45 minutes conducting airwork, and then returned for some practice landings and takeoffs, the first three of which were uneventful. According to the pilot, the ASOS (automated surface observing system) broadcast stated that the wind was from 220 degrees at 10 knots, so he selected runway 16 for the fourth landing. The pilot reported that he lost directional control during the landing rollout due to a wind disturbance. The airplane exited the east side of the runway, struck a ditch, and upended onto its nose. The right outboard wing was substantially damaged. Initial review of the recorded hourly wind data suggested that the winds were variable. The high resolution wind data revealed that the wind speed and direction both varied significantly in the minutes surrounding the accident time. The pilot reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.